Environmental control of greenhouse crop transpiration

Abstract

An application of the control of crop transpiration in the environmental management of a greenhouse tomato crop is described. The climate control algorithm is not based on prescribed day or night setpoints for temperature and humidity within a greenhouse but on a setpoint for crop transpiration. The setpoints for temperature and humidity corresponding to the transpiration setpoint can be deduced from within the algorithm. These values are used as setpoint values for the climate control in the next 2 min. The procedure is based on a model of the transpiration of greenhouse crops, whereby the transpiration is calculated from the effects of the microclimate on the canopy. In this particular instance the microclimate is assumed to be determined by the temperature and humidity of the air surrounding the canopy and the amount of incoming global radiation. The canopy itself is assumed to be totally defined by the efficiency with which it is able to exchange energy (whether sensible, latent, or radiative). This simple model is able to assess the transpiration rate of a real greenhouse crop with an accuracy of about 10%. It follows then, that if a “transpiration setpoint” and the quantity of incoming radiation are assumed as given, the transpiration model can be inverted in order to yield the combinations of air temperature and humidity that could cause the desired transpiration rate. In fact, many different combinations of the two can provide the same transpiration rate; the choice of final climate setpoints has to be dictated, therefore, by additional criteria, e.g. minimize heating requirement, or (more far fetched) maximize assimilation. Results show that control of crop transpiration is possible, as far as greenhouse environment can be manipulated. Failure to realize the setpoint of humidity prevented, in some instances, the achievement of the desired transpiration level; trials in a double-glazed house, moreover, showed that scarcity of radiation limits the attainable transpiration rate. Some preliminary results described here show that, to the extent that a climate control system is able to sustain different transpiration levels under a given radiation, some difference in crop development may be expected.