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Universität Hohenheim (2012)

Physiological and growth responses of Jatropha curca L. to water, nitrogen and salt stresses

Rajaona, Arisoa Mampionona

Titre : Physiological and growth responses of Jatropha curca L. to water, nitrogen and salt stresses

Auteur : Rajaona, Arisoa Mampionona

Université de soutenance : Universität Hohenheim

Grade : Doktor der Agrarwissenschaften (Dr. sc. agr. / Ph.D. in Agricultural Sciences) 2012

Résumé
This thesis provides necessary and complementary information for an improved understanding of jatropha growth to guide further research to evaluate the response of jatropha to abiotic stressors and for designing plantations adapted to the plants ? requirements. Given the fact that jatropha is claimed to grow on marginal lands, we studied effects of water supply, salt stress, nitrogen and air humidity as major abiotic stressors on gas exchange parameters and biomass production followed by management options for pruning the trees to positively influence biomass productivity and to contribute to optimize resource use. The effects of water availability (rainfed versus irrigated) on growth and gas exchange parameters were investigated for 4-year old jatropha grown in a semi-arid environment at a plantation site in Madagascar in 2010. The results confirmed that 1250 mm water in addition to a 500 mm rainfall did not affect biomass production and instantaneous gas exchange. Nevertheless, leaf light responses of irrigated plants were higher than that of rainfed plants. The study showed to what extent salt stress affected water use, canopy water vapour conductance, leaf growth and Na and K concentrations of leaves of 3-year old and young jatropha plants. 3-year old plants were exposed to seven salt levels (0-300 mmol NaCl L-1) during 20 days and young plants to five salt levels (0-200 mmol NaCl L-1) during 6 days. In both experiments, plants responded rapidly to salt stress by reducing water loss. The threshold value of responses was between 0 and 5 dS m-1. Leaf area increment of young jatropha had a threshold value of 5 dS m-1 implying that jatropha is sensitive to external salt application in terms of canopy development, conductance and CO2 assimilation rate. Transpiration of plants in both experiments was reduced to 55% at EC values between 11 and 12 dS m-1 as compared to non-stressed plants. These findings indicate that jatropha responds sensitive to salt stress in terms of leaf elongation rate and consequently canopy development, and to immediate physiological responses. Leaf gas exchange characteristics of jatropha as affected by nitrogen supply and leaf age were intensively studied, as carbon assimilation is one of the central processes of plant growth and consequently a key process embedded in modelling approaches of plant productivity. This study showed that N supply effects on leaf gas exchange of jatropha leaves were small with only the treatment without nitrogen resulting in lower rates of CO2 assimilation rate and light saturated CO2 assimilation rate, nevertheless, effects of N supply on biomass formation were pronounced. Instantaneous rates of leaf gas exchange of different leaves subject to variable air humidity (atmospheric vapour pressure deficit (VPD)) were investigated. This study showed that CO2 assimilation rate (A) and stomatal conductance (gs) were correlated in a hyperbolic fashion, and that gs declined with increasing VPD. Maximal stomatal conductance of jatropha was in the range of 382 mmol m-2 s-1 and gs is predicted to be close to zero at 6 kPa. Effects of VPD, via stomatal conductance, by preventing high transpiration rates, have been demonstrated to be decisive on water use efficiency. Our findings are in this regard relevant for the estimation of water use efficiency of jatropha. The outcome further indicates favourable conditions at which stomatal opening is high and thereby allowing for biomass formation. This information should be considered in approaches which aim at quantifying leaf activity of field-grown bushes which are characterized by spatially highly diverse conditions in terms of microclimatic parameters. Microclimatic parameters can be modified by the tree structure. The reported field experiment on 4-year old jatropha indicated that the biomass production and canopy size depended mainly on primary branch length. A comparison of plants of different pruning types with regard to trunk height (43 versus 29 cm) and total length of primary branches (171 versus 310 cm), suggest that higher biomass production and greater leaf area projection was realized by trees with short trunks and long primary branches. Growth of twigs and leaves was positively correlated with total length of branches. Relative dry mass allocation to branches, twigs and leaves, length of twigs per cm of branches and specific leaf area were not affected by pruning and water supply. Trees with shorter branches had a higher leaf area density. As opposed to an allometric relationship between the average diameter of primary branches and total above ground biomass, our data suggest that these traits were not constantly correlated. Our data indicate that the length of newly formed twigs, where the leaves are attached to, can be related to the total length of already established branches. Leaf area density and relative dry mass allocation to leaves were not affected by the two pruning techniques, indicating that pruning differences in leaf area size were proportionally converted to corresponding pruning differences in the canopy volume exploited by plants. The results reported in this study are relevant for understanding jatropha growth. It helps farmers first for a better plantation management and researchers as well as contribution to future modelling purpose concerning jatropha growth under variable climatic conditions. Additionally, it should complement information for a better set of priorities in research, contribute indirectly to breeding programs and adjust agricultural policies in terms of encountering global change.

Mots Clés : Abiotischer Stress , Purgiernuss , Wasserdampfdruckdefizit , Photosynthese ,

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Page publiée le 13 septembre 2012, mise à jour le 23 septembre 2017