Update on Nighttime Stomatal Conductance in Plants Nighttime Stomatal Conductance and Transpiration in C3 and C4 Plants 1[W]

Incomplete stomatal closure during the night is observed in a diverse range of C3 and C4 species (Fig. 1; Supplemental Table S1) and can lead to substantial nighttime transpirational water loss. Although water loss is an inevitable consequence of stomatal opening for photosynthetic carbon gain, nighttime stomatal opening is unexpected because carbon gain is not occurring and the need to cool leaves is reduced or absent. Most species have the ability to close stomata more than is commonly observed at night, as demonstrated by reduced nighttime leaf conductance (gnight) in response to water stress, abscisic acid (ABA), and other treatments reviewed in this Update. The magnitude of water loss occurring during the night depends on both gnight and the vapor pressure difference (VPD) between leaves and the air, as well as canopy structure and atmospheric mixing. While gnight has been recorded at up to 90% of daytime conductance, nighttime VPD is typically much lower than daytime. Thus, nighttime transpiration rates (Enight) are typically 5% to 15% of daytime rates, although sometimes as high as 30%, based on gas exchange measurements of individual leaves, whole-plant sap flow, and field scale lysimetry (Benyon, 1999; Snyder et al., 2003; Bucci et al., 2004, 2005; Daley and Phillips, 2006; Scholz et al., 2007). While some methods are more accurate and/or have less uncertainty than others, a few studies have compared methods, generally finding agreement even across measurement scales (e.g. leaf versus whole plant; Green et al., 1989). Drawbacks for each method must be recognized, particularly when comparing species or environments. For example, leaf-level gas exchange typically includes cuticular as well as stomatal components of leaf conductance to water vapor, while sap flow methods typically have attendant uncertainties as to the proportion of the measured flux resulting in bole refilling rather than transpiration from the canopy. Nevertheless, there is broad agreement among the methods and scales that stomata of many species remain partially open during the night. Measurements of minimum leaf conductance induced by ABA application and by drying excised leaves to wilting have been used to separate stomatal (gstomatal) and cuticular (gcuticular) conductance (Rawson and Clarke, 1988; Howard and Donovan, 2007). Conductance measured at maximal stomatal closure can be functionally defined as gcuticular because it is not under guard cell regulation. For a few species, this would include the effect of dust or stomatal plugs that prevent complete closure (Feild et al., 1998). In general, gcuticular estimates range from 0.004 to 0.020 mol m 22 s (Rawson and Clarke, 1988; Nobel, 1991; Kerstiens, 1995; Boyer et al., 1997; Burghardt and Riederer, 2003; Howard and Donovan, 2007), far lower than most estimates of gnight (Fig. 1; Supplemental Table S1). Thus, most reported values of gnight are largely influenced by gstomatal. Although awareness of gnight and Enight has recently been growing, little is understood about the phenomena. In particular, the costs and benefits of high gnight and Enight remain largely unknown. However, patterns of occurrence and relationships of these processes with plant physiology are emerging. This Update reviews the occurrence of gnight in C3 and C4 species, plant and environmental factors that affect gnight, and both documented and hypothesized implications of gnight and Enight (Fig. 2).