# Constraints to participatory plant breeding and participatory variety selection of tropical root and tuber crops: a review

Vincent Lebot, Carlos Viquez-Zamora

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Vincent Lebot, Carlos Viquez-Zamora. Constraints to participatory plant breeding and participatory variety selection of tropical root and tuber crops: a review. Euphytica, 2025, 221, pp.111. 10.1007/s10681-025-03561-0. hal-05130040

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# Constraints to participatory plant breeding and participatory variety selection of tropical root and tuber crops: a review

Vincent Lebot Ⓓ · Carlos Viquez-Zamora Ⓒ

Received: 24 February 2025 / Accepted: 28 May 2025
© The Author(s) 2025

**Abstract** The tropical root and tuber crops (cassava, sweet potato, yams and aroids) share common biological traits. Cultivars are allogamous, heterozygous, with erratic flowering, and are asexually propagated. Breeding is based on phenotypic recurrent selection of genotypes evaluated on-station through successive clonal generations, and through multi-locational trials once there is enough clonal material. The distribution of selected clones to farmers is constrained by their low multiplication rate and the absence of a seed industry. Quality aspects are critical and improved genotypes are often rejected because they do not correspond to the stringent consumers' expectations. Considering the socio-economic characteristics of smallholders, participatory plant breeding (PPB) and participatory variety selection (PVS) seem to be well adapted. This global review covers 35 studies related to 19 projects conducted over the last three decades. It attempts to clarify why PPB and PVS implementation is practically complex. Several constraints are identified: the heterogeneity of the clonal propagules, the need for simple and robust experimental field layouts, the necessary but difficult sensory evaluation, and the absence of breeders and farmers' reward systems.

Because of the significant GEI, most conventional breeding programs are now attempting to increase the number of sites as early as the single row and preliminary trials. We suggest that installing early seedling trials in different environments could allow farmers to be involved in the single row trial evaluation and claim ownership on the selected clones. These could be propagated on-farm, further evaluated, and returned to breeders for new cycles.

**Keywords** Asexual propagation · On-farm evaluation · Phenotypic recurrent selection · Variety adoption

## Introduction

Broadly, participatory plant breeding (PPB) covers all approaches strengthening breeders, farmers and end-users' collaboration to contribute to genetic improvement. It can be implemented through farmers' participatory variety selection (PVS) among improved genotypes or through selection within segregating populations (Almenkinders and Elings 2001). PVS is considered as distinct from PPB, depending on when farmers' participation starts but the process is similar in its objectives and varies with its level of advancement, the species (self- or cross-pollinated and/or asexually propagated), and the means available (Weltzien et al. 2003). When conducted in low-inputs cropping systems, PPB can develop varieties that

<footnote>V. Lebot (✉) · C. Viquez-Zamora
CIRAD, UMR AGAP Institut, 34398 Montpellier, France
e-mail: vincent.lebot@cirad.fr</footnote>

<footnote>V. Lebot · C. Viquez-Zamora
UMR AGAP Institut, University of Montpellier, CIRAD,
INRAE, Institut Agro, Montpellier, France</footnote>
outperform those selected on-station with conven-
tional breeding (Sperling et al. 2001; Weltzien et al.
2000). PPB has been identified as a practical solu-
tion to address the needs of smallholders in develop-
ing countries (Witcombe et al. 1996). It necessitates,
however, the use of simple and robust field designs
for multi-locational trials and the control of genotype
x environment interactions (GEI) (Atlin et al. 2001,
2017).

PPB is a decentralized approach complementing
the international system conducted in major research
centres (Morris and Bellon 2004). It present obvious
advantages: new varieties are released faster than in
conventional breeding and are better adapted to farm-
ers' needs (Ceccarelli and Grando 2007). As different
genotypes, selected in different locations, are readily
adopted, PPB contributes to agrobiodiversity (Cec-
carelli 2015; Ceccarelli and Grando 2022; Noru et al.
2024). In a few cases, PPB has been considered as
more expensive than conventional breeding because
of the travel costs involved in visits to farmers but the
two are in fact comparable (Mangione et al. 2006).
Despite these advantages, in many countries the
adoption of PPB is constrained by the difficulties of
the seed industry, and the absence of rewards for pub-
lic system breeders. National Agricultural Research
Systems (NARS) are also favouring more centralized
and scientist-driven approaches and are often reluc-
tant to develop PPB projects (Kholová et al. 2024).

The tropical root and tuber crops are an important
group of species in developing countries where they
play a major role for food security. Cassava (Mani-
hot esculenta), sweet potato (Ipomoea batatas), yam
(Dioscorea spp.) and aroids (Colocasia esculenta and
Xanthosoma sagittifolium) are cultivated by small-
holders for their starchy underground organs and for
their leaves rich in proteins (except for yam). Cassava
is the most important with 320 million tonnes pro-
duced worldwide, followed by sweet potato (90 Mt),
yam (80 Mt) and aroids (12 Mt) (FAOSTAT 2023).
As they are often intercropped, or planted in small
dispersed fields, some countries do not provide offi-
cial statistics, or national statistics are approximate,
and their real global contribution is probably under-
estimated. Cassava, sweet potato, and yams are under
the mandate of the CGIAR system while aroids are
orphan crops.

These species share common biological traits.
They are asexually propagated, are heterozygous,

allogamous with asynchrony, have variable ploidy
levels, and many cultivars do not produce flowers.
However, for each species, hundreds of local cul-
tivars exist and result from spontaneous, but rare,
cross-pollinations in farmers’ fields. Local cultivars
and improved varieties are clones of hybrids (Cebal-
los et al. 2015). Plants resulting from self-pollina-
tions are often discarded because of their poor per-
formances (Duputié et al. 2009). Volunteer seedlings
are tested, selected, and clonally propagated when
they satisfy farmers’ needs, especially regarding
taste. Local cultivars are often of very high qual-
ity and satisfy the diverse but stringent consumers’
requirements. Women play a key role in traditional
selection. Quality aspects are complex and vary with
different cultural backgrounds and societies but they
remain essential for farmers’ adoption of a new vari-
ety (Alamu et al. 2024). Most farmers are therefore
using ancient cultivars rather than new improved gen-
otypes. In many countries where they are important
today, these species have been clonally introduced
and present narrow genetic bases. The distribution of
selected clones is constrained by their low multiplica-
tion rate, the geographical isolation and distribution
of smallholders, and the absence of a private sector-
led seed industry. Consequently, the adoption of a
new clone is high in the vicinity of the research cen-
tres and tends to decrease with the increasing distance
from these centres (Alamu et al. 2024; Ceballos et al.
2015, 2021; Thiele et al. 2021). The international
movement of germplasm is difficult for phytosanitary
reasons.

Considering the socio-economic characteristics of
farmers and end-users, PPB and PVS seem to be well
adapted. A review of studies conducted over 36 years,
has shown that PPB and PVS have been used mostly
for major crops (rice, maize, bean, sorghum, barley,
potato, wheat and millet) in a few countries and only
seven studies were listed for cassava, four for sweet
potato, and one for taro (Ceccarelli and Grando
2020a, b). It is unknown if this situation is due to the
technical complexity of conducting crosses, to the
bulkiness and perishable nature of the propagules, to
the difficulties of establishing reliable experimental
field layouts, to cultural constraints from farmers, or
to the lack of interest from breeders.

The objectives of the present study are: (i) to
review PPB and PVS experimental projects con-
ducted globally on root and tuber crops over the last
thirty years, (ii) to compare and discuss their specific constraints, and (iii) to outline future research perspectives. It is expected that lessons learnt from one species might be useful for others.

## Materials and methods

We conducted a literature search through Agritrop, Google Scholar, Medline (PubMed), Science direct, and Web of Science databases. In each, we typed: “participatory plant breeding” and “participatory variety selection” followed by the common and species names of each tropical root and tuber crops. We also retrieved references listed in the studies examined. All papers were analysed to confirm that they were addressing PPB and PVS experimental work. Those addressing mostly participatory surveys and rural appraisals of farmers’ preferred traits or institutional, organisational, or policy issues, were consulted but not retained for this review. We deliberately decided to focus on the technical and practical constraints related to the field implementation of PPB and PVS.

## Results

Overall, and globally, 35 studies were retained and analysed, including 16 studies for cassava, 11 for sweet potato, 3 for yams and 5 for taro. These studies corresponded to 19 different research projects (10 for cassava, 3 for sweet potato, 3 for yams and 3 for taro) (Table 1). Hereafter, we use the word ‘cultivar’ to refer to farmers’ clones and the word ‘hybrid’ to describe an asexually propagated seedling obtained through pollinations conducted on-station by breeders. An improved ‘variety’ correspond to a new hybrid evaluated through successive clonal generations by breeders or farmers, and finally selected because performing better than the controls, often local cultivars. Hence, a variety is a new hybrid formally released and officially registered in national catalogues.

### Conventional breeding

For all species, on-station conventional breeding is based on phenotypic recurrent selection of hybrids

evaluated for few traits of agronomic interest. Heterozygous parents are selected on their individual value and are inter-crossed to generate full- or half-sib progenies. After germination, individual seedlings corresponding to different hybrids are asexually propagated through successive clonal generations by breeders. Every year, hundreds of hybrids are evaluated through phenotypic mass selection. Before an accurate assessment of their agronomic potential can be done it is often necessary to increase the number of clonal plants per hybrid to assess pests and diseases pressure (Fig. 1). Hybrids are raised in year 1 in a $F_1$ seedling trial (1 plant/hybrid). In year 2, a single row trial is established (5–10 plants/hybrid/1 site), followed in year 3 by a preliminary trial (10 plants/hybrid×2 reps/2 sites). In year 4, an advanced yield trial (20 plants/hybrid×3 reps/3 sites) finalize the on-station evaluation before going, in year 5, to multi-locational trials (MLTs) (20 plants/hybrid×3 reps/4 sites). MLTs are critical as GEI are known to be significant. PPB starts at the seedling trial stage, when farmers are invited to select visually the hybrids they will further evaluate in successive clonal trials. PVS is usually based on hybrids selected at the advanced yield trial and evaluated on-farm in different sites during the MLTs phase. With some minor modifications, depending on the programs means and objectives, this breeding scheme is used for cassava (Kawano 2003; Hershey 2020; Ceballos et al. 2021), sweet potato (Lebot 2010; Mwanga et al. 2017), yam (Darkwa et al. 2020) and aroids (Lebot and Ivančič 2022; Lebot et al. 2024).

### Cassava

In the early 1990s, the PVS approach was tested by CIAT (Centro Internacional de Agricultura Tropikal) breeders in northern Colombia. Ten new hybrids were tested in 28 communities with ten farmers participating in each community. Breeders conducted their own evaluation of the hybrids and compared their results with farmers’ evaluation. A major constraint was soon observed: the comparison between local cultivars and introduced hybrids could be done only if cuttings were produced in the same environmental conditions to avoid differences in propagules vigour. Hence, nurseries were established where cuttings from cultivars and hybrids were produced from a common plot. The record of information was also
<table><caption>**Table 1** Outcomes of PVS and PPB projects for cassava, sweet potato, yams and aroids (1993–2024)</caption><thead><tr><th>Species</th><th>Country</th><th>Outcomes</th><th>References</th></tr></thead><tbody><tr><td rowspan="10">Cassava</td><td>Colombia<br>North coast</td><td>Two varieties released after eight years breeding cycle. Cheaper data point cost compared to conventional breeding</td><td>Hernández (1993)<br>Hernández and Saad (2004) López et al. (2007)</td></tr><tr><td>Brazil<br>Northeast states</td><td>Ten varieties released after 8 years breeding cycle. 44% of farmers willing to pay for new varieties</td><td>Saad et al. (2005)</td></tr><tr><td>Brazil<br>State of Roraima</td><td>Four cultivars with high probability of being adopted after testing for two years</td><td>Souza et al. (2022)</td></tr><tr><td>Brazil<br>Federal District</td><td>Three yellow-fleshed, 3 cream-fleshed and 8 pink-fleshed varieties released after 8 years breeding cycle</td><td>Vieira et al. (2018), (2019)</td></tr><tr><td>Ghana</td><td>29 hybrids selected after four years breeding cycle</td><td>Gibson 2003, (2006)<br>Manu-Aduening et al. (2006)</td></tr><tr><td>Nigeria</td><td>Nine varieties released after four years breeding cycle</td><td>Dixon et al. (2007)</td></tr><tr><td>Kenya</td><td>Root weight of new hybrids were thrice higher than parents after two years of trials</td><td>Kamau et al. (2011)</td></tr><tr><td>Kenya</td><td>Development of a selection index based on farmers' preferred traits</td><td>Woyengo et al. (2014)</td></tr><tr><td>Uganda</td><td>0–5 varieties adopted by farmers after 6 years breeding cycle</td><td>Kawuki et al. (2011)</td></tr><tr><td>Uganda</td><td>Three varieties recommended with tricot approach during two years of trials</td><td>Nanyonjo et al. (2024)</td></tr><tr><td rowspan="3">Sweet potato</td><td>Uganda</td><td>One variety released after four years breeding cycle</td><td>Gibson et al. (2008), (2011) Mwanga et al. (2011)</td></tr><tr><td>Uganda</td><td>Farmers participating in PPB and PVS were 37 and 7 times more likely to adopt improved varieties</td><td>Kiiza et al. (2012)<br>Obong et al. (2017)<br>Okello et al. (2022)</td></tr><tr><td>South Africa</td><td>Seven cream-fleshed and three orange-fleshed varieties were recommended for six sites</td><td>Laurie and Magoro (2008)</td></tr><tr><td rowspan="3">Yams</td><td>Ghana</td><td>Three varieties released after five years breeding cycle</td><td>Otoo and Asiedu (2009)</td></tr><tr><td>Benin</td><td>More precise selection criteria for breeders based on farmers' needs</td><td>Vernier and Dansi (2006)</td></tr><tr><td>Nigeria</td><td>Three new hybrids evaluated by 461 farmers were confirmed as suitable for release</td><td>Asfaw et al. (2023)</td></tr><tr><td rowspan="3">Taro</td><td>Samoa</td><td>Five varieties released after 13 years of participative recurrent selection</td><td>Ebert and Waiqanabete (2018) Alexandra et al. (2020)</td></tr><tr><td>Hawaii</td><td>Four hybrids accepted by farmers after four years of trials</td><td>Paudel et al. (2022)</td></tr><tr><td>14 countries*</td><td>New hybrids outperformed local cultivars in most of the participating countries</td><td>Lebot et al. (2017)<br>Donkor et al. (2020), (2021)</td></tr></tbody></table>

problematical as farmers and breeders used different words for the same trait. So, information was standardized and a simple format (good, average, poor) was used. At the end of the project, farmers' evaluation of hybrids resulted in the release of two new

varieties in northern Colombia. Breeders had a better understanding of the traits of interest to farmers, especially regarding dry matter content (DMC) as farmers were selling their cassava as dried chips. Another interesting result was the financial estimation

<footnote>*Costa Rica, Nicaragua, Cuba, St Vincent, Burkina Faso, Ghana, Nigeria, South Africa, Madagascar, India, Indonesia, the Philippines, Papua New Guinea, Vanuatu</footnote>
<figure><img src="image_2.png" /><figcaption>**Fig. 1** On-station conventional breeding of root and tuber crops. In year 1, hybrids from half- or full-sib progenies are raised in a seedling trial and their number is drastically reduced (line in red), through selection on morphological traits and tolerance to pests and diseases. The number of clonal plants per hybrid (line in blue) is increased through successive clonal generations</figcaption></figure>

of the data point collected throughout the project: 0.5 USD for the on-farm evaluation and 0.8 USD for the on-station evaluation. Two steps were considered as essential: a phase of calibration and homogenization of planting material before field evaluation and an impact assessment of the method used for the project (Hernández 1993; Hernández and Saad 2004; López et al. 2007).

In Northeast Brazil, the main constraints are cassava root rots (Phytophthora spp., Fusarium spp., Pythium spp.) and cassava witches' broom disease (Xanthomonas campestris pv. campestris). Several communities in the States of Bahia, Sergipe, Pernambuco and Ceará, were involved in PVS during eight years. Demonstration plots of new hybrids were established on communal land and 122 farmers selected those to cultivate on their own land and compared them to their best cultivars used as local checks. Overall, more than 300 participatory trials were conducted. Ten varieties were formally released and a number of others were identified as potentially acceptable by farmers. Nearly half of the farmers adopted the new varieties and 44% were willing to pay for the cuttings, confirming their interest for these new hybrids (Saad et al. 2005). A similar approach was used in the state of Roraima, to test the adoption of nine introduced cultivars. On-farm trials, composed of small plots of nine lines of 50 plants per cultivar (1.0×0.6 m) without repetition, were maintained by farmers. Nine months after planting, cultivars were ranked by farmers from 1 to 9 (most preferred) for yield, root size and shape, stem height,

quality of the cooked root, starch content, taste, presence of fibre, and cooking time. Finally, four cultivars had high probability of being adopted (Souza et al. 2022).

In the Federal District around the capital Brasília, the Embrapa breeding program used PVS to release new varieties of sweet type cassava (cyanide content < 100 mg/kg). Seedlings were selected on-station through two successive clonal generations for their yellow or cream root colour flesh, high first branching and bacterial blight resistance. Selected hybrids were then evaluated through a third trial and 13 hybrids were selected and evaluated on-farm in 27 MLTs and compared to the same local cultivar during three seasons in small plots (5 lines×10 plants, 1.2×0.8 m). The plants were cultivated following farmers' techniques and the hybrids were ranked in decreasing order of acceptance. Three new varieties were selected with yellow flesh and one with cream flesh (Vieira et al. 2018). The same approach was used for pink-fleshed hybrids. After on-station evaluation, eight sweet pink-fleshed hybrids were selected for PVS conducted over three years in 19 participatory tests. Farmers evaluated the eight hybrids for initial vigour for weed control, resistance to pests and diseases, root yield, cooking time, and culinary quality, and two varieties were officially released (Vieira et al. 2019).

Two villages representing the Forest and the Forest-Savannah Transition Zones of Ghana were involved in a PPB project over five years. Seeds from 16 half-sib families were imported from IITA
(International Institute of Tropical Agriculture, Ibadan, Nigeria). The objective was to introduce a greater genetic diversity in farmers' fields. The families were selected for their CMD (Cassava Mosaic Disease) resistance, high yield, and Ghanaian or West African origins. Farmers were invited to list the attributes they would select: early vigour to cover weeds, early maturing, plants able to be intercropped with maize, high root yields, good cooking qualities, and good marketability. Six hundred and fifty seedlings were given to 44 farmers in the Forest zone and 700 to 40 farmers in the Forest-Savannah. The first year, 160 and 139 hybrids were selected, respectively. In the second year, 62 and 65 hybrids were selected. In the third year, 13 and 16 hybrids were selected and in the fourth year, the 29 hybrids selected in the third year were bulked together for MLTs. The rapidity of PPB was confirmed by the selection of 29 hybrids from 1350 seedlings within only three years. In Ghana, conventional breeding conducted on-station, takes approximately eight years and does not result automatically into farmers' adoption of new varieties. PPB was therefore considered as fast and cost efficient (Gibson 2003, 2006; Manu-Aduening et al. 2006).

In 2002, IITA raised a threat alarm regarding the arrival in Nigeria of a new CMD virus strain (EACMV-Ug2). A fast-track approach was developed to replace local cultivars susceptible to CMD with new CMD tolerant hybrids with high yield and high DMC, a major quality trait. Forty new hybrids were compared to three local cultivars in 176 on-farm trials, 50 demonstration trials and 32 MLTs, over two years. For each MLT, the results were analysed to compare the mean performance of each genotype per location and across locations. Plants were scored for yield, stem weight, root flesh colour, root shape, puree colour, taste, swelling capacity, hand feel, and stickiness. Assessment was done on a scale of 1–3 (best) and over 1500 farmers participated. In 2006, nine new varieties were officially released combining tolerance to the new virus strain and an acceptable quality (Dixon et al. 2007).

In Kenya, four local cultivars were crossed with six genotypes introduced from IITA. The best hybrids were selected for CMD resistance. After the seedling and single row trials, 225 hybrids were selected and planted on-station with the ten parents (two rows of 12 plants each, 1×1 m). Sixty-five farmers, including

34 women, were invited to participate in the selection process for early bulking, root size, appearance, taste and fibre content. Seven months after planting, farmers selected 30 hybrids that they considered with suitable quality. Eight months after planting an additional 21 hybrids were selected. The selection of 51 hybrids (early + medium bulking) resulted in a selection pressure of 22% (over 225 hybrids). A selection index was developed based on criteria with different weights (between brackets): root yield (5), DMC (4), harvest index (3), cyanide content (-3), number of roots (2) and shoot weight (1). The comparison of the ten best hybrids with the ten original parents, gave an indication of the progress. For example, the mean root weight of the ten best new hybrids was thrice the mean root weight of the parents (Kamau et al. 2011).

Another study in Kenya compared farmers and breeders' evaluation methods using 15 hybrids (3 sites, 3 reps, 3 rows×5 plants/hybrid). Farmers' traits included: yield, resistance to pests and diseases, plant height, branching level, cyanide content, and taste of cooked roots. Each trait was scored from 0 (absent) to 5 (satisfactory level). Breeders' evaluation was done using quantitative measurements and ranking was based on an index integrating CMD tolerance, fresh roots yield, DMC, cyanide score, number of roots per plant, plant height and first branching. Farmers and breeders rankings were compared and, interestingly, farmers' qualitative method of evaluation was found equally efficient than breeders' quantitative method. Differences were, however, observed between farmers and breeders rankings and farmers from different sites selected different hybrids (Woyengo et al. 2014).

CMD is a serious problem in Uganda where 9365 seeds from nine half-sib families were produced in a polycross block involving five IITA CMD tolerant genotypes and four local cultivars, susceptible to CMD but of good quality. In year two, 4080 seedlings were evaluated in a single row trial focusing on CMD tolerance and in year three, 1014 hybrids were evaluated in a non-replicated evaluation trial on a unique site using a selection index combining DMC, fresh root yield, cyanide content and CBSD (Cassava Brown Streak Disease) tolerance. In year four, a replicated uniform yield trial was conducted in six different sites (with 22 to 29 hybrids per site) with each site having different sets of hybrids. Selected hybrids were adopted at the respective sites (Kawuki et al. 2011). More recently, the triadic comparison of
technology options (tricot) allowed the evaluation of different hybrids in ten different sites in Uganda (van Etten et al. 2020). The tricot method compared eleven hybrids the first year and nine the second year (with a common control in the two years) and involved 294 men and 320 women farmers. The tricot approach was also used to evaluate hybrids for consumers' preferences for a boiled product and a flour-based meal. The different locations ranked the hybrids differently. The number of days with precipitation > 20 mm significantly influenced the ranking of hybrids between sites. The combination of agronomic and quality traits allowed the identification of three new officially released varieties which could be recommended to farmers (Nanyonjo et al. 2024).

To sum up, for cassava, PPB and PVS studies conducted in six countries (Brazil, Colombia, Ghana, Kenya, Nigeria, Uganda) encountered similar difficulties due to the limited volume of planting material available to establish robust trials with sufficient replications (and blocks) in different sites (Table 1). However, the conclusions of these studies were all encouraging and recommended the use of these two approaches for cassava. The need for new, clean, genotypes corresponding to particular taste and local dishes was emphasised and so was the role of women. Cassava research is often more advanced because of its economic importance, but comparative analysis with other root and tuber crops is possible, as the biological constraints are similar, even if the means invested are of different proportions. A greater number of PPB and PVS projects were conducted on cassava (Table 1) but other species projects indicate strong similarities.

## Sweet potato

Sweet potato viruses are easily transmitted by the stem cuttings and are transmitted from one plant to another by aphids and whiteflies. The control of viruses through genetic resistance, quality planting material and crop management is a priority in all producing countries. In the absence of a regular supply of sanitized planting material, smallholders select plants resulting from the germination of seeds as they are often more vigorous, at least in the early clonal generations. In the region around the Lake Victoria, in East Africa, the major constraint faced by sweet potato farmers is SPVD (Sweet Potato Virus

Disease). Surveys revealed that 11% of farmers were aware of seedlings germinating spontaneously in their fields but considered that they were inferior to their cultivars. It was suggested to approach the problem via PPB (Gibson et al. 2000). Similar surveys conducted in Indonesia (Widyastuti and Minantyorini 2001), and Papua New Guinea (Roullier et al. 2013) revealed that farmers were regularly selecting new seedlings because they appear healthier and more vigorous than old cultivars. These observations encouraged the use of PPB and PVS for sweet potato (Grüneberg et al. 2009).

In Uganda, sweet potato conventional breeding started in the early 1990s and successfully released improved varieties to farmers, the most successful being *NASPOT 1*. In 2003, about 6000 seeds were collected in crossing blocks combining local cultivars and introduced varieties. The seeds were given to one farmers' association who evaluated 2382 seedlings and in three successive clonal generations selected 163, 67 and finally 13 hybrids. These 13 hybrids were then combined with 11 hybrids similarly obtained by another farmers' association. In the fourth clonal generation, all 24 hybrids were tested by both associations. Nine of these hybrids were compared to improved varieties including *NASPOT 1*. Local cultivars were used as controls in MLTs composed of single plots with 6–8 new hybrids plus a plot of two cultivars selected by farmers (Gibson et al. 2008). Not less than 52 attributes were scored qualitatively by farmers as: very good (2), good (1), adequate (0), and bad (−1) or very bad (−2). The most important attributes ranked by farmers in decreasing order of importance were: yield, roots sweet when cooked, big roots, drought resistance and roots mealy when cooked. At the end of the experiment, a new hybrid was selected and spread very rapidly through informal channels under the name *Tomulabula*, performing better than *NASPOT 1* (Gibson et al. 2011). *Tomulabula* was then released as an official variety under the name *NASPOT 11* (Mwanga et al. 2011) (Table 1).

Following this success, a study was undertaken to evaluate the impact of PPB and PVS in Uganda. Three groups of farmers were chosen from three districts: one involved in PPB, another involved in PVS and a third not participating to the two approaches. Farmers who participated in PPB and PVS were 37 and 7 times more likely to adopt new hybrids than
those who had not. The weaker adoption after PVS
might be explained by unsuitable skin and flesh col-
our, taste, or maturity period. Farmers who partici-
pated to PPB felt that they owned the new hybrids
since they were involved in the seedling selection
stage (Kiiza et al. 2012). Participatory variety dem-
onstration trials were also organized and confirmed
that farmers adopt readily new hybrids (Laban et al.
2015; Obong et al. 2017). Hence, a large survey
was conducted in Uganda to better inform breeders
about farmers’ preferred traits (984 farmers, 52 seed
multipliers, 134 traders and 147 consumers were
surveyed). Vine survival at planting, weevil resist-
ance and SPVD resistance were farmers’ most pre-
ferred traits while consumers ranked highly sensory
quality traits (Okello et al. 2022).

Hybrids developed in South Africa were com-
pared to imported varieties using PVS to assess
their acceptability in low input cropping systems.
Overall, 22 local hybrids, six local cultivars and
eight imported varieties, were tested in six dif-
ferent sites, with local cultivars used as controls.
Cuttings were first propagated from a virus-tested
source. Each trial compared 15 genotypes (3 reps
of 30 plants, 1.0×0.3 m in RCBs). Planting and
maintenance was done by research technicians,
agricultural extension officers and farmers. Entries
were scored for skin and flesh colours, shape,
defects, yield, and taste evaluated during farmers’
days with cooked sweet potatoes. Imported varie-
ties performed poorly compared to local hybrids.
The strength of the PVS approach was a rapid and
cost effective way to assess genotypes but the lack
of trials uniformity (variation in soil fertility, poor
weeding) were real difficulties. Hence, to reduce the
number of genotypes for final on-farm evaluation, it
was suggested to conduct on-station trials over two
years before establishing MLTs (Laurie and Magoro
2008).

In Ethiopia, five local hybrids with orange flesh
were evaluated on two different sites (RCB, three
reps, plots of 3×3 m, with 1.5 m between plots). At
harvest, hybrids were scored by farmers for major
agronomic traits and taste. The analysis of variance
showed significant differences among hybrids for
plant height, root diameter, root yield and dry matter
content. However, the sole hybrid that outperformed
others for yield was discarded by farmers based on
taste after boiling. (Abewoy et al. 2022).

Results obtained from these PPB and PVS stud-
ies on sweet potato confirmed the efficiency of these
approaches to evaluate new genotypes. It seems nec-
essary, however, to select hybrids with sufficiently
distinct morphological traits to ease identification and
traceability by farmers. The need for a source of virus
free cuttings is critical to homogenize the planting
material before trial establishment. The practical con-
straints involved in the establishment of sweet potato
PPB and PVS trials are very similar than those faced
by cassava breeders. The most critical ones are the
limited number of plants per hybrid, the homogeniza-
tion and sanitation of propagules.

Yams

The development of varieties that satisfy consumers
taste, palatability and cooking requirements is the
most complex part of a yam breeding program. Yam
can be prepared boiled, pounded, fried, roasted or
in flour, and there are local preferences and cultural
factors necessitating farmers’ participatory evalu-
ation (Alamu et al. 2024). In Ghana, 36 genotypes
(including local cultivars and hybrids) were evalu-
ated on-station over three years with farmers’ partici-
pation, and further evaluated on-farm in 44 villages,
with local cultivars used as controls in RCBs. The
best 11 genotypes (5 cultivars and 6 hybrids) for yield
and pests and diseases tolerance were further evalu-
ated (3 ecological zones, 6 villages/zone, 2 farmers/
village). Appreciation of boiled yam was done by
farmers scoring on a scale of 1–5 for: colour, texture,
aroma, taste and perception, after-taste and general
acceptance. Environment had no effect on farmers’
preferences but gender differences were observed
for the appreciation of cooked yam texture and taste.
Two cultivars and one hybrid were the most preferred
regarding sensory evaluation and were officially
released as recommended varieties in 2005 (Otoo and
Asiedu 2009).

In Benin, indigenous knowledge associated to 310
local cultivars was recorded using individual inter-
views and group discussions in 70 villages. Accord-
ing to farmers, the major criteria were: tuber quality
and ability to give good pounded yam, yield, earli-
ness, market value, storage ability and seed qual-
ity (Vernier and Dansi 2006). As a significantly low
rate of adoption of improved varieties by produc-
ers and consumers was observed, another study was
undertaken in 20 villages and 16 markets to identify preferences. Results indicated that an improved variety should have high scores for texture, softness, elasticity, and colour, as well as tolerance to yam mosaic virus disease, anthracnose and nematodes, confirming the complexity of breeders' tasks (Loko et al. 2022). Thirteen *D. alata* hybrids introduced from IITA in Nigeria were compared to three local cultivars across four sites in Benin. Local cultivars performed better than the introduced hybrids for yield and quality of pounded yam. It was suggested to collect more local cultivars and to evaluate them further (Dansi et al. 2024).

However, in Nigeria, four new hybrids considered as ready for official release were evaluated on-farm in 15 sites and compared to local cultivars. Depending on locations, 30 to 55 farmers with different age and gender were involved and overall, 461 farmers participated in the evaluation. In each site, the trials were planted and managed by farmers with the best local cultivars used as controls. Farmers scored the genotypes for tuber yield, tuber appearance and tuber shape, and for quality of boiled and pounded yam. One *D. alata* hybrid and two *D. rotundata* hybrids were found suitable for release as new varieties (Asfaw et al. 2023) (Table 1).

The very low propagation ratio of major yam species is a technical constraint to produce sufficiently homogenised planting material on-station before establishing MLTs. The need to combine tolerance to severe diseases (anthracnose for *D. alata* and YMV for *D. rotundata*) with acceptable cooking quality traits, is a very complex task and limits opportunities for PPB and PVS. As yams are usually propagated using tuber fragments, their clonal propagation ratio is lower than those of cassava and sweet potato. Stem cuttings can be used to speed up the process but not all genotypes are amenable to this technique.

## Aroids

Cocoyam (*X. sagittifolium*) breeding started in the late 1970s in Puerto-Rico and Cameroon. Thousands of seedlings were produced with the objectives of releasing virus-free plants and hybrids resistant to cocoyam root rot disease but PPB and PVS were not used (Lebot et al. 2024). Taro (*C. esculenta*) breeding started in the 1970s in Fiji with on-station evaluation but also involved MLTs with on-farm scoring to

compare 15 hybrids with two local cultivars. Selected hybrids were introduced in vitro to Samoa for field evaluation and compared well with local cultivars (Sivan and Liyanage 1993). In 1993 *Phytophthora colocasiae* (taro leaf blight, TLB) was introduced to Samoa and devastated a flourishing industry exporting taro corms to New Zealand and Australia. A breeding program was implemented to generate hybrids tolerant to TLB. As all cultivars and hybrids from Fiji, were susceptible to TLB, exotic genotypes from Micronesia and Asia were introduced, cross-pollinated with local cultivars and thousands of seedlings were screened on-station for tolerance to TLB. However, farmers were concerned by the slow pace of the breeding program and it was decided to speed up the process through PVS. In 1995, exotic cultivars were evaluated on-farm by 25 farmers who ranked them on a scale of 1–4 for growth vigour, yield, resistance to TLB, production of suckers and quality. In 1999, 200 hybrids were evaluated in a single village. Farmers ranked them following the same qualitative scale and selected 25 hybrids (Hunter et al. 2001).

The best Samoan cultivars were crossed with TLB resistant cultivars from Indonesia and Malaysia, seedlings were produced, screened through five successive clonal generations, and distributed to farmers for further evaluation. Forty-five farmers were involved and were responsible for the planting, labelling, maintenance, selection and evaluation of culinary traits. Every year, each farmer was supplied with up to 100 seedlings each, and about 6000 hybrids were scored and successfully evaluated on-farm. Farmers selected the hybrids without any advice from the breeders and gave them back cuttings of their best selections. Farmers had the right to exchange or sell any planting material from their selections (Iosefa et al. 2013). The recurrent phenotypic selection process took approximately 13 years, and five varieties (Samoa-1 to -5) were officially released in 2009 using PVS (Ebert and Waiqanabete 2018; Alexandra et al. 2020).

In Hawaii, over three decades, hybrids were produced by cross pollinations between local cultivars and introduced varieties resistant to TLB and were evaluated on-station by the University of Hawaii. From 2016 to 2020, 14 MLTs (RCBs) were conducted in four different environments with varying number of hybrids and replicates per trial. Overall, 18 hybrids were compared to the best two local cultivars. Each hybrid and cultivars were planted in plots of 30
plants (3 rows × 10 plants, spaced 0.3 × 0.6 m). After nine months, only the inner row was harvested to avoid border effects, and scored for fresh yield, DMC, tolerance to TLB. At harvest, farmers evaluated corm quality for stickiness, sweetness, flavour, and overall acceptance. Although the trials results were highly unbalanced, a Bayesian approach was used to account for distribution and identified four hybrids resistant to TLB, high yielding with a taste acceptable by farmers (Paudel et al. 2022).

A global experiment was conducted by the Inter-
national Network for Edible Aroids (INEA) in 14
countries from America, Africa, Asia and the Pacific.
A set of 50 indexed genotypes composed of hybrids
and cultivars was sent in vitro to partners. After two
years of on-station agronomic evaluation trials, the
best genotypes were distributed to farmers for PVS.
Hybrids and exotic cultivars tolerant to TLB out-
performed local cultivars in most countries (Lebot
et al. 2017). In Ghana, four hybrids introduced by
INEA were registered by the National Seed Coun-
cil and released as official varieties named Huogbe-
lor (hybrid: SM158), Asempa (SM115), Agyenkwa
(SM151) and Y'anyawoa (SM16) (NSC 2019). These
new varieties were used as parents in controlled
crosses with local cultivars and new locally produced
hybrids were then evaluated (Donkor et al. 2020,
2021) (Table 1).

As taro is not under the mandate of an international
research centre, it was necessary to use PVS and PPB
to accelerate the distribution of improved hybrids to
farmers, especially as it is a smallholders' crop in
countries with limited means devoted to breeding
(Lebot and Ivančič 2022). The hybrids selected by
farmers were returned to breeders to be used as par-
ents in the next breeding cycle. This system appeared
highly efficient for taro and could be used for cassava,
sweet potato and yam.

Discussion and comparative analysis
between species

We have briefly reported here 19 different PPB and PVS projects conducted on root and tuber crops over the last three decades at the global level (Table 1). Considering the importance of these species for food security in different countries, and the advancement of conventional and molecular breeding activities

conducted on these species by international centres,
the number of participatory projects is comparatively
low. This is surprising because the projects analysed
were successful and resulted in new varieties being
adopted by farmers. PPB and PVS seems to be well
adapted to root and tuber crops because they rapidly
satisfy the need for new genotypes in countries or
regions were these crops have been introduced clon-
ally and harbour narrow genetic bases. With forth-
coming climatic changes, PPB and PVS are seen for
most crops, as an efficient mean of introducing allelic
diversity in farmers' fields (Ceccarelli and Grando
2020a, b, 2022) and this is critical for root and tuber
crops.

There are, however, several technical and practi-
cal constraints that impede the development of such
projects. The heterogeneity of propagules and their
low propagation ratios, the GEI, the need for simple
but robust field layouts, the necessary but complex
sensory evaluation, and the absence of breeders and
farmers' reward systems. We discuss, hereafter, these
constraints within a comparative analysis between
species.

Heterogeneity of clonal propagules

Most pests and diseases are easily transferred through
infested propagules. As these species are cultivated
using cuttings, tubers, suckers, or corms, saved
from the previous crop, many factors impact sprout-
ing and plant establishment. The physiological age,
and the origin, size, and weight of the propagule,
directly influence the plant vigour. The DMC of the
propagule, as well as its nutrients content may affect
sprouting. Higher DM and nutrients levels in the
propagule represent more reserves allocated to vigor-
ous young tissues growth and to faster development
of aerial parts and canopy. These factors combined
lead to heterogeneous clonal populations for a given
genotype (hybrid or cultivar). Likewise, storage con-
ditions of the propagules strongly influence their
sprouting ability, their subsequent plant vigour, and
the final yield of the plant.

When MLTs are established in farmers’ fields, new
hybrids are compared with local checks, very often
the best local cultivars. These cultivars are planted
with clonal material which is often heavily infected
with viral particles or endophytes which dimin-
ish their performances. The new hybrids which are
introduced for comparison in on-farm trials, result from a few clonal generations (3–5) after the germination of the seed and therefore present a comparatively low viral or bacterial load. When farmers adopt new genotypes given to them via a PVS project, it is unknown if they adopt them because they perform well as a result of clean planting material or if it is due to their genetic performances. As the two are often combined it is difficult to differentiate them. The demand for clean material is often so high, that farmers may accept for trial every new hybrid presented to them. As the vegetative propagation ratio is rather low, 1: 5–10 for cassava and sweet potato and 1: 3–5 for yam and taro, it takes several years for a farmer to establish a clonal population of a new hybrid, comparable to the one he has for local cultivars. For PVS projects, the production of planting material under controlled conditions in the target region is of course preferable, but it implies anticipation, extra time, and additional means.

When planting material is bulked to satisfy the needs of PVS trials, especially when there is a need for replicates, there is a risk to produce non calibrated and heterogeneous clonal material. Farmers are especially interested in propagules with high survival rate, rapid sprouting, and fast canopy development to cover rapidly the weeds. For a given genotype, sprouting performance of the propagule is variable, depending on its origin on the plant. For cassava for example, it is often recommended to take cuttings from the middle and lower parts of the stems showing a high volume density, which approximately correspond to a survival rate over 80–90%. Soft cuttings, taken from the terminal part of the stems are more fragile, with lower density, and present a lower survival rate. For sweet potato, the distal part of the stem performs better. For taro, the ‘head set’ corresponding to the base of the stem and head of the main corm, outperforms suckers and is the most appreciated propagule.

The situation is rather problematical for yams. The high plant variability, between different clones of the same genotype and between the distal, central and proximal parts of the same tuber, affect the early crop development. For all species, plants with different physiological age, or plants resulting from propagules with different size and weight, have a different response. This often leads to intra-clonal variation being greater than inter-genotype variation. When the genotype is the treatment, within plot variation

is often greater than between plots variation. Skewed distributions are then rendering statistical analysis of the trials problematical. Highly variable plant size within a plot of a given genotype induces large coefficient of variation of the means, especially with plots composed of a small number of individual plants. To reduce these effects on the experimental precision of the field layout, the size of the experimental plot or the number of replications is usually increased but this is laborious, expensive and might be problematical if the propagules are not calibrated. It has been suggested than cohorts (group of individuals characterized by the same physiological age) would be a suitable experimental unit for yams (Cornet et al. 2014).

## Poor correlations between clonal generations

The phenotypic plasticity of root and tuber crops is another major challenge. Hybrids that are identified as good performers in the single row trial will not necessarily perform well several clonal generations later. Along the hybrid selection process, from the single row trial to the MLTs, there are often significant experimental errors. GEI, different responses of heterogeneous propagules and changes in the association between major traits, such as fresh root yield and DMC, are serious difficulties faced by breeders. For cassava, the nutritional, physiological and health status of the propagule on the vigour of the clonal population is highly significant and could explain the poor correlations between year 1 and year 5 (Joaqui et al. 2016). For taro, correlations are poor between major traits measured in the seedling trial and the first two clonal generations, even when propagules have been properly calibrated and homogenized (Soulard et al. 2016).

## Genotype x environment interactions (GEI)

Although this is not specific to root and tuber crops, GEI are highly significant and the extremely low propagation ratio (1: 3–10) is a practical constraint when attempting to increase the number of sites for testing new hybrids as early as possible. For example, in Nigeria, trials were established to evaluate eight new cassava hybrids with farmers and processors during four years in seven sites, with two replications on large plots (320m², 1×0.8 m). Hybrids were scored
for growth vigour, root number, fresh yield, and DMC and a highly significant GEI effect was observed for all studied traits. The highly significant variation observed for most traits across all locations confirmed genetic variation. Three hybrids presented high DMC for processors and farmers but only one out of eight was found stable across locations (Ogwuche et al. 2023). The situation is comparable for sweet potato (Lebot 2010; Mahmud et al. 2021; Mwanga et al. 2017), yam (Darkwa et al. 2020) and aroids (Lebot and Ivančič 2022; Lebot et al. 2024).

In year 4, when the stage of the advanced trial is reached on-station, there is no guarantee that the selected genotypes will perform well on different sites. The trends in genetic gains for the IITA yam breeding program for West Africa have been estimated using the data from advanced trials from 2010 to 2022 (31 sites for *D. rotundata* and 28 sites for *D. alata*). The genotypes evaluated were hybrids developed by IITA, except for the controls, often local cultivars or hybrids from NARS. The genetic gain for fresh tuber yield was 2.18% per year for *D. alata* and 1.38% for *D. rotundata*. Negative trends were observed for tuber DMC in both species, indicating a lack of improvement in DMC, a major trait related to quality. Overall, the results suggested that, while the breeding program has succeeded in enhancing fresh tuber yield, it might have reduced DMC and compromised disease resistance (Asfaw et al. 2024). It is unknown, however, if the measured gains were due to the use of clean planting material compared to local controls harbouring an old varietal age, corresponding very often to decades of clonal propagation. It is also unknown if the new hybrids will continue to perform well when the number of clonal generations will accumulate with their load of endophytes and viral particles. For all root and tuber crop species, most breeders observe vigour in the first clonal generations of the recently produced hybrids and a less spectacular performance years after.

## Simple and robust experimental field designs

In the absence of a tissue culture laboratory, the field propagation and calibration of planting material resulting from true botanical seed is a rather slow process for most NARS. In five years, field propagation can result in 180–300 stem cuttings for cassava and sweet potato, and to 90–120 propagules

for yam and aroids. Micro-propagation (single node cuttings for cassava and sweet potato, mini setts for yams and taro) can speed up the process, but it is difficult to have homogeneous propagules in a sufficient number for robust on-farm trials before year 5. The limited access to sufficient volume of calibrated and homogenized planting material may explain the small number of MLTs established and the small numbers of replications, plants per hybrid, and plants per trial (Singh et al. 2006). As plots are necessarily small, it is important to increase the number of repetitions in different sites. Small plots are easy to manage by farmers and need to be bordered to avoid inter genotypes competition but the need for borders increase the number of plants needed per hybrid, reducing the number of plots that can be established and the number of plants scored per hybrid. Most PPB and PVS projects reviewed here, used plots with 15 to 30 plants per plot, rarely more, and the increase in the number of plants per plot was always associated with a reduction in the number of repetitions and sites. For all species, the availability of homogeneous and calibrated planting material is the major constraint to a robust experimental design.

## Sensory evaluation

Sensory evaluation, often associated to blind panel testing, is somewhat laborious to organise in the field and is often done in a few supervised sessions, when communities need at least several weeks or months of regular consumption to assess a new hybrid and compare its quality to local cultivars (Moyo et al. 2021). The association of women, who play a determinant role in final selection and adoption of new hybrids, is obviously of utmost importance and was recognized in most projects reviewed here. Some hybrids are adopted only once they have been processed into a particular local dish (*fufu* or *gari* in West Africa) and women are the most critical evaluators. For cassava, triploid varieties often out performs diploids and are rich in DMC (Sardos et al. 2009) but are not appreciated by women who are looking for genotypes easy to peel.

The development of new cassava hybrids rich in carotene generated enthusiasm about two decades ago, but their adoption was poor. In Rwanda, introduced yellow-fleshed hybrids resistant to CMD with early bulking (8 months) and high yield were tested
on-farm. Farmers reported that these hybrids were low in DMC, with a poor flour colour, no taste, non-desirable odour, poor storage in the field and not cooking well. It was suggested that the absence of PPB was responsible for the poor adoption of these new hybrids (Nduwumuremyi et al. 2016). In Uganda, despite these drawbacks, the adoption of yellow-fleshed hybrids was considered as possible if the new hybrids had disease-resistance and if on-farm trials were established (Esuma et al. 2019). However, in Vanuatu, local yellow-fleshed hybrids are appreciated and readily adopted because they produce an attractive bright yellow *laplap* (a local dish similar to the African *fufu*), an essential trait for consumers (Lebot et al. 2014). It therefore, appears that the same trait (yellow flesh) has different levels of acceptation in different cultural backgrounds, but different genotypes of yellow-fleshed hybrids and cultivars also perform differently depending on the diseases pressure levels. This implies that the multiplication of sites is a necessary pre-requisite prior adoption but remains practically a major constraint for the reasons explained above.

## Breeders and farmers’ reward systems

The sequencing of cassava cultivars collected from farmers' fields in Africa revealed the impact of very ancient breeding programs. In the 1930–40 s, a breeding program conducted in Tanzania used the related species *M. glaziovii* as a source of CMD resistance and generated inter-specific hybrids. Nowadays, local cultivars in Eastern Africa harbour *M. glaziovii* haplotypes and these were naturally introgressed by farmers who selected resistant cultivars from volunteer seedlings (Bredeson et al. 2016). Although, these genes of resistance came from national breeding programs, the adopted hybrids were not officially released by breeders. Farmers finalised the introgression and selected the best hybrids in their fields, and use them as traditional cultivars nowadays. It is however, difficult to imagine a rewarding financial system for breeders and farmers involved in the development of new hybrids. For all root and tuber crops, genotypes are clonally propagated, and in the absence of a private sector led ‘seed’ industry, new genotypes are often distributed and exchanged through informal channels and it is practically impossible to trace them to claim ownership, and royalties.

In the Netherlands, potato (*Solanum tuberosum*) PPB is successful. In practice, farmers act as breeders and collaborate with the seed industry. Every year, farmers receive between 300 and 2000 seedlings, or clones of seedlings (F1C1), produced by private companies. These hybrids are prepared in green houses and the farmer choose the ones he will evaluate, based on the characteristics of the parents to make sure that they harbour interesting traits. The farmer assess these new hybrids on his farm and screen them for disease resistance, tuber shape and other traits. After farmers’ selection, about 1% of the selected hybrids are returned to the breeders who assess their potential. It is of course difficult to imagine that this system can be transposed directly to developing countries because Dutch farmers are often highly qualified (Almekinders and Elings 2001; Almekinders et al. 2014). However, if Dutch potato farmers can participate early in the selection of hybrids, it is because these can be cloned through vegetative propagation, a biological trait identical to the tropical root and tuber crops.

## Future research perspectives

Most breeding programs are presently attempting to increase the number of sites as early as the single row and preliminary trials (from 1 to 2 sites) (Naidoo et al. 2024). This present the advantage of building up populations adapted to different sites from where they will be propagated for different on-farm trials conducted in different ecological zones. One of the solutions to strengthen the PPB process might be to install different seedling trials in different environments and to start PPB with farmers on different sites as early as the single row trial (Fig. 2).

As one major constraint is the transport of bulky and perishable propagules, the distribution of seedlings (F1), or their clones (F1C1), might represent a practical solution. This geographical distribution of allelic diversity could be used as a practical approach to increase local agrobiodiversity and to broaden genetic bases. This would not be very different to what farmers do in their traditional cropping systems (Lebot et al. 2005; Duputié et al. 2009). In order to be efficient, however, the system relies on trust between breeders and farmers (Kholová et al. 2024). In Samoa, the process has been quite successful for taro because farmers accepted to give back to
<figure><img src="image_3.png" /><figcaption>**Fig. 2** Current breeding schemes can be optimized, incorporating on-farm selection earlier in the process. F1: Seedling trial, F1C1: Cloned seedling trial, SRT: single row trial, AYT: Multi-locational trial, FT: Farmer's trials. * Selection of plants with storage roots or tubers. Upward arrows represent insertion of parents in new cycles</figcaption></figure>

breeders propagules of the best hybrids they selected on their farm. Breeders inserted those in new cycles of recurrent selection conducted on-station to generate new progenies (Iosefa et al. 2013). The potential of this shuttle between farmers and breeders, deserve attention as it could allow the identification of outstanding families and therefore ease breeders’ work. For heterozygous and asexually propagated species, a few families deserve particular attention (Simmonds 1996; Ceballos et al. 2021).

Increased farmers’ participation in F1C1 could accelerate the breeding process because farmers are more able to judge good quality hybrids, the major constraint to adoption, in the first clonal generation. If farmers could share their selected hybrids with breeders, as already done in the Netherlands (for potato) and in Samoa (for taro), then breeders could introduce these in their phenotypic recurrent selection scheme and continue to capture additive effects by recombinating them with those selected for other traits, such as tolerance to pests and diseases. This type of PPB

could be easily implemented by NARS as a complementary approach to the on-going conventional and molecular breeding programmes conducted by the international centres for cassava, sweet potato and yams. Aroids being under-researched. There is also a cost efficiency issue. The on-station characterization and evaluation of hybrids is a fairly expensive and a laborious operation for NARS.

Parents are selected on their individual value. Quite often, however, the best genotypes do not flower. Genetically improved genotypes of cassava and taro have lost their flowering ability due to the selected ideotypes (no branching, single stem or single corm) (Ceballos et al. 2021, Lebot V, Ivančič 2022) and there is a need for flowering induction techniques (pruning, hormonal treatment), as well as for pollen freeze drying and storage protocols to cope with asynchrony. Many interesting sweet potato parents do not flower naturally and need special techniques (Mwanga et al. 2017) and the situation is even more complex with dioecious *Dioscorea* spp. (Mondo
et al. 2020). If the protocols aiming at producing large full-sibs and half-sibs progenies could be optimized, then the collaboration with farmers would represent a good return on investment, especially in a context of accelerating the adaptation to climatic changes (Fig. 2).

## PPB vs PVS

For all root and tuber crop species, the choice between PPB and PVS depends, just like for other plant species, on the knowledge associated to the genotypes under evaluation and to the degree of advancement of their evaluation. However, there are several reports of improved varieties, known for their high performance in some sites or countries, which were selected for on-farm evaluation and were rated lower than local cultivars, not only for taste but also for agronomic performance (Laurie and Magoro 2008; Dansi et al. 2024). PVS involving cultivars or hybrids selected in environments distant from farmers' conditions might appear disappointing. PVS trials using hybrids produced on-station not too distant pedoclimatically from the targets have, of course, better chances of performing well but this implies that the number of targets is reduced as the number of stations where hybrids are developed is often limited. Farmers might be more interested in the genotypes they have contributed to select early in the PPB process, but the number of farmers involved in such PPB activities is rather humble as there are practical constraints in inviting farmers to visit breeders' plots. These farmers are often from the vicinity of the research stations. Successful open field days are often organised where farmers can select hybrids under evaluation and can hand-carry back to their farms, propagules of the most interesting hybrids. The choice between PPB vs PVS, is often due to financial constraints, or oriented by the number of plants available, and/or the farmers willing to participate and assisted by extension specialists.

## Conclusions

PPB and PVS are insufficiently used for root and tuber crops because of the low propagation ratios, the heterogeneity of clonal propagules, uncertain

correlations between clonal generations, GEI, and the need for simple but robust experimental field layouts. Despite these major technical constraints, it appears that most projects have been successful resulting in new hybrids being adopted by farmers. Because of the biological characteristics of these species, farmers are always demanding new genotypes. Although these species have been historically introduced clonally in most countries, farmers are now cultivating hundreds of local cultivars selected mostly for their outstanding quality, satisfying consumers' requirements. It is observed that hybrids developed by breeders are high yielding and resistant to pests and diseases but often fail to satisfy consumers' taste, resulting in very low adoption rate. This review of 19 participatory projects conducted over the last three decades suggest that seedlings trials could be established on-farm in an increased number of sites. This implies that the techniques for mass production of hybrid seeds are optimized to generate every year large half-sibs or full-sibs progenies. Different progenies could be tested on different sites by farmers who will be able to claim ownership as they will be involved sooner in the selection process. Farmers will focus on the most critical and difficult part of a breeding program, the evaluation of the quality and taste of the hybrid. If they can share propagules of their best hybrids with breeders, the recurrent phenotypic selection will be strengthened and accelerated through the capture of additive effects controlling adaptation and quality. This approach might represent for NARS a cost-efficient and rapid system to inject allelic diversity in farmers' fields to cope with forthcoming changes.

**Acknowledgements** The authors are gratefully acknowledging farmers who over the years took the time to explain to them their appreciation of new hybrids submitted to their critical evaluation.

**Author contributions** Both authors contributed equally to the study conception and design. Material preparation, data collection and analysis were performed by VL and CV-Z. The first draft of the manuscript was written by VL and both authors commented on previous versions of the manuscript. Both authors read and approved the final manuscript.

**Funding** Open access funding provided by CIRAD. The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

**Data availability** No datasets were generated or analysed during the current study.
## Declarations

**Competing interests** The authors declare no competing interests.

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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