Effects of CO2on leaf conductance and canopy transpiration of greenhouse grown cucumber and tomato

Abstract

The effects of carbon dioxide (CO2) on stomatal opening and canopy transpiration were investigated in cucumber (Cucumis sativus L., cv. Jessica) and tomato (Lycopersicon esculentum Mill., cv. Calypso). Stomatal opening (i.e. leaf conductance, g) was measured with a porometer, and canopy transpiration rate (E) with weighing lysimeters on intact plants in large greenhouses. Regression analysis was applied to account for the effects of radiation, air humidity, leaf temperature and CO2 on g. The effect of CO2 on E, which is primarily through g and secondarily through adjusted air humidity, was investigated by combining the regression equation for g with the Penman-Monteith equation for E. The relative effect of CO2, as calculated with the fitted regression equations, was a decrease of about 4% in g for cucumber and of about 3% for tomato, per 100 μol mol-1 increase in CO2, in the range of about 300 to 1200 μmol mol−1 CO2. The effect of CO2 on E was smaller than on g and the extent of the effect depended on the conditions, mainly ventilation rate. The ratio K (relative change in calculated E divided by relative change in calculated g) was estimated at less than 0.2, except at low radiation. In reality, K will be even lower, because feedback mechanisms enforce the reduction in g and counteract the reduction in E. So the reduction of the transpiration rate of greenhouse cucumber and tomato caused by moderate CO2 enrichment is small and mostly negligible, except under low light conditions.