Occurrence, patterns, and implications of nighttime stomatal conductance and transpiration in C3 and C4-plants

C3 and C4 plants are generally expected to close stomata at times when photosynthetic carbon gain does not occur, such as during the night. However, many C3 and C4 plants maintain stomatal opening throughout the night and, depending on nighttime conditions (i.e., VPD), substantial transpirational water loss can occur. In a comparison of three methods spanning leaf to canopy scales of nighttime water loss in field-grown tomato, stomata were open and 6-10% of daytime water loss rates were observed at night. Gas exchange measurements of various Solanaceous species and accessions of Arabidopsis showed that substantial genetic variation in nighttime leaf conductance (gnight) exists among and within species. Furthermore, variation in gnight in Arabidopsis was highly correlated with daytime gas exchange traits, including daytime leaf conductance (gday) and photosynthesis. Although g night was also highly correlated with VPD of the accessions' native environments, whether there is a direct or indirect (via the correlation with gday) basis for the relationship is unknown. The implications of gnight and Enight include various costs (e.g., reduced predawn plant water status) as well as benefits (e.g., enhanced nutrient supply), so that the occurrence and magnitude of gnight and transpiration (Enight) may depend on the relative tradeoff of these implications in a particular environment. Experimentally suppressing nighttime water loss with high relative humidity treatments had no effect on leaf nutrient content (Na, K, Ca, N) or final plant biomass of Arabidopsis. Reduced nitrogen supply resulted in smaller plants and lower gnight, despite maintenance of relatively high leaf N. Thus, there was no evidence of a benefit in terms of enhanced nutrition by transpiring at night in Arabidopsis . Future investigations will need to focus on how plants regulate stomata during the night and provide more direct quantification of the implications of these phenomena to obtain an understanding of the ecological and evolutionary consequences of gnight and Enight for C3 and C4 plants.