Transpiration from geranium grown under high temperatures and low humidities in greenhouses

Most transpiration studies of greenhouse crops have been undertaken under cool conditions in northern and central Europe. These conditions are greatly exceeded in most greenhouses of the Mediterranean basin or in other zones where the climate is warmer. This article presents some measured transpiration data of Geranium, Pelargonium zonale grown under high vapour pressure deficit (VPD) (up to 3.4 kPa) and temperature (up to 36°C). Calculated transpiration values from the Penman–Monteith equation averaged over 30 min intervals were compared with measured transpiration by weighing plant on an electronic balance. To calculate the energy used in transpiration, the radiation term of the Penman–Monteith equation was set equal to the net radiation above the plant canopy times (1−exp(kL)), where k is the extinction coefficient and L is the leaf area index. The best correlation was obtained for k=0.61 (R2=0.93). Leaf resistance was estimated as a function only of solar radiation, since leaf resistance derived from porometer measurements showed no dependence on VPD or temperature. The agreement between measured and calculated transpiration was good for the wide range of temperature, radiation and humidity levels that occurred during the 42 days of the test. The effect of air humidity and temperature on canopy resistance was estimated from measurements of crop water use by rearranging the Penman equation. No reduction of canopy resistance was found for VPD values from 1.4 to 3.4 kPa and ambient temperature from 26 to 36°C. A simplified model of daytime transpiration based on easily measured variables is also presented.