Background/Question/Methods Substantial nighttime conductance (g) has recently been discovered in a broad range of C₃ plant species. However, little is known about its regulation. Experimental manipulations of leaf-to-air vapor pressure deficit (VPD_leaf) have established a clear pattern of stomatal responses during the day, but nighttime responses have been largely inferred from correlative field data. We provide one of the first comparisons of daytime and nighttime g responses to VPD_leaf using experimental manipulations that eliminate potential confounding effects.

Results/Conclusions Solanum lycopersicum var. cerasiforme and Populus balsamifera spp. trichocarpa had substantial nighttime g (~ 0.2 mol m^-2 s^-1 at 0.5 kPa VPD_leaf) and nighttime g did decrease as VPD_leaf was increased to 2.5 kPa. For S. lycopersicum the nighttime sensitivity of g to VPD_leaf (assessed as slope/intercept) was substantially lower than daytime sensitivity. In contrast, P. balsamifera nighttime and daytime sensitivity to VPD_leaf were similar. We are currently testing additional environmental conditions and species to see if a general pattern emerges. Our data thus far support the conclusions that nighttime g does decline with increasing VPD_leaf, but that species may differ in whether the nighttime sensitivity is lower than that of daytime.