Mechnisms of drought resistance in Melia volkensii and M. azedarach

The aim of studies reported in this thesis was to investigate 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 underlie these differences. The underlying assumption was that variations in adaptation to different environmental conditions within the natural ranges of these two species exist. M. volkensii and M. azedarach are potentially important multipurpose tree species for planting in the semi-arid zones. Knowledge of characteristics that confer the ability to resist water stress in seedlings is vital in the understanding of their ophysiology, and identifying genotypes best suited for planting on specific sites. To address this objective, both glasshouse and field experiments were conducted. The natural ranges of M. volkensii (Kenya, Somalia and Tanzania) and M. azedarach (east coast of Australia) are characterized by variable climatic conditions, which also include seasonal droughts. Seeds were collected from three provenances along climatic gradients in Kenya (M. volkensii) and northern Queensland, Australia (M. azedarach). The number of trees from which seed was collected at each site ranged from 4 to 10, giving a total of 22 families of M. volkensii and 12 families of M. azedarach. Owing to the large amount of experimental material available, it was necessary to screen for genotypic variations at the beginning of these studies in order to identify families and/or provenances exhibiting the most contrasting responses to deficit irrigation for further detailed investigations. Two approaches were used: (a) isozyme analysis, and (b) glasshouse screening experiments in which seedlings of each species were raised from all available seed and subjected to full and deficit irrigation treatment in two separate glasshouse experiments in small pots. Six families from each species with the most contrasting responses to deficit irrigation were further selected on the basis of experiment 1 and 2 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 conOIuons. Po. runner rounn gtassnouse expenmern m wmcn IWO conrrasung IamitieS from each species were subjected to three irrigation regimes was conducted - full irrigation (IRR), recurring deficit irrigation (RDI), and no irrigation (DRY). The aim of the fourth glasshouse experiment was to determine the physiological basis of seedling responses observed in the glasshouse and field plot experiments. The isozyme technique failed to adequately resolve the question of genetic differences because of the very low levels of isozyme variation detected in either species. Overall, water stress caused a significant reduction in all growth parameters assessed in both species and an adjustment in dry weight allocation among roots and shoots. The most striking difference between species was in the morphology of the taproot. M. volkensii seedlings were characterized by fleshy swollen roots which were absent in M. azedarach. The development of the fleshy swollen root in M. volkensii and lack of it in M. azedarach was the basis for physiological differences between the species. Differences in M. volkensii were stronger at the family level and weaker at the provenance level while an opposite response pattern was observed in M. azedarach. The most important difference between species was 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 higher water stress integral observed in seedlings of M. azedarach were a consequence of lower leaf pre-dawn water potential (-2.5 MPa) compared to higher values observed for M. volkensii (-0.9 MPa). Water loss in both species was reduced by a reduction in leaf area. Long-term water use efficiency 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 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 possibly 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; comparisons with 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.