Chloroplast acclimation to low osmotic potential

Abstract

Photosynthetic potential of isolated chloroplasts was investigated during in situ water deficits. An eight day stress cycle imposed on spinach plants reduced leaf ψ_w by 0.57MPa, and leaf ψ_π by 0.50MPa, resulting in partial turgor maintenance during the stress cycle. Pressure/volume curves confirmed the occurrence of osmotic adjustment. Leaf ψ_π depression was associated with an altered response of chloroplasts to low ψ_π in vitro. Optimum reaction medium ψ_π for photosynthesis shifted from −1.04 to −1.57MPa, and low ψ_π was not as inhibitory to photosynthesis of plastids pre-exposed to stress in situ. These data indicate that chloroplasts acclimate to low external ψ_π in response to leaf water deficits. This response was still evident four days after a stress cycle ended, but was nearly reversed eight days after stress. Repeated stress cycles in situ did not increase the degree of chloroplast acclimation to low ψ_π in vitro. Fast dehydration of leaves did not induce this apparent chloroplast acclimation.