Estimation of the effect of photoinhibition on the carbon gain in leaves of a willow canopy.

The occurrence of photoinhibition of photosynthesis in leaves of a willow canopy was examined by measuring the chlorophyll-a fluorescence ratio of F V/F M (FM is the maximum fluorescence level of the induction curve, and FV is the variable fluorescence, F V=F M-F 0, where F0 is the minimal fluorescence). The majority of the leaves situated on the upper parts of peripheral shoots showed an afternoon inhibition of this ratio on clear days. This was the consequence of both a decrease in F M and a rise in F O. In the same leaves the diurnal variation in intercepted photosynthetic photon flux density (PPFD) was monitored using leaf-mounted sensors. Using the multivariate method, partial least squares in latent variables, it is shown that the dose of PPFD, integrated and linearly weighted over the last 6-h period, best predicts photoinhibition. Photoinhibition occurred even among leaves that did not intercept PPFDs above 1000 μmol·m(-2)·s(-1). Exposure of leaves to a standard photoinhibitory treatment demonstrated that the depression in the F V/F M ratio was paralleled by an equal depression in the maximal quantum yield of CO2 uptake and a nearly equal depression in the rate of bending (convexity) of the light-response curve of CO2 uptake. As a result, the rate of net photosynthesis is depressed over the whole natural range of PPFD. By simulating the daily course in the rate of net photosynthesis, it is estimated that in the order of one-tenth of the potential carbon gain of peripheral willow shoots is lost on clear days as a result of photoinhibition. This applies to conditions of optimal temperatures. Photoinhibition is even more pronounced at air temperatures below 23° C, as judged from measurements of the FV/FM ratio on clear days: the afternoon inhibition of this ratio increased in a curvilinear manner from 15% to 25% with a temperature decrease from 23° to 14° C.