Technical Abstract: Nighttime stomatal conductance and transpiration can lead to substantial water loss occurring at non-photosynthetic times in C3 and C4 plants. Water loss without simultaneous carbon gain is intuitively wasteful, however it is possible that either there is no cost when water supply is sufficient or that it provides a benefit under certain conditions. For example, the transpiration stream is largely responsible for supplying soil-mobile nutrients (e.g. nitrate) to plant roots, and maintenance of water flux through the soil-plant-air continuum through nighttime transpiration (NT) may enhance nutrient supply to plant roots. In a growth chamber study with 3 ecotypes of Arabidopsis thaliana, nighttime relative humidity was manipulated to suppress or allow NT in plants with sufficient or limited N supply. N-limited plants grown under high nighttime relative humidity (~90%) showed decreased plant fitness relative to plants in which NT was not suppressed (~55% RH), indicating that under non-water-limiting conditions, NT benefited N-limited plants. Results for non-N-limited plants varied among ecotypes, suggesting that for some ecotypes relative costs and benefits depended on nutrient supply. In a separate field study, Helianthus anomalus plants given nutrient additions showed decreased day and night conductance and transpiration. To explore the extent to which NT may affect the supply of mobile nutrients to plant roots, the Barber-Cushman model was used to investigate depletion of nitrate at various distances from a rooting zone under different levels of water flux through the system. Results showed that increasing water flux through the system (as with NT) increased N supply to plant roots. Another field study using the desert shrub Sarcobatus vermiculatus showed that plant uptake of 15NO3- decreased slightly when NT was suppressed, although differences were not significant due to low sample sizes. Taken as a whole, there is growing evidence that the magnitude of NT may affect nutrient acquisition of plants.