# Vegetative and reproductive bud burst in adult mango trees: effects of light and growth unit architectural traits

Emma Carrié, Isabelle Grechi, Frédéric Boudon, Elzbieta Frak, Didier Combes, Frédéric Normand

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Emma Carrié, Isabelle Grechi, Frédéric Boudon, Elzbieta Frak, Didier Combes, et al.. Vegetative and reproductive bud burst in adult mango trees: effects of light and growth unit architectural traits. Scientia Horticulturae, 2026, 357, pp.114650. 10.1016/j.scienta.2026.114650. hal-05523853

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# Vegetative and reproductive bud burst in adult mango trees: effects of light and growth unit architectural traits

Emma Carrié<sup>a,b,1</sup> <sup>D</sup>, Isabelle Grechi<sup>a,b,2</sup>, Frédéric Boudon<sup>c,d</sup>, Elzbieta Frak<sup>e</sup>, Didier Combes<sup>e</sup>, Frédéric Normand<sup>a,b,*</sup> <sup>D</sup>

## ARTICLE INFO

**Keywords:**

Architectural trait

Bud burst

Growth unit

Flowering

Light

Mangifera indica

Vegetative growth

## ABSTRACT

Tree growth and reproduction results from the balance between internal and environmental factors. Knowledge is lacking on this balance in evergreen fruit trees characterised by a permanent contrasted light environment within the crown. This study hypothesised that architectural traits and light environment of terminal growth units (GUs) affected their vegetative and reproductive bud burst in the mango tree. Bud burst was monitored on terminal GUs of four adult mango trees, cv. 'Cogshall', during vegetative growth and flowering. Some GUs architectural traits (diameter, apical vs lateral position, outer vs inner location within the crown) and their light environment (quantity: relative transmitted PPFD, TrPPFD, and quality: red to far-red ratio, ζ) were measured at sampling. Factors affecting the occurrence, intensity and dynamics of bud burst were evidenced using generalised linear mixed models. The occurrence of vegetative and reproductive bud burst increased with ζ. For vegetative buds, it also increased with GU diameter at medium to high ζ values. The occurrence of reproductive bud burst was higher in the outer than the inner part of the crown, and on the apical than the lateral GUs. Two vegetative and reproductive flushes occurred. The dates of vegetative and reproductive bud burst were globally delayed by low ζ and high TrPPFD. For GUs that burst during the main reproductive flush, bud burst occurred earlier in the outer than the inner part of the crown. The interplay between the signals leading to the observed spatial and temporal patterns of bud burst is discussed.

## 1. Introduction

Architectural variables govern plant growth processes (Barthélémy and Caraglio, 2007; Kawamura, 2010; Alla et al., 2013), and the mango tree (Mangifera indica L.) is an excellent case study in this respect. Mango is an evergreen tropical tree characterised by rhythmic vegetative growth and terminal flowering (Davenport, 2009). The leafy axis produced by a bud during an uninterrupted period of growth is called a growth unit (GU), which represents the basic unit of the tree woody structure (Normand and Lauri, 2018). Terminal GUs, localised at the extremities of branches, play a major role in tree development and reproduction because new GUs and inflorescences mainly originate from

their buds. The morphological (e.g., diameter), topological (apical vs. lateral position) and temporal (e.g., date of birth) characteristics of terminal GUs affect their ability for vegetative growth and reproduction (Scholefield et al., 1986; Normand et al., 2009; Ramírez et al., 2014; Dambreville et al., 2013; Normand and Lauri, 2018). In addition, the fate (reproductive vs. vegetative) of the terminal GUs, which represents their reproductive effort, impacts subsequent vegetative growth (Capelli et al., 2016; Normand et al., 2016).

Plant growth and physiology also respond to environmental factors such as light. Plants are affected by both light quantity, represented by light intensity and duration (photoperiod), and light quality (spectral composition) perceived by their phytoelements. Light quantity,

<footnote><sup>a</sup> CIRAD, UPR HortSys, F-97455 Saint-Pierre, La Réunion, France</footnote>

<footnote><sup>b</sup> HortSys, Univ. Montpellier, CIRAD, Montpellier, France</footnote>

<footnote><sup>c</sup> CIRAD, UMR AGAP Institut, F-34398 Montpellier, France</footnote>

<footnote><sup>d</sup> UMR AGAP Institut, Univ. Montpellier, CIRAD, INRAE, Institut Agro, Montpellier, France</footnote>

<footnote><sup>e</sup> INRAE, UR P3F, F-86600 Lusignan, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: frederic.normand@cirad.fr (F. Normand).</footnote>

<footnote n="1">¹ Present address: INRAE, BioSP, 84914, Avignon, France.</footnote>

<footnote n="2">² Present address: CIRAD, UPR HortSys, F-34398 Montpellier, France.</footnote>
expressed as photon flux density over the range of photosynthetically-active radiation (PPFD, 400-700 nm), acts as a source of energy for carbon fixation in leaves by photosynthesis. On the other hand, light quality acts as a signal to regulate some key processes related to plant growth and development. Different classes of photoreceptors absorb specific wavelengths over the range of Morphogenetically Active Radiation (MAR, 350-800 nm) and induce photomorphogenic responses in plants (Paradiso and Proietti, 2022). Cryptochromes (cry), phototropins (phot), and ZTL-type receptors absorb UVA-blue radiation (350-500 nm). Phytochromes (phy) absorb red (peak absorption at around 660 nm) radiation in their Pr form and far-red (peak absorption at around 730 nm) radiation in their Pfr form. It is the phytochrome photo-equilibrium, defined as the relative concentration of the two interconvertible forms Pr and Pfr, that enables plants to sense variations in the red to far-red region of the light spectrum.

Within the tree crown, leaf optical properties, geometry and spatial arrangement result in a wide gradient of PPFD and transmitted PPFD (TrPPFD), leading to adaptive leaf traits with local adjustment of photosynthesis (Niinemets and Valladares, 2004; Niinemets et al., 2015; Poorter et al., 2019). Bud burst can be triggered via the inductive effect of sugars (Girault et al., 2010; Schneider et al., 2019; Bertheloot et al., 2020), and may therefore vary with the light quantity received locally (Takenaka, 2000; Niinemets and Lukjanova, 2003; Chen and Sumida 2018). Leaves preferentially absorb UVA-blue (350-500 nm) and red (600-700 nm) radiations, while reflecting and transmitting most far-red (700-800 nm) radiation (Ustin and Jacquemoud, 2020). Therefore, not only light quantity but light quality as well is modified within the crown because of specific wavelength absorption. In particular, the ratio between narrowband red (Rn, 655-665 nm) and narrowband far-red (FRn, 725-735 nm) radiation, referred to as the red to far-red ratio or ζ, decreases from the outer to the inner parts of the crown with induced shade and is nonlinearly related to TrPPFD (Leuchner et al., 2007; Escobar-Gutiérrez et al., 2009; Carrié et al., 2023). A low ζ, indicative of decreased phytochrome photoequilibrium, reduces vegetative bud burst in Arabidopsis (Holalu et al., 2021), wheat (Skinner and Simmons, 1993; Evers et al., 2006) and roses (Girault et al., 2008). Furthermore, depending on the species, low ζ can either accelerate or delay the timing of bud burst (Demotes-Mainard et al., 2016). While other wavelengths e. g., UVA-blue, can influence aspects of plant development such as hypocotyl elongation or stomatal behaviour, their direct effects on bud burst appear variable across species and have been understudied (Leduc et al., 2014; Demotes-Mainard et al., 2016).

In fruit trees, studies have long established relationships between light quantity and leaf functional traits (DeJong and Doyle, 1985; Rosati et al., 1999; Le Roux et al., 2001) or fruit growth and quality attributes (Yu et al., 2016). By contrast, few studies on the effects of light on bud burst have been published (Bastías and Corelli-Grappadelli, 2012), and the light treatments were always applied at the whole tree scale (Erez et al., 1966; Morgan et al., 1985; Baraldi et al., 1998; Pitchers et al., 2021; Tiffon-Terrade et al., 2023), not at the bud scale. The results of the studies vary widely. Natural shade received by apple trees grown in an agroforestry system, corresponding to a simultaneous reduction of TrPPFD and ζ, reduced vegetative and reproductive bud burst (Pitchers et al., 2021). Artificial shading treatments applied to dissociate TrPPFD and ζ showed that a reduction of TrPPFD, and not ζ, reduced vegetative and reproductive bud burst in kiwi and grapevine (Morgan et al., 1985). For peach, vegetative and reproductive bud burst were not affected by either TrPPFD or ζ (Baraldi et al., 1998). However, these studies did not account for the variability of light quantity and quality within the tree crown that could locally affect bud burst. This local impact of light is important to consider, in particular, for evergreen fruit trees that provide a year-round contrasted light environment around their vegetative and reproductive buds. Additionally, these studies did not consider the possible effect of tree architecture on bud burst that could interact with the effects of light.

The crown of the evergreen mango tree is characterised by a high

variability in light quantity and quality around terminal GUs, with a permanent gradient from the outer to the inner parts of the crown (Carrié et al., 2023). We hypothesise that light variability locally affects bud burst and therefore shapes, to some extent, the spatialisation of bud burst. As shown above, architectural traits of the terminal GUs also affect structural and temporal characteristics of vegetative and reproductive bud burst. The mango tree is therefore an interesting model to study the local effects of architectural traits, light environment and their interaction on bud burst. Two questions were addressed in this study: (i) What is the variability of light quantity and quality among terminal GUs in a mango tree crown prior to vegetative growth and flowering? (ii) What are the effects of GU light environment and architectural traits on vegetative and reproductive bud burst? These two questions were investigated by measuring light environment and architectural traits of terminal GUs on adult mango trees, and by monitoring their vegetative and reproductive bud burst.

## 2. Materials and methods

### 2.1. Field site and plant material

The study was carried out in an experimental orchard located at the French Agricultural Research Centre for International Development (CIRAD) research station at Saint-Pierre, Reunion Island (21°06'S, 55°32'E, 125 m a.s.l.). The soil is a brown ferralitic soil, with a depth of about 1.5 m, a low spatial heterogeneity and a good available water capacity (130 mm.m⁻¹) (Raunet, 1991). Reunion Island, located near the Tropic of Capricorn, experiences a subtropical climate, characterized by a hot and rainy season from January to March (austral summer) and a cool and dry season from May to November (austral winter), with April and December being transitional months.

Climate of the field site was recorded by a weather station located 2 km from the orchard. Monthly climate data over a 10-year period (2015–2024) is presented below and in Appendices, Fig. A1. During austral summer, mean daily temperature per month was on average 25.9 °C (range: 25.7-26.1 °C) and monthly cumulative precipitation was on average 156 mm (range: 127-200 mm). During austral winter, mean daily temperature per month decreased to 21.3 °C (range: 20.2-22.6 °C) and monthly cumulative precipitation decreased to 52 mm (range: 30-67 mm). During austral summer months, daily global solar radiation was on average 21.6 MJ.m⁻² (range: 20.2-22.4 MJ.m⁻²), while during austral winter months it was on average 17.4 MJ.m⁻² (range: 14.4-21.6 MJ.m⁻²). These seasonal variations in received light are related to changes in solar elevation (lower in winter) and day length, which varies over the year, with the longest days around the December solstice (~13 h 15 min of sunlight) and the shortest days around the June solstice (~10 h 50 min of sunlight).

Trees were planted in 2004 with a spacing of 5 m between trees along the row and 6 m between rows. Row orientation was North East-South West (34° from the North). The orchard was composed of 153 mango trees of the Floridian cultivar ‘Cogshall’ grafted onto the polyembryonic rootstock ‘Maison Rouge’. Trees were pruned every two years starting from 2012. Irrigation was applied with a drip system on an evapo-transpiration basis (about 5 mm.d⁻¹ on average), from flowering until rainy season establishment.

### 2.2. Sampling design and data collection

Vegetative and reproductive bud burst were recorded during two phenological periods, respectively: (i) vegetative growth occurring after tree pruning following harvest, from February to March 2020; and (ii) flowering occurring after vegetative rest following vegetative growth, from July to September 2020. Trees were heavily pruned on February 3, 2020. The experiment was carried out on four randomly selected trees in the orchard, which differed between the two periods. The same experimental design was used to study vegetative and reproductive bud burst.
Before the periods of vegetative growth and flowering, on February 3 and July 1, 2020, respectively, 110 terminal unpruned GUs were sampled on each tree. The mango tree crown is formed by a peripheral layer, about 0.8-1 m thick, composed of leafy GUs surrounding a central leafless space corresponding to the older woody structure. The outer part is the external surface of the leafy layer and the inner part is the internal surface of the leafy layer. They are characterised by contrasted light quantity and quality (Carrié et al., 2023), as well as by visually higher flowering on the terminal GUs in the outer part of the crown than those in the inner part. Two subsamples of 80 and 30 terminal GUs were randomly sampled in the outer and the inner part of each tree crown, respectively. This sampling design, which considered the location of the GU within the crown as a factor, enabled the specific effect of location to be assessed beyond its indirect effect through light. Sample size differed between the two locations to avoid overrepresentation of GUs in shaded areas since terminal GUs in the outer part of the crown may also be in a shaded location. The two subsamples were each composed of half apical and half lateral terminal GUs. For each GU, the basal diameter and the light environment (see §2.3) were measured at sampling, and bud burst was monitored twice a week following sampling date. The date of burst (phenological stage C for vegetative and reproductive buds (Dambreville et al., 2015) was recorded for each bud of the sampled GU that produced a new GU (referred to as a daughter GU) or an inflorescence during vegetative growth or flowering, respectively.

## 2.3. Light environment measurement

Light spectral irradiance (µW.cm⁻².nm⁻¹) was measured above each sampled GU at regular intervals (08:00, 10:00, 12:00, 14:00 and 16:00) during a clear day, using a JAZ spectrometer (Ocean Insight, USA), with wavelengths ranging from 200 to 1100 nm and a spectral resolution of 1 nm. During the measurement, the sensor of the spectrometer was maintained horizontally above the apical bud of the GU with a gimbal stabiliser. At the same time, the incident PPFD (µmol.m⁻².s⁻¹) received horizontally above the orchard was measured in an open field at about 100 m from the experimental orchard using a PQS1 PAR quantum sensor (Kipp & Zonen, the Netherlands). The sensor was connected to a CR1000X data logger (Campbell, USA) to measure over the day the incident PPFD at 5-s intervals and averaged every 30 s.

Photon flux density (µmol.m⁻².s⁻¹) was calculated for each wavelength from spectral irradiance using a series of conversions described in Carrié et al. (2023). Photosynthetic photon flux density (PPFD), narrowband red (Rn) and narrowband far-red (FRn) were then calculated (µmol.m⁻².s⁻¹) by integrating the photon flux density over the range 400-700 nm for PPFD, 655-665 nm for Rn and 725-735 nm for FRn. The red to far-red ratio ζ (dimensionless), representing light quality, was calculated as the ratio between Rn and FRn. The relative transmitted PPFD (TrPPFD), representing light quantity, was calculated as the ratio between the PPFD measured with the spectrometer above each sampled GU and the incident PPFD. TrPPFD and ζ were calculated for each time of light measurement (i.e., at 08:00, 10:00, 12:00, 14:00 and 16:00), and the five values were then averaged to obtain a mean daily value for each variable, characterizing the light environment surrounding a GU.

Specific morphogenetically-active radiations, like UVA-blue, Rn or FRn, were not considered to characterize light quality because they are linearly related to light quantity (PPFD) (Escobar-Gutiérrez et al., 2009; Carrié et al., 2023) and their effects are therefore confounded with natural light. On the opposite, the ratio ζ is non-linearly related to light quantity (Leuchner et al., 2007; Escobar-Gutiérrez et al., 2009; Carrié et al., 2023), justifying to consider both TrPPFD and ζ in this study. These variables are moreover the most studied light signals involved in bud burst regulation (Schneider et al., 2019; Bertheloot et al., 2020; Holalu et al., 2021).

## 2.4. Data analysis

### 2.4.1. Response variables for vegetative and reproductive bud burst

Several response variables were calculated from the data recorded at the scale of each sampled GU to characterise vegetative and reproductive bud burst. The appearance of at least one daughter GU (resp. inflorescence) was recorded as a binary variable (0/1) indicating the occurrence of vegetative (resp. reproductive) bud burst. For sampled GUs where bud burst occurred, the intensity and dynamics of vegetative (resp. reproductive) bud burst were then quantified. The number of daughters GUs (resp. inflorescences) produced per GU expressed the intensity of vegetative (resp. reproductive) bud burst. The dynamics of vegetative (resp. reproductive) bud burst was based on the analysis of the distributions of burst timing at the scale of the populations of sampled GUs. The bud burst timing of a GU was set to the date of appearance (expressed in days from the date of sampling) of its first daughter GU (resp. inflorescence). Distributions of vegetative and reproductive burst timing were composed of two flushes, with the second flush being the most important (see the Results section). Because the Gamma distribution is defined for positive real numbers and is commonly used for modelling temporal variables, we modelled the two flushes as a weighed sum of two Gamma distributions. The parameters were estimated from the data with a non-linear method that minimises log-likelihood. Each GU was classified into the first or second flush, according to its bud burst date and the threshold date separating the two flushes, fixed as the intersection point of the two Gamma distributions. Therefore, the dynamics of bud burst was described by three variables: a binary variable (0/1) corresponding to the occurrence of bud burst during the second flush, and two temporal variables corresponding to the bud burst timing, for the GUs that burst during the first or the second flush, respectively.

### 2.4.2. Effects of the environmental and architectural variables on GU bud burst

Statistical analyses were performed at the GU scale with R software, version 4.0.3 (R CoreTeam, 2020). Because of the large sample size and to highlight the main variables and their interactions affecting bud burst, the significance level in statistical tests was set at *P* = 0.01.

First, we evaluated how the distributions of light variables TrPPFD and ζ differed (i) between the outer and inner parts of the crown for each of the vegetative growth and flowering periods; and (ii) between the two periods for each part of the crown. Light distributions were compared using the Kolmogorov-Smirnov test with the ks.test() function (Birnbaum and Tingey, 1951).

Secondly, we investigated how GU light environment and architectural traits affected vegetative and reproductive bud burst. A generalised linear mixed model (GLMM) was built for each response variable, with all the variables characterising GU light environment (TrPPFD, ζ) and architectural traits (diameter, position, location) and all their second-order interactions as predictors. A random effect for the tree was considered to account for the clustered structure of the data. The binary response variables describing bud burst occurrence were associated with a binomial distribution. The count response variables describing bud burst intensity were associated with a negative binomial distribution. The temporal response variables describing bud burst timing were associated with a Gamma distribution. GLMMs were built with the glmer() function of the 'lme4' package (Bates et al., 2015). The following two-step approach was then applied to select the variables and their interactions that most significantly affected each response variable. First, an automated model selection procedure performed with the dredge() function of the 'MuMin' package (Bartoń, 2022) was applied to the complete GLMM to identify the minimal adequate model, based on the Akaike Information Criterion (AIC). Second, a type III deviance test (or type II if there was no significant interaction term selected in the previous step) with backward stepwise selection was performed on the selected minimal model, and the model with only significant terms was
retained.

# 3. Results

## 3.1. Distribution of terminal GU light environment prior to vegetative growth and flowering

Distributions of the mean daily TrPPFD (hereafter referred to as TrPPFD) of the sampled GUs significantly differed between the inner and outer parts of the crown prior to both vegetative growth (D = 0.56, P < 0.001; Fig. 1A) and flowering (D = 0.65, P < 0.001; Fig. 1C), with smaller values in the inner part of the crown. In addition, TrPPFD distributions significantly differed between the two periods for both the outer (D = 0.33; P < 0.001) and inner (D = 0.59, P < 0.001) parts of the crown, with smaller values prior to flowering. Prior to vegetative growth, TrPPFD ranged from 0.006 to 0.90, and was on average (± standard error) 0.33 (± 0.01) in the outer part of the crown and 0.12 (± 0.01) in the inner part (Fig. 1A). Prior to flowering, TrPPFD ranged from 0.001 to 0.81, and was on average 0.20 (± 0.01) in the outer part of the crown and 0.03 (± 0.005) in the inner part (Fig. 1C).

Distributions of the mean daily red to far-red ratio (hereafter referred to as ζ) of the sampled GUs significantly differed between the inner and outer parts of the crown prior to both vegetative growth (D = 0.48, P < 0.001; Fig. 1B) and flowering (D = 0.52, P < 0.001; Fig. 1D), with smaller values in the inner part of the crown. In addition, ζ distributions significantly differed between the two periods for both the outer (D = 0.25, P < 0.001) and inner (D = 0.43, P < 0.001) parts of the crown, with smaller values prior to flowering. Prior to vegetative growth, ζ ranged from 0.05 to 1.24, and was on average 0.65 (± 0.02) in the outer part of the crown and 0.39 (± 0.02) in the inner part (Fig. 1B). Prior to flowering, ζ ranged from 0.04 to 1.16, and was on average 0.52 (± 0.02) in the outer part of the crown and 0.22 (± 0.01) in the inner part (Fig. 1D).

## 3.2. Effects of terminal GU light environment and architectural traits on bud burst occurrence

### 3.2.1. Vegetative bud burst occurrence

The mean vegetative bud burst occurrence of terminal GUs was 0.68 (n = 440). A significant interaction appeared between GU diameter and ζ (χ² = 7.88, df = 1, P < 0.01; Table 1), showing that the relationship between GU diameter and bud burst occurrence varied with ζ (Fig. 2A). Vegetative bud burst occurrence was stable or slightly decreased with GU diameter for ζ lower than about 0.43, and increased with GU diameter for ζ higher than 0.43. Moreover, for a given diameter, vegetative bud burst occurrence increased with ζ (Fig. 2A).

### 3.2.2. Reproductive bud burst occurrence

The mean reproductive bud burst occurrence of terminal GUs was 0.71 (n = 440). Reproductive bud burst occurrence was significantly affected by GU location (χ² = 46.20, df = 1, P < 0.001), ζ (χ² = 20.35, df = 1, P < 0.001) and GU position (χ² = 19.32, df = 1, P < 0.001) (Table 1; Fig. 2B). Reproductive bud burst occurrence was higher in the outer part of the crown (0.86 ± 0.03) than in the inner part (0.33 ± 0.06). It increased with ζ and was higher on apical (0.83 ± 0.03) than on lateral (0.61 ± 0.05) terminal GUs (Appendices, Fig. A2). Interactions were not significant.

## 3.3. Effects of terminal GU light environment and architectural traits on bud burst intensity

### 3.3.1. Vegetative bud burst intensity

GUs on which vegetative bud burst occurred (n = 301) produced 1 to 6 daughter GUs. This intensity of vegetative bud burst was significantly affected by ζ (χ² = 16.62, df = 1, P < 0.001) and GU position (χ² = 8.84, df = 1, P < 0.01) (Table 1; Fig. 3). The intensity of vegetative bud burst increased with ζ and was higher on apical (1.36 ± 0.12) than on lateral

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Relative frequency distribution of the light quantity (mean daily relative transmitted PPFD, TrPPFD) (graphs A, C) and light quality (mean daily red to far-red ratio, ζ) (graphs B, D) measured on terminal growth units (GUs) located in the inner (green bars) and outer (white bars) parts of the crown of four adult mango trees cv. 'Cogshall' prior to vegetative growth (graphs A, B) and flowering (graphs C, D). Bars represent the proportion of observations per bin within each group. Vertical lines indicate the mean TrPPFD and ζ values for GUs located in the outer part (dashed line, n = 320) and the inner part (solid line, n = 120) of the tree crowns. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
<caption>**Table 1**</caption>

Effects of light and architectural explanatory variables on response variables describing vegetative and reproductive bud burst at the scale of terminal growth units (GUs) of four adult mango trees cv. 'Cogshall'. For continuous explanatory variables (GU diameter, TrPPFD (mean daily relative transmitted PPFD) and ζ (mean daily red to far-red ratio)), + and – indicate a significant positive or negative effect, respectively, on the response variable. For discrete explanatory variables (GU location (outer, inner) and GU position (A: apical, L: lateral)), > and < indicate the hierarchy between significantly different levels on the response variable. The notation variable1: variable2 indicates a significant interaction between variable 1 and variable 2. ns: non-significant (*P* > 0.01); **: *P* < 0.01; ***: *P* < 0.001. n is the number of sampled GUs.

<table><thead><tr><th rowspan="2">Response variable</th><th rowspan="2">Bud fate</th><th rowspan="2">n</th><th colspan="5">Light and architectural variables</th></tr><tr><th>GU location</th><th>GU position</th><th>GU diameter</th><th>TrPPFD</th><th>ζ</th></tr></thead><tbody><tr><td rowspan="2">Occurrence of bud burst</td><td>Vegetative</td><td>440</td><td>ns</td><td>ns</td><td>GU diameter: ζ **</td><td>ns</td><td>GU diameter: ζ **</td></tr><tr><td>Reproductive</td><td>440</td><td>outer > inner ***</td><td>A>L ***</td><td>ns</td><td>ns</td><td>+ ***</td></tr><tr><td rowspan="2">Intensity of bud burst</td><td>Vegetative</td><td>301</td><td>ns</td><td>A>L **</td><td>ns</td><td>ns</td><td>+ ***</td></tr><tr><td>Reproductive <sup>a</sup></td><td>314</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td></tr><tr><td rowspan="2">Occurrence of bud burst during the second flush</td><td>Vegetative</td><td>301</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td></tr><tr><td>Reproductive</td><td>314</td><td>ns</td><td>ns</td><td>ns</td><td>+ ***</td><td>-. ***</td></tr><tr><td rowspan="2">Timing of bud burst in the first flush</td><td>Vegetative</td><td>133</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>-. ***</td></tr><tr><td>Reproductive</td><td>121</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td></tr><tr><td rowspan="2">Timing of bud burst in the second flush</td><td>Vegetative</td><td>168</td><td>ns</td><td>ns</td><td>ns</td><td>+ ***</td><td>-. ***</td></tr><tr><td>Reproductive</td><td>193</td><td>outer < inner ***</td><td>ns</td><td>ns</td><td>ns</td><td>ns</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Vegetative bud burst occurrence of terminal growth units (GUs) according to GU diameter and GU light quality (mean daily red to far-red ratio, ζ) (graph A), and reproductive bud burst occurrence in relation to GU light quality (ζ) and GU location within the crown (graph B) in four adult mango trees cv. 'Cogshall'. Lines represent simulated data from the GLMM estimation (±95% confidence interval). Points represent the observed data: for visualization purposes, continuous variables ζ (graphs A and B) and GU diameter (graph A) have been divided into classes of equal size, and points represent class means for x-axis and y-axis variables. Class size was 48-49 GUs in graph A, and 40 for GUs located in the inner crown and 64 for GUs located in the outer crown in graph B. In graph A, ζ have been divided into three classes first, within which GU diameter have been subdivided into three classes.</figcaption></figure>

(1.08 ± 0.03) GUs. The difference between apical and lateral GUs increased with ζ (Fig. 3). Interactions were not significant.

### 3.3.2. Reproductive bud burst intensity

The intensity of reproductive bud burst was not analysed since 98% of the GUs that flowered produced only one inflorescence, indicating that the studied explanatory variables did not affect this response variable.

## 3.4. Effects of terminal GU light environment and architectural traits on bud burst dynamics

### 3.4.1. Vegetative bud burst dynamics

Vegetative bud burst occurred in two main flushes, with a median value of 17 d and 30 d after GU sampling, respectively, with 56% of bud burst occurring during the second flush (Fig. 4A). The two flushes showed contrasting patterns. The first flush was spread out over time (interquartile range, IQR = 9 d), whereas the second flush had a sharper peak (IQR = 2 d). None of the studied variables explained the occurrence of bud burst during the second flush (Table 1). By contrast, the timing of vegetative bud burst for the terminal GUs that burst during the first flush

was significantly affected by ζ (χ² = 15.35, df=1, *P* < 0.001), with a negative effect. That means that vegetative bud burst was earlier with increasing values of ζ (Appendices, Fig. A3). Despite a sharp peak during the second flush, the timing of vegetative bud burst for the terminal GUs that burst during this flush was significantly affected by ζ (χ² = 24.02, df=1, *P* < 0.001) and TrPPFD (χ² = 17.13, df = 1, *P* < 0.001), with a negative and positive effect, respectively (Table 1). For a given TrPPFD (resp. ζ) level, the timing decreased (resp. increased) with increasing values of ζ (resp. TrPPFD) (Fig. 5A). Interactions were not significant.

### 3.4.2. Reproductive bud burst dynamics

Reproductive bud burst occurred in two main flushes, with a median value of 14 d and 60 d after GU sampling, respectively, with 62% of bud burst occurring during the second flush (Fig. 4B). The first flush was spread out over time (interquartile range, IQR = 15 d), as was the second flush, but with a more pronounced peak (IQR = 7 d). The occurrence of bud burst during the second flush was significantly affected by ζ (χ² = 39.70, df = 1, *P* < 0.001) and TrPPFD (χ² = 23.39, df = 1, *P* < 0.001), with a negative and positive effect, respectively (Table 1). For a given TrPPFD (resp. ζ) level, the occurrence of bud burst during the second flush decreased (resp. increased) with increasing values of ζ (resp.

<footnote><sup>a</sup>: Intensity of reproductive bud burst was not analysed since 98% of the GUs that flowered produced only one inflorescence.</footnote>
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Vegetative bud burst intensity of terminal growth units (GUs) in relation to GU light quality (mean daily red to far-red ratio, ζ) and GU position in four adult mango trees cv. 'Cogshall'. Lines represent simulated data estimated from the GLMM estimation (±95% confidence interval). Points represent the observed data: for visualization purposes, the continuous variable ζ has been divided into classes of equal size for each GU position (54 for apical GUs and 46 for lateral GUs), and plotted points represent class means for x-axis and y-axis variables.</figcaption></figure>

TrPPFD) (Fig. 5B). The interaction was not significant. The timing of reproductive bud burst for the terminal GUs that burst during the first flush was not affected by the explanatory variables (Table 1). By contrast, the timing of bud burst for the terminal GUs that burst during the second flush was significantly affected by GU location (χ² = 18.67, df = 1, P < 0.001; Table 1). The timing (± standard deviation) was delayed in the inner (62.7 ± 5.5 d) compared to the outer (58.0 ± 6.2 d) part of the crown.

# 4. Discussion

The study is the first to record the variability of light quantity and quality surrounding individual terminal GUs in the crown of adult mango trees, and to highlight their effects on vegetative and reproductive bud burst in terms of occurrence, intensity and dynamics. The results showed the marked effect of the red to far-red ratio (ζ) on bud burst in the mango tree, together with GU architectural traits and, to a lesser

extent, light quantity (TrPPFD).

## 4.1. Light distribution within the mango tree crown

The mean transmitted PPFD (TrPPFD) and red to far-red ratio (ζ) within a mango tree crown hide a large variability at the level of individual terminal GUs. This variability was structured by the GU location within the crown and the phenological period. Few studies have assessed the variability of mean light quantity within the crown of different mango cultivars (Schaffer and Gaye, 1989; Sharma et al., 2006; Bally et al., 2020; Mahmud et al., 2023). They showed that light variability was structured according to the height (vertical gradient), the depth (radial gradient) and the cardinal orientation within the crown. TrPPFD values measured in 'Cogshall' before vegetative growth and before flowering were consistent with mean values found in these studies. In addition, our results showed that within our orchard conditions, the entire range of possible values for TrPPFD (0–1) and ζ (~0–1.3) was encountered at the scale of the GUs in the outer crown. Neighbouring GUs, adjacent trees and cardinal orientation were probably important sources of variability in the light quantity and quality around them. On the contrary, distributions of TrPPFD and ζ values in the inner crown only encompassed about half of the range of possible values.

The difference in light quantity and quality distributions between the two phenological periods was probably a consequence of the pruning that the trees received before the study, resulting in a more open crown at the first date of experiment, before vegetative growth. After vegetative growth and before flowering, tree crowns closed, limiting light diffusion to the inner crown. This resulted in a shift of light distributions toward the small values of TrPPFD and ζ. This shift was more pronounced for TrPPFD than for ζ, suggesting that light quantity was more impacted than light quality. The larger leaf density after vegetative growth probably resulted in higher absorption and scattering that shape the nonlinear relationship between TrPPFD and ζ. According to this relationship, ζ varies broadly between 0.02 and 0.5 for the smallest values of TrPPFD between ~0 and 0.05 (Carrié et al., 2023).

## 4.2. Vegetative and reproductive bud burst occurrence and intensity

### 4.2.1. Vegetative bud burst

Persello et al. (2019) showed a positive effect of GU diameter on vegetative bud burst occurrence of terminal GUs in 'Cogshall' trees. Our results further showed that this positive effect holds only for ζ >~0.43. For lower ζ values, bud burst was low and independent of GU diameter.

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Vegetative (graph A) and reproductive (graph B) bud burst dynamics of terminal growth units (GUs) in four adult mango trees cv. 'Cogshall'. Histograms represent the observed distributions of burst dates expressed in days since GU sampling. Solid black curves are a weighted sum of two Gamma distributions (dashed lines) fitted on observed data. Vertical red dashed lines indicate the boundary between the first and the second flushes defined by the intersection of the two Gamma distributions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Timing of vegetative bud burst (expressed in days since sampling) of terminal growth units GUs that burst during the second flush (graph A), and occurrence of reproductive bud burst during the second flush (graph B) in relation to GU light quality (mean daily red to far-red ratio, ζ) and quantity (mean daily relative transmitted PPFD, TrPPFD) in four adult mango trees cv. 'Cogshall'. Lines represent simulated data from the GLMM estimation (±95% confidence interval). Points represent the observed data: for visualization purposes, continuous variables ζ and TrPPFD have been divided into classes of equal size, and points represent class means for x-axis and y-axis variables. Class size was 14 GUs in graph A and 26 GUs in graph B. Note that TrPPFD have been divided into four classes first, within which ζ have been divided into three sub-classes.</figcaption></figure>

The negative effect of low ζ values on bud burst has been shown in various species. For example, ζ < 0.25 inhibits vegetative bud burst in roses (Girault et al., 2008) and tillering in wheat stops when ζ < 0.35-0.40 (Evers et al., 2006). In the mango tree crown, ζ values lower than about 0.43 correspond to dark areas where TrPPFD < 0.1 (Carrière et al., 2023). The lack of effect of the GU location on vegetative bud burst occurrence indicates that terminal GUs, whether located in the inner or outer part of the crown, had the same ability to burst if their light environment was favourable. Tree pruning about 10 days before the beginning of vegetative bud burst modified the light environment around GUs, in particular in the inner crown. The observed effect of ζ on GU bud burst therefore suggests that the integration time of the light signal perception was short (i.e., a few days), as generally observed in photomorphogenic responses (Demotes-Mainard et al., 2016; Barillot et al., 2021).

Terminal GUs that burst and were in a light environment with ζ <~ 0.5 produced an average of one daughter GU, independently of their position, suggesting that low ζ alters branching in mango, as recorded in several, generally non-woody species (Demotes-Mainard et al., 2016). In environments with ζ > 0.5, the intensity of bud burst increased with ζ, in particular for apical GUs. This result therefore implies that the effect of GU position on bud burst intensity (i.e., higher for apical than lateral GU; Dambreville et al., 2013) is mainly expressed in high light environments.

In unpruned 'Cogshall' trees, GU position affects both the occurrence and intensity of vegetative bud burst (Normand et al., 2009; Dambreville et al., 2013). In pruned trees, our results and those of Persello et al. (2019) showed that GU position affected bud burst intensity as well, but not bud burst occurrence. This suggests that the triggering effect of tree pruning (Oosthuys, 1994; Campbell and Wasielewski, 2000; Davenport, 2006; Persello et al., 2019) on bud burst of unpruned terminal GUs surpasses the effect of GU position, contrary to the intensity of bud burst that still remain dependent on GU position.

### 4.2.2. Reproductive bud burst

The occurrence of reproductive bud burst was lower for terminal GUs located in the inner rather than in the outer part of the crown for a same ζ value. If we assume that photosynthesis and sugar levels were higher in terminal GUs located in the outer part of the crown because of globally higher TrPPFD, the higher occurrence of reproductive bud burst in the outer crown could be related to the positive, dose-dependent effect of sugar on bud burst (Girault et al., 2010; Barbier et al., 2015; Schneider

et al., 2019; Bertheloot et al., 2020). This effect was not demonstrated for vegetative bud burst, probably because of the triggering effect of pruning and the higher TrPPFD in the inner crown after pruning than before flowering.

Another hypothesis for the GU location effect is the probable need of sugar for the translocation to the buds of the florigenic promoter synthesized in mango leaves under cool temperatures (Wilkie et al., 2008). In the outer part of the crown, a higher proportion of terminal GUs are located in favourable TrPPFD, ensuring adequate photosynthetic activity and allowing the translocation of the florigenic promoter with sugar to the buds of these GUs and neighbouring GUs (Davenport et al., 2006). This could lead to a globally higher and more homogeneous distribution of sugar and florigenic promoter within GUs in the outer crown, suggesting that the occurrence of bud burst on these GUs was more closely related to the positive effect of ζ on reproductive bud burst than to a limitation of flowering by low TrPPFD. On the other hand, the occurrence of reproductive bud burst in the inner part of the crown was probably more limited by TrPPFD.

The results confirmed the higher occurrence of flowering on apical than on lateral GUs (Normand et al., 2009; Dambreville et al., 2013; Grechi et al., 2022). Contrary to vegetative bud burst, the significant effect of GU position was probably related to the lack of pruning as a triggering event before flowering. Functional hypotheses have been proposed to explain the effect of GU position on flowering (Normand et al., 2009), but they still remain unclear.

### 4.3. Vegetative and reproductive bud burst dynamics

Vegetative and reproductive bud burst both occurred in two flushes. The first vegetative flush, which was spread out, could result from the natural initiation of bud burst of the GUs after harvest. The second vegetative flush, which was more concentrated in time, could be the response to the triggering effect of tree pruning realized just before the experiment. The beginning of each flush could be related to sudden and heavy rainfall, as suggested by Persello et al. (2019), with 60 mm between 4-7 days after sampling and 58 mm between 24-27 days after sampling, and no rainfall in between. The two reproductive flushes could be related to the two flushes of vegetative growth in the orchard since the date of reproductive burst of a GU is related to its own date of birth (Dambreville et al., 2013).

Vegetative and reproductive bud burst dynamics were almost exclusively affected by light-related factors, indicating the importance
of light in the timing of bud burst. Terminal GUs with higher ζ burst earlier, either by shortening the timing of bud burst during the two flushes for vegetative growth, or by decreasing bud burst occurrence during the second flush for flowering (Table 1). The effect of low ζ on the timing of bud burst is a photomorphogenic response that depends on the species and the experimental conditions (Demotes-Mainard et al., 2016). It can advance, delay or have no effect on bud burst timing. The delaying effect of low ζ on bud burst timing observed in the mango tree is also reported on vegetative (Morgan et al., 1985; Wan and Sosebee, 1998) or reproductive (Lorenzo et al., 2019; Qin et al., 2022) buds in other woody and non-woody species.

Despite a positive nonlinear relationship between ζ and *TrPPFD* (Carrié et al., 2023), these two factors had opposite effects on the timing of both vegetative and reproductive bud burst. This suggests that light quantity and light quality are signals affecting different mechanisms related to bud burst in the mango tree. While higher ζ advanced the timing of bud burst, higher *TrPPFD* delayed the timing of bud burst. The delaying effect of *TrPPFD* occurred either by increasing the timing of bud burst during the second flush for vegetative growth, or by increasing bud burst occurrence during the second flush for flowering. This was opposite to the expected results since higher *TrPPFD* suggests higher photosynthesis and sugar concentration, which should lead to earlier bud burst (Barbier et al., 2015; Bertheloot et al., 2020).

Only the timing of reproductive bud burst during the second flush was affected by factors other than light. As for the occurrence of reproductive bud burst, the earlier bud burst in the outer part of the crown could be explained by the positive effect of sugar on the timing of bud burst, as detailed above. However, discrepancies shown in results interpretation suggest that the sugar-based hypothesis alone, where increased light quantity promotes bud burst through enhanced carbohydrate availability, does not fully explain the timing of bud burst. Further investigation is needed to elucidate factors and processes affecting the timing of bud burst in adult mango trees.

## 5. Conclusion

This study showed the major effects of light, in particular the red to far-red ratio ζ, and of architectural traits of terminal GUs on vegetative and reproductive bud burst in the mango tree. The effects of light and architectural factors were mostly additive. Light quantity (*TrPPFD*) and light quality (ζ) surrounding GUs were highly variable, in particular, in the outer part of the crown. They also differed before vegetative and reproductive bud burst, in relation to tree phenology and vegetative growth that occurs before reproductive growth. *TrPPFD* and ζ appeared as signals for distinct physiological mechanisms. Low ζ (resp. high ζ) reduced (resp. stimulated) the occurrence and intensity of bud burst and delayed (resp. advanced) bud burst timing. On the opposite, *TrPPFD* was only directly involved in bud burst dynamics, and high *TrPPFD* tended to delay bud burst timing. The GU architectural traits (diameter, position, location) mainly affected the occurrence and intensity of vegetative and reproductive bud burst, and to a lower extend reproductive bud burst dynamics.

These results are useful to improve pruning and training techniques of the mango tree, in particular, to better consider the role of ζ and the advantage of the outer vs. inner part of the crown for reproduction. The results also pave the way for new research questions to decipher the respective roles and the interactions of environmental factors (light quality and quantity), architectural traits and cultivation practices (pruning, crown management) on important phenological processes in the evergreen mango tree such as bud burst and GU mortality. The in-depth analysis of the influence of light on the development and production of fruit trees is challenging for promising and resilient mango orchard design and management such as high-density orchards, agroforestry, or agrivoltaic systems.

## Declaration of generative AI in scientific writing

The authors declare not having used generative AI for scientific writing.

## Funding

This work was supported by the CIRAD DPP COSAQ (activities 2018-2021) and CIRAD DPP SADUR (activities 2022-2026) agronomical research programmes funded by the European Community (ERDF fund) and the Conseil Régional de la Réunion. The funding sources were not involved in study design, in the collection, analysis or interpretation of data, in the writing of the report, or in the decision to submit the article for publication.

## CRediT authorship contribution statement

**Emma Carrié:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Isabelle Grechi:** Writing – review & editing, Resources, Methodology, Investigation, Funding acquisition, Conceptualization. **Frédéric Boudon:** Writing – review & editing, Resources, Methodology, Conceptualization. **Elzbieta Frak:** Writing – review & editing, Resources, Methodology, Conceptualization. **Didier Combes:** Writing – review & editing, Resources, Methodology, Conceptualization. **Frédéric Normand:** Writing – review & editing, Resources, Methodology, Investigation, Funding acquisition, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

The authors would like to thank Emmanuel Jouen for the loan of the spectrometer, and Raymond Nativel for his help in programming the Campbell datalogger and installation of PAR quantum sensors.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.scienta.2026.114650.

## Data availability

Data will be made available on request.

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