All terrestrial plants face the same major dilemma that the opening of stomata facilitates both carbon assimilation and water loss.  Thus, a major selective force in nearly all land plants is maximizing carbon gain while minimizing water loss. The optimization of plant carbon and water relations suggests plants should close their stomata to conserve water when there is no opportunity for carbon fixation, such as during the night for C3 and C4 plants. Moreover, nighttime water loss should be a maladaptive trait, particularly in arid environments where water is often the most limiting resource, and atmospheric demand can be very high.  Given the expected negative implications of nighttime stomatal opening and the importance of water conservation in arid ecosystems, we assessed nighttime stomatal conductance (g_night) and water loss for 10 plant species, representing different functional types in the Chihuahuan, Great Basin, Mojave, and Sonoran Deserts.  We used a hierarchical Bayesian framework to synthesize datasets from the four deserts and quantify (1) how stomatal conductance responds to key environmental drivers (e.g., temperature, soil water, vapor pressure deficit); (2) how this response differs between daytime and nighttime periods; and (3) the importance of nighttime transpiration to whole-plant water loss over daily and seasonal time-scales.

Results/Conclusions

Contrary to the conventional stomatal control paradigm, we found substantial g_night in all 10 desert species, but the degree of nighttime opening was not related to plant functional type or desert.  At night, desert plants operated like mesic species with isohydric behavior, but during the day, they were functionally isohydric under wet conditions and anisohydric under dry conditions. Our model results suggest that nighttime water fluxes represent a substantial proportion of the cumulative whole-plant water loss over a 24-hour period, with nighttime transpiration representing 17-26% of total daily water loss in 9 out of 10 species. Relative nighttime water loss in desert species is comparable to that in plants from coastal, temperate deciduous, and tropical evergreen forests, where nighttime transpiration is 10-30% of the maximum daytime transpiration. Over the course of a growing season, nighttime transpiration in desert plants has the potential to account for up to 29% of the precipitation inputs during this period. Current theories of plant functional behavior suggest losing large proportions of water at night would put plants at a selective disadvantage, yet natural selection has not resulted in the strict water conservation expected in arid areas.