Abstract:

Canopy transpiration of crops grown in greenhouses affects greenhouse air temperature and humidity and is therefore important for the optimization of greenhouse climate control. In this study, cucumbers (Cucumis sativus) were grown in a subtropical modern greenhouse under winter climate conditions, canopy transpiration was measured every 30 minutes and greenhouse microclimatic factors were sampled with 1 second interval and 30 minutes average values were saved in a datalogger and mean values of canopy resistance rc and boundary layer resistance ra were estimated based on experimental measurements made in the greenhouse. The Penman-Monteith equation was used to estimate canopy transpiration rates. The results showed that the diurnal time course of canopy transpiration rates followed that of the net radiation above the canopy with the daily maximum value occurring at 12∶00. In contrast, maximum water vapor pressure deficit (VPD) occurred between 13∶00 and 14∶00. These results indicate that under winter climate conditions of China, subtropical greenhouse microclimate conditions are characterized by low VPD level and that daytime variations of crop canopy transpiration rates depend primarily on net radiation above the canopy. The daily maximum values of net radiation and VPD above the canopy and crop canopy transpiration rates were below 350 W·m－2, 2.0 kPa and 200 W·m－2, respectively; the ratio of latent heat caused by crop transpiration to the net radiation above the canopy in winter averaged 46%; and rc and ra were 100 s·m-1 and 600 s·m-1, respectively. The crop canopy transpiration rate and total cumulative crop transpiration calculated by Penman-Monteith equations using both the actual rc and ra values and the mean values of rc and ra agreed well with the measured results. The determination of the ratio of latent heat to the net radiation above the canopy (46%) and the characteristic values of rc (=100 s·m-1) and ra (600 s·m-1) facilitates the development of a crop transpiration model to optimize greenhouse climate and irrigation control systems. More experiments are needed to determine if these values are valid for other sites and types of greenhouses.