Effects of photoinhibition on whole-plant carbon gain assessed with a photosynthesis model

A canopy photosynthesis model was modified to assess the effect of photoinh ibition on whole-plant carbon gain, Photoinhibitory changes in maximum quan tum yield of photosystem II (F-v/F-m) could be explained solely from a para meter (Lflux) calculated from the light microenvironment of the leaves. Thi s relationship between F-v/F-m and the intercepted cumulative light dose, i ntegrated and equally weighted over several hours was incorporated into the model. The effect of photoinhibition on net photosynthesis was described t hrough relationships between photoinhibition and the shaping parameters of the photosynthetic light-response curve (quantum use efficiency, convexity, and maximum capacity). This new aspect of the model was then validated by comparing measured field data (diurnal courses of F-v/F-m) with simulation results. Sensitivity analyses revealed that the extent of photoinhibitory r eduction of whole-plant photosynthesis was strongly dependent on the struct ural parameters (LAI and leaf angle). Simulations for a Mediterranean everg reen oak, Quercus coccifera, under climatic conditions which cause mild pho toinhibition revealed a daily loss of 7.5-8.5% of potential carbon gain in the upper sunlit canopy layers, a 3% loss in the bottom canopy, and an over all loss of 6.1%. Thus, this canopy photoinhibition model (CANO-PI) allows the quantitative evaluation of photoinhibition effects on primary productio n.