Vol. 73 No: 6

Title:
Gas Exchange, Stem Water Potential and Leaf Orientation of Rubus Idaeus L. are Influenced by Drought Stress

Authors:
D.C. PERCIVAL, J.T.A. PROCTOR and J.-P. PRIVÉ

pp: 831-840

Abstract:
Rubus idaeusL. cv. Heritage raspberry plants were placed in controlled environment chambers (25°C, 14 h photoperiod, 2.0 kPa saturated vapour pressure deficit, CO₂ concentration of 380 µl l^-1) to study the effects of drought stress on leaf gas exchange and stem water potential. Whole-plant photosynthesis (Pn) and transpiration were sensitive to drought stress and gradually decreased from the second day of the study until rehydration. Stomatal aperture regulation occurred during 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 to the apex) leaves decreasing at a faster rate than the older, proximal leaves (i.e. close to crown). Evidence of increased mesophyll resistance to drought stress was apparent with intercellular CO₂ concentration (c1) remaining either constant or increasing, while Pn and carboxylation efficiency simultaneously decreased. Therefore, an optimum balance between water loss and uptake of c_i existed, and an alteration in these rates represented an adjustment in stomatal conductance to match the intrinsic photosynthetic capacity rather than a causal relation. Protection of the underlying photochemistry was evident with parahelionastic leaf movements which resulted in a reduction in the effective 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.

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