Modelling diurnal courses of photosynthesis and transpiration of leaves onthe basis of stomatal and non-stomatal responses, including photoinhibition

A mathematical model for photoinhibition of leaf photosynthesis was develop ed by formalising the assumptions that (1) the rate of photoinhibition is p roportional to irradiance; and (2) the rate of recovery, derived from the f ormulae for a pseudo first-order process, is proportional to the extent of inhibition. The photoinhibition model to calculate initial photo yield is i ntegrated into a photosynthesis-stomatal conductance (g(s)) model that comb ines net photosynthetic rate (P-N), transpiration rate (E), and g(s), and a lso the leaf energy balance. The model was run to simulate the diurnal cour ses of P-N, E, g(s), photochemical efficiency, i.e., ratio of intercellular CO2 concentration and CO2 concentration over leaf surface (C-i/C-s), and l eaf temperature (T-1) under different irradiances, air temperature, and hum idity separately with fixed time courses of others. When midday depression occurred under high temperature, g(s) decreased the most and E the least. T he duration of midday depression of g(s) was the longest and that in E the shortest. E increased with increasing vapour pressure deficit (VPD) initial ly, but when VPD exceeded a certain value, it decreased with increasing VPD ; this was caused by a rapid decrease in g(s). When air temperature exceede d a certain value, an increase in solar irradiance raised T-1 and the degre e of midday depression. High solar radiation caused large decrease in initi al photon efficiency (alpha)- P-N, E, and g(s) showed reasonable decreases under conditions causing photoinhibtion compared with non-photoinhibition c ondition under high irradiance. The T-1 under photoinhibition was higher th an that under non-photoinhibition conditions, which was evident under high solar irradiance around noon. The decrease in C-1/C-s at midday implies tha t stomatal closure is a factor causing midday depression of photosynthesis.