Leaf water characteristics and drought acclimation in sunflower genotypes

The responses of leaf water parameters to drought were examined using three sunflower (Helianthus annuus L.) genotypes. Osmotic potential at full wate r saturation (pi(100)), apoplastic water fraction (AWF) and bulk elastic mo dulus (BEM) were determined by pressure-volume curve analysis on well water ed or on water-stressed plants (-1.0 MPa <Psi(1) < -1.5 MPa) previously dro ught-pretreated or not. The drought-pretreated plants were subjected to a 7 -day drought period (predawn leaf water potential reached -0.9 MPa) followe d by 8 days of rewatering. In well watered plants, all genotypes in respons e to drought acclimation displayed a significantly decreased pi(100) associ ated with a decrease in the leaf water potential at the turgor-loss point ( decrease in Psi(tlp) was between 0.15 and 0.21 MPa, depending on the genoty pe). In two genotypes, drought acclimation affected the partitioning of wat er between the apoplastic and symplastic fractions without any effect on th e total amount of water in the leaves. As a third genotype displayed no mod ification of AWF and BEM after drought acclimation, the decreased pi(100) w as only due to the net accumulation of solutes and was consistent with the adjustment of the photochemical efficiency observed previously in this geno type in response to drought acclimation. In water-stressed plants, the osmo tic adjustment (OA) can increase further beyond that observed in response t o the drought pretreatment. However, the maintenance of photosynthetic rate and stomatal conductance at low leaf water potentials not only depends on the extent of osmotic adjustment, but also on the interaction between OA an d AWF or BEM. Adaptative responses of leaf water parameters to drought are thus quite contrasted in sunflower genotypes.