PHOTOSYNTHETIC INDUCTION STATE OF LEAVES IN A SOYBEAN CANOPY IN RELATION TO LIGHT REGULATION OF RIBULOSE-1-5-BISPHOSPHATE CARBOXYLASE AND STOMATAL CONDUCTANCE

Photosynthetic induction state, stomatal conductance and light regulation of ribulose-1,5-bisphosphate carboxylase (rubisco) were examined for leaves in a mature, closed soybean (Glycine max) canopy (leaf area index approximately 5) with the objective to determine the extent to which these factors may be limiting the capacity to respond to light transients during sunflecks. When sampled along a vertical gradient, leaves near the bottom of the canopy had lower rubisco contents and chlorophyll a/b ratios as compared with upper leaves. Leaves sampled at midcanopy showed a wide variation in photosynthetic induction state (ratio of the photosynthetic rate achieved after 1 minute exposure to high light to the steady-state assimilation rate achieved after 20 minutes exposure). Both photosynthetic induction state and the initial rubisco activity varied in parallel with stomatal conductance. By contrast there was no correlation between total rubisco activity and stomatal conductance. The results indicate that induction state, as determined by the light regulation of both rubisco activity and stomatal conductance, is an important limitation to the ability of leaves in a soybean canopy to respond to light transients that occur during sunflecks.