Optimal Control of Gas Exchange during Drought: Empirical Evidence

The optimal regulation model by Mäkelä, Berningerand Hari (Annals of Botany 77: 461–467, 1996) was appliedto data for photosynthesis and transpiration of Scots pine duringa 22-d drought period. There was a clear decrease in photosynthesisand transpiration during that period. The agreement betweenmodel and photosynthesis data was good. The residuals of photosynthesiswere not systematic with respect to temperature, irradianceor water vapour deficit. However, the model initially overestimatedtranspiration by 50%, although there was a clear linear relationshipbetween measured and estimated values. The results suggest thatthere was no decrease in photosynthetic capacity during theperiod, but a decrease in stomatal conductance was responsiblefor the changes in photosynthesis and transpiration. The observationsare similar to results in the literature. Transpiration; photosynthesis; stomatal conductance; drought; Pinus sylvestris