Nighttime water loss from C3 plants
Howard, Ava Rose
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Plants dominate many ecosystems and play a critical role in water cycling. Most plants have a C3 photosynthetic pathway and water losses at night can include both transpiration (Enight) from the canopy and hydraulic redistribution (HR) from roots. The magnitude of water lost in these pathways is highly variable and a better understanding of the relationship between these processes and potential environmental drivers is needed. In greenhouse studies, we examined the effects of manipulating soil water and nitrogen availability on instantaneous measures of leaf conductance (gnight) and Enight in three focal species: Populus angustifolia, P. balsamifera spp. trichocarpa and wild Helianthus annuus. For all species gnight was at least five times greater than minimum leaf conductance, suggesting most gnight and Enight may be regulated. Based on known daytime responses to water limitation, we hypothesized that gnight and Enight would decrease when soil water availability was limited. Results from five studies and all three species supported this hypothesis. Based on the potential for transpiration to increase mass flow of mobile nutrients to roots, we hypothesized that gnight and Enight would increase under limiting soil nitrogen. Results from eight studies and all three species rejected this hypothesis. Nitrogen limitation sufficient to reduce biomass did not affect gnight or Enight when potentially confounding water stress was eliminated. Based on reductions of HR when transpiration is artificially increased by nighttime lighting, we hypothesized that naturally occurring Enight would reduce the magnitude of HR. We tested this hypothesis in the greenhouse using Artemisia tridentata, Helianthus anomalus and Quercus laevis. Suppressing Enight with nighttime canopy bagging resulted in increased HR for A. tridentata and H. anomalus but not Q. laevis. Overall, our studies suggest that gnight and Enight vary in response to soil water but not nutrient availability and can affect the magnitude of HR. Variability in Enight should be incorporated into models of stand and ecosystem water flux. Increased understanding the variability in Enight should also be used to improve estimates of when and to what magnitude HR may play an important role in ecosystem hydrology.