Finding optimal temperatures to maximize leaf photosynthesis in a tomato or cucumber crop in heated glasshouse

The areas of semi-closed greenhouses have been greatly increasing in France for a couple of years. In this new generation of greenhouses, heating, cooling and dehumidification management, by both forced outside-air intake and inside-air mixing, leads to a better control of the ambient temperature. But while the optimal levels of light and CO₂ are rather well-known to obtain a high production of tomatoes or cucumbers, that of temperature is far from being clear and needs to be determined. Therefore the final aim of the study is to draw up a reference table of temperature set points (for heating, ventilating or cooling) to steer the greenhouse climate. That seems possible by identifying the “optimum” leaf temperatures, namely when the net photosynthesis is maximum, depending on available light and CO₂ levels. In this trial, the leaf owning the maximum photosynthetic rate (Pmax) in our semi-commercial conditions of soilless glasshouse crop, was determined and chosen to be thereafter the referent leaf for specific measurements. Various CO₂ concentrations (between 400 and 1200 µmol mol^-1) and photosynthetic irradiances (PPFD between 100 and 1600 µmol photons m^-2 s^-1) were then combined inside the leaf chamber of the LI-6400XT portable photosynthesis system (Li-Cor, Lincoln, NE, USA). For each combination, leaf photosynthetic activity was measured according to a gradient of temperature from 16 to 34°C. The first data already confirmed that it is really possible to define narrow ranges of target temperatures in which the maximal potential of photosynthesis can be reached. These findings require to be repeated and multiplied and should finally help the French greenhouse growers both to reduce their production costs and optimize the climate management while preserving the crop performance.