沙漠研究 19-1, 185- 188 (2009 )Journal of Arid Land Studies“Desert Technology IX ”
Refereed Paper

Drought and Defoliation Effects on Recoveryof Two Mediterranean Medicago Shrubs
Elkadri LEFI1, Josep CIFRE2, Javier GULIAS2, Hipolito MEDRANO2
Abstract: Soil water availability is one of the main limiting factors of growth and plant production indry lands. A field experiment was carried under Mediterranean climate. Medicago arborea andMedicago citrina plants were subjected to irrigation and rain fed conditions and two modes ofmanagement plants along the year: non-physically disturbed plants (NDP) and defoliated plants (DP).
A clear effect of drought, plant management and seasons on dry matter of shoots, stems, leaves,seeds production and leaf area. A similar pattern of two growing periods along the year wasobserved for both species and management modes; an important one in spring and a reduced one inautumn. M. arborea suffered total leaf senescence in summer, but M. citrina, with a better adaptation
to drought period, increases shoots production with water availability. In semi-arid conditions, M.
citrina is a more promising foraging species than the widely used M. arborea.
Keywords: Growth, Medicago shrubs, Mediterranean climate

1. Introduction
Drought affects plant growth and development processes, and seriously reduces forage production andits seasonal availability. This phenomenon is frequently accompanied by overgrazing, which causes
impoverishment and erosion of soils. Fodder shrubs constitute excellent craving resources with a highfood value, and easy to integrate in agro-pastoral systems (Le Houérou, 1993). Medicago arborea L. isone of the three Medicago species included in the section Dendrotelis (Vassilcz) Lassen. This species,together with M. strasseri Greuter, Matthäs & Risse (Endemic from Crete), is native to coastal areas andislands of the eastern Mediterranean. M. arborea is widely used as forage species, but it is sensitive todrought. The remaining species of the section, M. citrina (Font Quer) Greuter, is endemic of the western
Mediterranean (Balearic Islands, Spain).
Shoot production depends on the soil water availability and the plant management. On the one hand,under drought, physiological limitations, as reductions in photosynthesis and in transfer of assimilate,affect plant growth. In this context, morphological modifications are adopted as alternative indicators ofwater stress (Escós et al., 2000). On the other hand, plant defoliation represents a limitation for carbon
resources, and that affects shoots growth, but several authors suggest that defoliation improves the plantwater status, consequently improving the plant growth (Páez and González, 1995).
The aim of this work is to study the effect drought and defoliation on growth, dry matter allocation,and shoot production for Medicago arborea and Medicago citrina plants.
2. Materials and Methods
M. arborea and M. citrina were studied at the experimental field of the University of Balearic Islands(Spain) on two years-old plants. A parcel of M. arborea and M. citrina plants, in inserted lines (1.5/1.5m), was maintained under irrigation regime to keep the soil at 100% of field capacity. A second parcel
was maintained under rain fed conditions (230 mm/year, without irrigation). Along the year, the soil
water content was measured with the TDR instrument (Time Domaine-Reflectometry) for two soil horizons(0-0.5 m and 0.5-1 m of depth) on four tubes located at 0.5 m from the plant. Soil water availability(SWA) was calculated after calibration of TDR values with respect to all measurements of soil moisture,taking into account the soil water content at the permanent wilting point (12.8%).
Four randomly selected plants were defoliated every 45 days to study the growth of shoots. The
capacity of recovery was studied for both species, under irrigated and drought regimes. For two-year-oldnon-physically disturbed plants (NDP), shoots (Edible biomass: leaves and stems) were cut. After each
1

Faculté des Sciences de Gafsa, Département de Biologie, Sidi Ahmed Zarroug, 2112, Gafsa, Tunisie; Fax no. + 216
76211026, Tel. no. +216 97867839, E-mail: lefielkadri2@yahoo.fr
2
Universitat de les Illes Balears, Departament de Biologia, C/ Valldemossa km 7.5, 07122 Palma de Mallorca, Spain.

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sampling, shoots, for the same plant, were again defoliated (DP) after 120 days, according to Corleto et al.
(1993). Leaves, stems, and seeds were separated and dried in a ventilated oven (85 ˚C/48 hours). The
dry matter of shoots per plant (B), stems (SDM), leaves (LDM) and seeds production (Y) was calculated.
The Delta-T Devices instrument (∆T Area Meter) was used to calculate the plant leaf area (PLA, m2plant-1) using the specific leaf area. The meteorological data show a typical Mediterranean climate
spring-summer period, with high temperature (18-27 ˚C) and low relative humidity that generate aconsiderable water loss by evapotranspiration (6.35 mm j-1) during summer. Data were submitted to theanalysis of variance (ANOVA) with a factorial fixed model.
3. Results and Discussion
Under irrigation regime, the soil water availability (SWA) was superior to 80% along the year, but
under rain fed conditions, SWA reached 45 and 50% in spring, against 30 and 20% in summer for M.
arborea and M. citrina, respectively. This latter has the capacity to extract water from deeper horizons(Lefi et al., 2004). The general pattern of variation of shoots and these components depends on watersupply, plant management and seasons. The soil water deficit affects all studies parameters for bothspecies (Figs. 1, 2 and 3).
In the field, both species showed very similar values for all measured parameters in NDP underdrought. Slightly standing out were the higher seeds production for M. arborea in summer (Fig. 2). On
the other hand, M. citrina showed higher LDM in spring and summer. These seeds would respond to thecharacteristics of the flowering period of M. arborea, aimed to have a maximum of seeds yield in summer,
a common pattern in many Mediterranean species. The reproduction phase is crucial in starting leaf
senescence for M. arborea under drought. Oppositely, M. citrina produced a lower seeds yield, and neversuffered total leaf senescence during the summer (Figure 3a). Leaf maintenance in summer must respondto the positive energetic balance of the maintenance of leaves (Gulías et al., 2002).
3500(a)1200(b)

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20000Oct Dec Fev Mar May Jun Jul Aug Sep

MonthOctF
De ebcAFe prbJM un
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Fig. 1. Seasonal variations in shoots (B) for non-physically disturbed plants (a) and defoliated plants (b) of M. arborea(circle) and M. citrina (triangle) under irrigation regime (closed symbol) and rain fed conditions (open symbol).

3.1 Effects of the irrigation treatment on plant production
Dealing with the irrigation regime for NDP, B was improved (P<0.001). This would illustrate theaptitudes of Medicago species for crop management, increasing their production with water availability.
Seeds yield of M. citrina doesn’t show any improvement under irrigation (Figure 2a and b). In contrast,M. arborea showed a very conspicuous increase in Y from May to July.
Therefore, for NDP under irrigation, B reported differences between species in summer, raising M.
citrina as the most productive (Figure 1a and b). The lower B in summer corresponded to a high Y, and the187

800
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Seeds yield (g plant-1)

Seeds yield (g plant-1)

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Recovery periodMonthSeasonal variations in seeds yield (Y) for non-physically disturbed plants (a) and defoliated plants (b) ofM. arborea (circle) and M. citrina (triangle) under irrigation regime (closed symbol) and rain fed
conditions (open symbol).

Fig. 2.

86

Plant leaf area (m2 plant -1)

Plant leaf area (m2 plant-1)

6(a)4205(b)4321

Oct Dec Fev Mar May JunMonth

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ar-JulM
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Fig. 3. Seasonal variations in plant leaf area (PLA) for non-physically disturbed plants (a) and defoliated plants(b) of M. arborea (circle) and M. citrina (triangle) under irrigation regime (closed symbol) and rain fed
conditions (open symbol).

plant is focusing resources on seeds prior to summer latency, rather than in plant growth, and inversely than M.
citrina. This difference was also observed for SDM, being higher for M. citrina. This latter has also high
PLA for NDP under irrigation (Figure 3), responding to the common spring growth in most plants. M.
arborea showed a reduction in PLA (50%) in summer conditions (Figure 3a), but not to water deficit.
Finally, as stressed above, the seeds yield was more conspicuously different for species in NDP underirrigation, since water availability in summer induced a nearly six-fold seeds yield under irrigationcompared to drought for M. arborea. Under drought, M. citrina showed a low flowering during the year,
especially for defoliated plants (Figure 2b).

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3.2 Plant foraging production improvement through defoliation
Woody shrubs can respond to defoliation in concrete year periods showing an increase of produced
biomass, this is the case of M. arborea and M. citrina. However, responses of both species are different
with regards to the dry matter allocation. Along the year, defoliated plants, for both species, showed avery similar pattern for all parameters. As a rule, two maximum periods were observed: a higher one inspring showing more differences between species and treatments, and a lower one in autumn with moresimilarities in values between species and treatments.
As for B, defoliation produced a higher shoots for M. citrina than for M. arborea in spring.
Concerning the components relationship of shoots, M. arborea showed a high ratio leaves/stems in winter,reaching the 80% in spring. Similarly, for M. arborea, Corleto et al. (1993) reported ratios of 120, 80 and60%, at frequencies of defoliation of 60, 120 and 180 days, respectively.
Under irrigation, the results for LDM and for SDM showed a similar pattern to those for B and nodifferences between species were shown. Under drought, M. arborea showed a slightly better
performance than M. citrina in spring.
4. Conclusions
In semi-arid conditions, M. citrina is a more promising foraging species than the widely used M.
arborea. This is mainly due to its higher water use efficiency and leaf biomass (Lefi et al., 2003).
Moreover, its recovery capacity was considerable with water availability and it presented several
morphological traits favoring its adaptation to drought against M. arborea, as the water extraction fromdeeper horizons (Lefi et al., 2004). All these productive, morphological and physiological traits confer toM. citrina a particular agronomical and ecological interests.
References
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