Limitation to photosynthesis in water-stressed leaves: Stomata vs. metabolism and the role of ATP

Decreasing relative water content (RWC) of leaves progressively decreases stomata] conductance (g(s)), slowing CO(2) assimilation (A) which eventually stops, after which CO(2) is evolved. In some studies, photosynthetic potential (A(pot)), measured under saturating CO(2), is unaffected by a small loss of RWC but becomes progressively more inhibited, and less stimulated by elevated CO(2), below a threshold RWC (Type I response). In other studies, Apot and the stimulation of A by elevated CO(2) decreases progressively as RWC falls (Type 2 response). Decreased A(pot) is caused by impaired metabolism. Consequently, as RWC declines, the relative limitation of A by g, decreases, and metabolic limitation increases. Causes of decreased A(pot) are considered. Limitation of ribulose bisphosphate (RuBP) synthesis is the likely cause of decreased A(pot) at low RWC, not inhibition or loss of photosynthetic carbon reduction cycle enzymes, including RuBP carboxylase/oxygenase (Rubisco). Limitation of RuBP synthesis is probably caused by inhibition of ATP synthesis, due to progressive inactivation or loss of Coupling Factor resulting from increasing ionic (Mg(2+)) concentration, not to reduced capacity for electron or proton transport, or inadequate trans-thylakoid proton gradient (DeltapH). Inhibition of A(pot) by accumulation of assimilates or inadequate inorganic phosphate is not considered significant. Decreased ATP content and imbalance with reductant status affect cell metabolism substantially: possible consequences are discussed with reference to accumulation of amino acids and alterations in protein complement under water stress. (C) 2002 Annals of Botany Company.