Modification of Drought Resistance by Water Stress Conditioning inAcaciaandEucalyptus

Abstract

Plants of Acacia and Eucalyptus species were grown under different levels of shading, nutrition, and irrigation to assess the effect of these factors on plant water use. Water use per unit of leaf (phyllode) area was affected only by the irrigation treatment, control plants that had received water daily using appreciably more water than plants that had been repeatedly subjected to water stress. Water stress conditioning had little or no effect on plant height, leaf (phyllode) area, or minimum stomatal resistance in any of the species. Detailed study of the water stress conditioning of Eucalyptus robusta showed that controls used 46% more water than conditioned plants. Leaf area and plant height were unaffected by conditioning. Control of transpiration was not due to stomatal functioning, both sets of plants operating with the same leaf diffusive resistance under conditions of ready water availability. Hydraulic conductivity of the intact root system was lowered by conditioning and it is suggested that this was due, at least in part, to the effect that conditioning had on root xylem conductivity. Specific conductivity of stem sections was lowered by water stress conditioning. Water stress avoidance was also associated with a more pronounced tendency for stomata to close prior to wilting and with a higher level of leaf resistance which could be maintained at a low leaf water potential. Conditioned plants exhibited drought tolerance in their ability to control loss of water from the leaf at lower leaf water potentials than the controls.