| Repeated water deficits can alter the response of photosynthesis and leaf growth to subsequent water deficits, but the mechanisms by which these changes occur are unknown. This study was conducted to evaluate the mechanisms by which photosynthesis and leaf growth acclimate to low leaf water potential ((psi)(,l)). The response of photosynthesis to (psi)(,l), determined from gas-exchange and isopiestic thermocouple psychrometry, was shifted 3 to 4 bars towards lower (psi)(,l) in plants grown under a mild soil water deficit. Both stomata and chloroplasts contributed to the acclimation. Leaf diffusive resistance at low (psi)(,l) was less in acclimated plants than control plants. Chloroplast activity, demonstrated in situ with measurements of quantum yield, capacity to fix CO(,2) at all partial pressures of CO(,2) (pCO(,2)), and in the isolated organelle with measurements of photosystem II electron transport, was inhibited less at low (psi)(,l) in the acclimated plants than in the control plants. Analysis of the acclimation indicated that altered chloroplast sensitivity contributed more than altered stomatal response to the acclimation of photosynthesis to low (psi)(,l).;Leaf growth, measured with a radial variable displacement transducer, was less in acclimated plants than in control plants. Acclimated leaves grew more slowly and for less of the day than control leaves, but were able to grow at (psi)(,l) which completely inhibited growth in control plants. This ability to grow at low (psi)(,l) was attributable to the maintenance of turgor by osmotic adjustment. However, when rewatered, the growth of acclimated leaves increased but was still less than the growth of controls despite greater turgor in acclimated leaves. Therefore, factors other than turgor and osmotic adjustment must have limited growth in acclimated leaves and may have caused the low growth rates that were observed. Five potentially controlling factors were investigated and showed that acclimated leaves were less extensible and required more turgor to initiate growth than control leaves. The slow growth of acclimated leaves was not due to a collapse of the potential gradient for water uptake but changes in the apparent hydraulic conductivity of acclimated plants could have occurred. It was concluded that leaf growth acclimated to low (psi)(,l) by adjusting osmotically, and the concomitant maintenance of turgor permitted growth where none otherwise would occur. However, this was accompanied by generally slow growth in the acclimated plants due primarily to changes in the turgor threshold for growth and the tissue extensibility. |
