Whole-plant net photosynthesis, an indication of actual growth, a tool for greenhouse climate control: A case study with Ficus benjamina `Natasja'

Climate control of greenhouses can still be improved. The speaking plant concept, where the plant asks for the desired combination (optimal) of climatic parameters such as temperature, CO2 concentration, relative humidity and radiation is becoming reality. Whole-plant photosynthesis, which is well correlated with the growth of foliage plants, can be used as a parameter to control the greenhouse climate. Until now, in most of the experiments found in the literature net photosynthesis was measured at the level of the leaf. Then, these results were extrapolated to the level of the plant. This requires complicated models concerning for example the influence of leaf age on net photosynthesis and the radiation extinction in the canopy. When whole-plant photosynthesis is measured, complicated models are not required and errors are diminished. In this experiment, three plant cuvettes were developed to measure the gas exchange of whole plants simultaneously. The cuvettes were set up in a greenhouse. The effect of a real combination of climate parameters on whole-plant photosynthesis was measured. For the experiment we used Ficus benjamina `Natasja' with a leaf area of about 2 dm(2). Whole-plant photosynthesis was continuously monitored during six consecutive days. The first three days the CO2 concentration was kept at 350 mu l.l(-1), the following days at 600 mu l.l(-1) After correction for changed leaf area and respiration of the substrate a non-linear regression model was fitted. By using those two models tone for each CO2 concentration) we could draw conclusions concerning the net carbon-uptake for the different CO2 treatments for typical days. For implementation to the speaking plant concept more measurements of whole-plant photosynthesis are needed during the whole cultivation period of the plant.