Modelling the effect of air vapour pressure deficit on leaf photosynthesis of greenhouse tomatoes: The importance of leaf conductance to CO2

SummaryThe response of photosynthesis of leaves of greenhouse tomato plants to leaf position and vapour pressure deficit (VPD) was studied by modelling the effect of these on leaf conductance to CO2. The study was carried out in Avignon (southern France) on well-irrigated plants during spring and summer seasons, with VPD ranging from 0.7 to 3.4 kPa at midday, while the 24 h mean ranged from 0.6 to 1.8 kPa. Net photosynthesis (Pn) was measured on single leaves placed at three levels defined by the leaf position, and under different CO2 concentrations in the range of 200 to 1100 µmol mol–1. A model for leaf photosynthesis was used to evaluate the leaf conductance to CO2 transfer. The leaf conductance to CO2 transfer was maximum for top level leaves, and decreased with leaf depth in the canopy. Leaf conductance at the upper level was reduced when air VPD exceeded a threshold value of 1 kPa.