Optimal Control of Gas Exchange during Drought: Empirical Evidence

The optimal regulation model by Mäkelä, Berninger and Hari ( Annals of Botany77 : 461–467, 1996) was applied to data for photosynthesis and transpiration of Scots pine during a 22-d drought period. There was a clear decrease in photosynthesis and transpiration during that period. The agreement between model and photosynthesis data was good. The residuals of photosynthesis were not systematic with respect to temperature, irradiance or water vapour deficit. However, the model initially overestimated transpiration by 50%, although there was a clear linear relationship between measured and estimated values. The results suggest that there was no decrease in photosynthetic capacity during the period, but a decrease in stomatal conductance was responsible for the changes in photosynthesis and transpiration. The observations are similar to results in the literature.