The effect of night-time transpiration on the magnitude of hydraulic redistribution in four diverse species

Nighttime water losses by C₃ plants can include both transpiration (E_night) from the canopy and hydraulic redistribution (HR) from roots.  HR is thought to provide plants with nutrient and water benefits.  We tested the hypothesis that the magnitude of HR is inversely related to the magnitude of E_night in a greenhouse study using Artemisia tridentata and Quercus laevis.  Plants were grown with their roots split between two compartments.  HR was initiated by briefly withholding all water, followed by watering only one compartment.  HR was defined as the difference between initial and maximal soil water potential overnight as measured by screen cage psychrometers in the compartment from which water was withheld.  Under these conditions both A. tridentata and Q. laevis showed significant E_night (0.027 – 0.41 and 0.24 – 0.62 mmol m^-2 s^-1, respectively).  Bagging plant canopies over night to minimize E_night significantly increased the magnitude of HR by A. tridentata, but had no effect on HR by Q. laevis.  Magnitude of HR was additionally affected by environmental variables including nighttime vapor pressure deficit and photosynthetically active radiation the previous day.  Our results suggest that for some species E_night may have further water costs to plants beyond direct losses to the atmosphere and may also negatively impact plant nutrient acquisition.  Similar studies are currently underway with Helianthus anomalus and Populus balsamifera spp. trichocarpa.