The stomatal response to evaporative demand persists at night in Ricinus communis plants with high nocturnal conductance

Evidence is building that stomatal conductance to water vapour (g(s)) can be quite high in the dark in some species. However, it is unclear whether nocturnal opening reflects a mechanistic limitation (i.e. an inability to close at night) or an adaptive response (i.e. promoting water loss for reasons unrelated to carbon gain). Further, it is unclear if stomatal responses to leaf-air vapour pressure difference (D) persist in the dark. We investigated nocturnal stomatal behaviour in castor bean (Ricinus communis L.) by measuring gas exchange and stomatal responses to D in the light and in the dark. Results were compared among eight growth environments [two levels for each of three treatment variables: air saturation deficit (D-a), light and water availability]. In most plants, stomata remained open and sensitive to D at night. g(s) was typically lower at night than in the day, whereas leaf osmotic pressure (Pi) was higher at night. In well-watered plants grown at low D-a, stomata were less sensitive to D in the dark than in the light, but the reverse was found for plants grown at high D-a. Stomata of droughted plants were less sensitive to D in the dark than in the light regardless of growth D-a. Drought also reduced g(s) and elevated Pi in both the light and the dark, but had variable effects on stomatal sensitivity to D. These results are interpreted with the aid of models of stomatal conductance.