GAS-EXCHANGE, STEM WATER POTENTIAL AND LEAF ORIENTATION OF RUBUS-IDAEUS L. ARE INFLUENCED BY DROUGHT STRESS

Rubus idaeus L. cv. Heritage raspberry plants were placed in controlle d environment chambers (25 degrees C, 14 h photoperiod, 2.0 kPa satura ted vapour pressure deficit, CO2 concentration of 380 mu l l(-1)) to s tudy the effects of drought stress on leaf gas exchange and stem water potential. Whole-plant photosynthesis (Pn) and transpiration were sen sitive to drought stress and gradually decreased from the second day o f the study until rehydration. Stomatal aperture regulation occurred d uring the initial 48 h of the study with transpiration rates dropping in response to a decrease in stem water potential. Spatial differences in response to drought stress were found with values of leaf Pn, and water vapour and CO2 conductance in the younger, distal (i.e. closer t o the apex) leaves decreasing at a faster rate than the older, proxima l leaves (i.e. close to crown). Evidence of increased mesophyll resist ance to drought stress was apparent with intercellular CO2 concentrati on (c(i)) remaining either constant or increasing, while Pn and carbox ylation efficiency simultaneously decreased. Therefore, an optimum bal ance between water loss and uptake of c(i) existed, and an alteration in these rates represented an adjustment in stomatal conductance to ma tch the intrinsic photosynthetic capacity rather than a causal relatio n. Protection of the underlying photochemistry was evident with parahe lionastic leaf movements which resulted in a reduction in the effectiv e leaf area and associated heat loads. Despite stem water potential of the stressed plants returning to control levels after re-watering, Pn of the stressed plants never attained control values, presumably as a result of damage to photochemistry.