MEASUREMENT OF STOMATAL-RESISTANCE AND TRANSPIRATION IN A GREENHOUSE TOMATO CANOPY - COMPARISON BETWEEN A 1-LAYER AND A MULTILAYER MODEL

Transpiration for a greenhouse soilless tomato crop was determined with an electronic balance during summer periods and analyzed with simultaneous measurements of: - leaf stomatal resistances (both lower and upper faces), measured at 3 levels inside the canopy (figs 2, 3 and 4); - leaf temperature (fig 3); - leaf area index; - main interior climatic parameters: global radiation, air temperature, vapour pressure deficit. A simple method for estimating the canopy resistance r(s) to water vapour transfer from climate parameters was put forward on the basis of the multiplicative model proposed by Jarvis (1976). It was shown that r(s) is primarily dependent upon solar radiation level, G(o) (fig 6) and that residuals can be taken into account by either vapour pressure deficit, D, or temperature (air or leaf temperature). Comparison between a 1-layer model and a multilayer model was carried out, and showed that the 2 models gave similar values for computed values of r(s). The 1-layer estimations of r(s) were used to determine canopy transpiration course by mean of: - the direct formula, with calculation of the air-leaf vapour pressure deficit from leaf surface temperature data; - the combination method (Penman-Monteith formula), with estimation of the radiation absorbed by the canopy. Calculated and measured hourly transpiration values (fig 9) showed a better agreement when using the combination method (fig 10) than when using the direct method. Partition of the total transpiration flux between the radiative component and the advective component (fig 11) was discussed. It was concluded that the advective term including the influence of vapour pressure deficit cannot be neglected in the case of greenhouse conditions, and that it is necessary for irrigation and climate control purposes to take into account this parameter, in order to improve short-term predictions for canopy transpiration.