Photosynthetic rates and partitioning of absorbed light energy in photoinhibited leaves

Parameters for the evaluation of the effects of photoinhibition on photosynthetic carbon gain were studied in Chenopodium album leaves. The light-response curve of photosynthetic rate was determined at 36 Pa CO ₂ partial pressure and fitted by a non-rectangular hyperbola. Both the initial slope of the curve and the light-saturated rate decreased in photoinhibited leaves, although the decrease in the latter was small. The convexity of the curve was also smaller in photoinhibited leaves. The capacities of ribulose-1,5-bisphosphate carboxylation (V_cmax) and electron transport (J_max) were estimated from the CO₂-response curves. V_cmax and J_max decreased similarly with increasing photoinhibition. Energy partitioning in photosystem II (PSII) was estimated using chlorophyll fluorescence parameters. The fraction of energy that was consumed by photochemistry decreased with increasing photoinhibition. However, an increase in inactive PSII, decreasing energy partitioning to active PSII, relaxed the excitation pressure in PSII, and led to a reduction in the fraction of excess energy that was neither consumed by photochemistry nor dissipated as heat.