Non-Stomatal Inhibition Associated with Inactivation of Rubisco in Dehydrated Coffee Leaves under Unshaded and Shaded Conditions

Abstract

The effect of rapid dehydration due to withholding of irrigation on leaf photosynthesis in coffee (Coffea arabica L.) was studied by comparing the CO₂-dependent rate of photosynthesis in intact leaves (A/Ci curve), the amounts of Chl, total soluble protein and ribulose-l,5-bisphosphate carboxylase (Rubisco) in leaves, and the activities of photochemical reactions of isolated chloroplasts and of Rubisco under unshaded and shaded conditions. The CO₂-saturated rate of photosynthesis and the carboxylation efficiency (the initial slope of A/Ci curve) decreased with decreasing leaf water potential (ψ_w). The non-stomatal limitation of photosynthesis, whereby photosynthesis was depressed independent of a restricted supply of CO₂ to the leaf as a result of stomatal closure, was estimated from changes in A/Ci curves and accounted for about 65% and 60% of the total reduction in the rate of photosynthesis at a ψ_w of – 1.8 MPa in the unshaded and shaded leaves, respectively. When leaf dehydration became more severe (–3.0 MPa), the non-stomatal limitation of photosynthesis increased to more than 90% in both unshaded and shaded leaves. Levels of total leaf soluble protein, Rubisco and Chl and the activity of whole-chain electron transport were unaffected by dehydration, but the initial activity of Rubisco decreased significantly. The non-stomatal contribution to the decrease in photosynthesis in coffee leaves that had been exposed to rapid dehydration was associated with inhibition of the activity of Rubisco in vivo.