WATER-UPTAKE AND TRANSPIRATION CHARACTERIZATION OF NEW-GUINEA IMPATIENS

Greenhouse environmental control programs are beginning to implement i rrigation control strategies based on evapotranspiration models; howev er a lack of crop-specific parameters currently limits the utility of these strategies. The objectives of this study were to evaluate daytim e and nighttime water uptake and transpiration characteristics of New Guinea impatiens (Impatiens X hb.) 'Equinox', and to determine canopy resistance characteristics applicable for use in transpiration models. This growth-chamber experiment used all combinations of three day tem peratures (20 to 30 degrees C) and five irradiance levels (22 to 180 W m(-2), 285-2800 nm) to create a range of environments in which to stu dy water uptake and transpiration response. Water uptake was measured continuously by a weighing lysimeter Water uptake increased significan tly with temperature but not irradiance. In terms of the evapotranspir ation model parameters used, water uptake was better predicted by vapo r pressure deficit than total radiation, and still better by either le af vapor pressure deficit or vapor pressure deficit and total radiatio n combined Leaf temperature deficit and leaf assimilation chamber tran spiration measurements supported these results. Water uptake increased with predicted transpiration up to about 0.32 kg m(-2) h(-1), above w hich it was unable to increase with further increases in transpiration potential. Uptake was shown to continue throughout the dark period; t his uptake was shown to be related to the environmental conditions of the previous light period. These results suggest that for some crops, irrigation management may not need to be responsive to short-term chan ges in irradiance, and that nighttime water uptake and transpiration m ay be significant.