Water Uptake of Hydrangea Macrophylla and Gardenia Jasminoides In Response to a Gradually Drying Substrate

Due to the lack of quantitative data regarding specific water requirements of ornamental species, precision irrigation of these species is a difficult task for nursery growers. Substrate moisture release curves have been used to predict the amount of plant available water in many soilless substrates, yet there is no information about whether there are differences among species in their ability to extract water from substrates. The objective of this study was to determine how water uptake in Hydrangea macrophylla and Gardenia jasminoides was affected by decreasing substrate volumetric water content (VWC) due to withholding irrigation.  Plants were studied in growth chambers under continuous lighting to prevent diurnal fluctuations in water use from obscuring changes in transpiration occurring in response to decreasing VWC.  Whole plant transpiration rates were calculated by weighing pots with load cells every hour over a period of two and a half weeks, while the substrate gradually dried from container capacity to the point where the plants died.  The rate of weight loss was used to quantify evapotranspiration rate.  Evaporation from the substrate surface was minimized by covering the substrate surface.  Water use by H. macrophylla ‘Fasan’ started to decrease at a higher VWC (26% to 28% v/v) compared to that of G. jasminoides ‘Radicans’ (VWC of 18% to 20% VWC).  Plant water uptake stopped completely at a VWC of 18% to 19% in H. macrophylla and 12% to 14% VWC in G. jasminoides.  The results show that H. macrophylla is less adept at extracting water from a drying substrate compared to G. jasminoides.  Traditionally, plant available water in soilless substrates has been studied using substrate moisture release curves, but our data suggest that there are important differences among species that cannot be detected by utilizing moisture release curves alone.  The technique utilized in this study can be combined with water release curve data to develop a better understanding of species-dependent differences in water uptake, especially at low VWC.