EFFECTS OF ABSCISIC-ACID ON MORPHOLOGICAL AND PHYSIOLOGICAL-RESPONSES TO WATER-STRESS IN EUCALYPTUS-CAMALDULENSIS SEEDLINGS

The effects of abscisic acid (ABA) and a subsequent water stress on the physiological and morphological characteristics of seedlings of three provenances of Eucalyptus camaldulensis Dehnh, grown in pots in a glasshouse were compared 56 days after application of ABA. Seedlings from a high rainfall monsoon provenance in the dry tropics (Katherine, Northern Territory, Australia) were largely unaffected by ABA or by water stress in either physiological or morphological characteristics. Seedlings from a lower rainfall monsoon provenance in the humid tropics (Petford, Queensland) responded to water stress by reducing gas exchange while increasing photosynthetic capacity, indicated by increased leaf nitrogen and chlorophyll concentrations, and both short- and long-term leaf transpiration efficiency, indicated by changes in carbon-isotope discrimination. The changes were largest when ABA was also applied. Seedlings from semi-arid Tennant Creek (Northern Territory), which had the highest assimilation rates overall, responded to both ABA and water stress primarily by morphological changes including suppression of apical growth and increase in specific leaf weight. The results are discussed in terms of specific adaptations for establishment in increasingly dry environments and the possible role of ABA in mediating stress responses.