CANOPY PHOTOSYNTHESIS OF TOMATO, CUCUMBER AND SWEET-PEPPER IN GREENHOUSES - MEASUREMENTS COMPARED TO MODELS

Two models for canopy photosynthesis (modified versions of those of Acock et al., 1978 and of Thornley, 1976) were examined by comparison with experimental photosynthesis data of cucumber (Cucumis sativus L.), sweet pepper (Capsicum annuum L.) and tomato (Lycopersicon esculentum Mill.). The data were obtained in six large-scale, long-term, semi-commercial cultivations in greenhouses (Nederhoff and Vegter, 1994). Measured environmental conditions and measured LAI were input to the model. The emphasis was on the models' sensitivity to the prevailing CO2 concentration. The (modified) Acock model with 'standard' (originally published) parameters underestimated the photosynthesis rate. This model tuned to one of our experimental data sets did not fit very well to the other data sets. As expected, if the model was tuned to each particular data set, it was fairly in agreement with the measurements, but the fitted parameter values were sometimes questionable. With the (modified) Thornley model it was obligatory to estimate or tune the light extinction. The model performed reasonably if all parameters were tuned and also if only the light extinction was tuned. The modified models were considered usable for practical applications, after parameter tuning. As the sensitivity to CO2 was not always equal among the models and the measurements, care should be taken when applying the models for CO2 supply control.