Effects of the glasshouse environment on leaf temperature of pot chrysanthemum and dieffenbachia

Studies were carried out to determine if the direct monitoring or modelling of leaf or canopy temperature could be used to aid the optimization of glasshouse environments in the summer. Chrysanthemum and dieffenbachia pot plants were grown in glasshouse compartments with air temperature regulated either by the operation of vents or by air-conditioning, and showing differences of up to 13 degreesC on extreme summer days. Irradiance treatments were applied within compartments by continuously shading plots with neutral filters, and forced air re-circulation was used to ensure that all shading treatments within a compartment were subjected to similar air temperatures and vapour pressure deficits. Leaf temperatures were monitored using thermocouples attached to the undersides of leaves. Excellent fits to observed leaf temperatures (variance accounted for >90%) were obtained in both species using simple, empirical, linear-response models utilizing only air temperature and irradiance. Increases in either air temperature or irradiance always gave greater increases in leaf temperature in dieffenbachia than in chrysanthemum. Dieffenbachia proved particularly sensitive to changes in irradiance. The findings indicated the need for different strategies in the two species to ameliorate the effects of the summer environment. Actual leaf - air temperature differences were compared to predictions given using a mechanistic model based on tomato which took account of irradiance, air temperature, vapour pressure deficit, leaf size and stomatal conductance. The model gave a good fit for chrysanthemum but not for dieffenbachia. Infra-red thermographic measurement showed that changes in leaf temperature due to changes in air temperature and in irradiance were extremely rapid and that the leaf temperature gradient across an individual leaf could be as great as 5 degreesC. Trials indicated that wide-angle infra-red sensors could give accurate determinations of average canopy temperatures for vent and screen control.