Drivers of nocturnal stomatal conductance in C and C plants.

Nocturnal water losses were for long considered negligible, but it is now known that incomplete stomatal closure during the night leads to significant water losses at leaf, plant and ecosystem scales. However, only daytime transpiration is currently accounted for in evapotranspiration studies. Important uncertainties on the drivers of nocturnal water fluxes hinder its incorporation within modelling frameworks because some studies indicate that night-time stomatal drivers may differ from day-time responses. Here, we synthesise the studies on nocturnal stomatal conductance (g) to determine underlying drivers through a systematic literature review and, whenever possible, meta-analytical techniques. Similar to daytime responses, we found negative effects of vapour pressure deficit, predawn water potential, air temperature, and salinity on g across the plant species. However, the most apparent trend was an increase of g from the beginning until the end of the night, indicating significant and widespread endogenous regulation by the circadian clock. We further observed how neither elevated CO nor nutrient status affected g significantly across species. We also did not find any significant associations between g and elevated ozone or increasing plant age. There was a paucity of studies on climatic extremes such heat waves and also few studies connected g with anatomical features such as leaf specific area or stomatal density. Further studies are also needed to address the effects of plant sex, abscisic acid concentrations and genotypic variations on g. Our findings solve the long-term conundrum on whether stomatal responses to daytime drivers are the same as those that during the nighttime.