In Situ Measurements of the Inhibitory Effects of Low Leaf Water Potentials on Photophosphorylation

Abstract

Photosynthetic activity declines as water becomes limited and the leaf water potential (Ψ_l) decreases. While these losses in photosynthesis performance are due in part to partial stomatal closure, in many cases direct effects of the water deficits on chloroplasts are more important. For instance in sunflower (Helianthus annuus L. cv. 1S894), although an increase in leaf diffusive resistance from about 2 to about 8 sec-cm^-1 was observed to accompany a decrease in Ψ_l from -0.33 to -1.63 MPa, this stomatal closure did not result in a decline in the intercellular CO2 concentration (1). Clearly the 75% reduction in rate of net photosynthesis induced by this drop in Ψ_l was not due to the stomatal closure. Two major contributors to the chloroplast level inhibition have been implicated, water oxidation and photophosphorylation. Previous work on isolated coupling factor (CF₁) indicated that the losses in photophosphorylation activity associated with low Ψ_l may result from direct effects of dehydration-induced increases in the stromal concentration of cations (2, 3). In the work reported here we have extended the earlier rn vitro studies on the effects of Ψ_l on photophosphorylation to the level of intact leaves.