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Accueil du site → Doctorat → Australie → 1994 → Mechanisms of drought resistance in Melia volensii and M. azedarach

Australian National University (1994)

Mechanisms of drought resistance in Melia volensii and M. azedarach

Milimo, Patrick Baraza Wamalwa

Titre : Mechanisms of drought resistance in Melia Volkensii and M. azedarach.

Auteur : Milimo Patrick B.

Université de soutenance : Australian National University

Grade : Doctor of Philosophy (PhD) 1994

The aim of studies reported in this thesis was to investigate through glasshouse field trials differences in drought resistance in seedlings from different families and provenances of Melia volkensii and M. azedarach var australasica, and to determine the morphological and physiological characteristics that underline these differences. The underlying assumption was that genetic variations exist in adaption to different environmental conditions within the natural ranges of these two species. M. volkensii and ?M. azedarach are potentially important multipurpose tree species for planting in the semi-arid zones. The natural ranges of M. volkensii (Kenya, Somalia and Tanzania) and M. azedarach ?(east coast of Australia) are characterized by variable climate including seasonal drought. Seeds were collected along a range of climatic gradients and from 4 to 10 parents per collection site at three provenances in Kenya (M. volkensii) and northern Queensland, Australia (M. azedarach). The total collection represented 22 families of M. volkensii with 12 families of M. azedarach. Given the large amount of experimental material available it was decided to screen for genotypic variations at the beginning of these studies in order to identify, for detailed investigation, families and/or provenances exhibiting the most contrasting responses to deficit irrigation. Two approaches were used : (a) isozyme analysis, and (b) glasshouse screening experiments in which seedlings of each species raised from each family were subjected to full and deficit irrigation treatment in two separate glasshouse experiments in small pots. From each species, six families with the most contrasting responses to deficit irrigation were further selected, on the basis of experiment 1 and 3 results (small pots), for further testing in the glasshouse and in field plots in Queensland. The aims of the third glasshouse experiment and field plot trials were to determine whether seedling responses observed in glasshouse experiments 1 and 2 were influenced by small soil volumes, and also whether they could be replicated under field conditions. A fourth glasshouse experiment in which two contrasting families from each species were subjected to three irrigation regimes was conducted full irrigation (IRR), recurring deficit irrigation (RDI), and no irrigation (DRY). They aim of this experiment was determination of the physiological basis of seedling responses observed in prior glasshouse and field plot experiments. The isozyme technique failed to adequately resolve the question of genetic differences because in both species very low levels of isozyme variation detected. In both species, water stress significantly reduced all assessed growth parameters, particularly the shoot dry weight ratio. M. volkensii seedlings typically had fleshy swollen roots a characteristic absent in M. azedarach. This striking difference in morphology may account for many of the physiological differences between the species. Differences in M. volkensii were stongest and between families and weakest between provenances while the opposite pattern was observed in M. azedarach. The species differed in their ability to maintain high tissue water potentials under water stress. M. azedarach seedlings endured lower mean average water stress integrals (-102.1 Mpa-days) than M. volkensii seedlings (-35.0 Mpa-days). The greater water stress integral observed in seedlings of M. azedarach were due to lower leaf pre-dawn water potential (-2.5 Mpa) compared to (-0.9 Mpa) for M. volkensii. Water loss in both species was reduced by a reduction in leaf area. Long-term water use efficiency (WUE) as measured by both dry weight (g DW/kg H2O transpired) and 13C/12C isotopic ratio was the same for both species under all treatments. The 13C/12C isotopic ratio was lowest (hence WUE was the highest) in seedlings grown on RDI and highest in those grown in the IRR treatment. The differences between treatments were significant and the stem apex 13C/12C isoptoic ratio was positively correlated with WUE determined on the basis of dry matter produced per unit water transpired, indicating that WUE may be useful in predicting the most productive genotypes. Both species responded to water limitation by an avoidance’ strategy rather than a tolerance’ strategy in that water limitation resulted in reductions in leaf area, at high leaf water potentials and relative water contents. In neither species was there evidence of osmotic adjustment except possible in the Tableland provenance of M. azedarach. In spite of similarities in some responses of M. volkensii and M. azedarach to water limitations significant differences between species were also observed. The water content of M. azedarach roots, stems and leaves decreased with water stress, and with prolonged stress seedlings of M. azedarach died. In contrast the water content of M. volkensii roots, stems, and leaves increased with increasing water stress leading to an increase in succulence in all plant components. The responses of M. volkensii are discussed in terms of tissue capacitance, i.e. the ability of stored water in the plant tissues to supply water requirements at times of stress, mechanisms of drought avoidance exhibited by desert succulents, and the possibility of M. volkensii seedlings switching from a C3 to a CAM pathway of photosynthesis under severe water stress.

Subject words : Melia Effect of drought on • Melia Drought tolerance • Melia Genetics • Melia Water requirements

Annonce (National Library of Australia)

Forestry Reposit-ry

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