Short-term effects of falling leafwater potential on photosynthesis and carbohydrate âccumulation in the leaves of four NS-sunflower hybrids were studied. Water stress was experimentally induced by holding the leaf and petiole submerged for 24 hours in 0, 0.05, 0.1, 0.2, 0.5, and 1.0 M sorbitol solutions. Increased concentrations of sorbitol, up to 0.2 M, induced a rapid decrease of relative water content (RWC) in the leaves of sunflower hybrids NS-H-26-RM, NS-Helios, NS-H-43, and NS-H-101. Rapidly applied water stress significantly reduced CO2 and light-saturated photosynthesis in the leaves already at 0.05 M sorbitol. The decline in net photosynthesis at decreased RWC in leaves was accompanied by a substantial decline in the apparent quantum yield. Primary photochemical reactions and electron trânsport appeared to be less affected by low RWC in leaves. Accumulation of soluble carbohydrates was increased under the effect of rapid water stress. The increased sucrose content could affect, through a feedback mechanism, the inhibition of photosynthesis. External wâter deficit induced the largest decline in the RWC and examined photosynthetic processes in NS-H-26-RM leaves, while the changes were the smallest in NS-H-101 leaves.