Endogenous regulation of night-time water relations in hybrid aspen grown at ambient and elevated air humidity

Although night-time water relations have been studied in different plant species in the last decade, there is limited information about the impact of climate variables on endogenous regulation of night-time water relations in trees. The aim of the current study was to elucidate how long-term exposure to increased air humidity impacts night-time gaseous and liquid phase conductance and gas exchange in cut shoots of hybrid aspen (Populus tremula L. × P. tremuloides Michx.) sampled from the free air humidity manipulation experimental site and measured at constant air relative humidity (RH) level in a growth chamber. Neither the early-night leaf conductance (gn1) nor canopy conductance (gc) differed (P > 0.05) between the humidification (H) and control (C) treatments. However, there was a significant (P < 0.01) difference in predawn leaf conductance (gn5) and shoot hydraulic conductance (K5) between the treatments, with the shoots from the H treatment opening their stomata more efficiently before dawn. Both the early-night dark respiration (R1) and height increment of stump sprouts were significantly higher (P < 0.01) in the control than in the H treatment. Although the relationship between gn5 and predawn dark respiration (R5) was statistically significant (P < 0.01) in the H treatment, the gn5 did not depend on R5 in the C treatment. Our findings suggest that endogenous increase in predawn water flux associates with decreased growth rate of hybrid aspen grown at elevated RH. Thus, regional changes in air humidity may potentially impact night-time water relations in fast-growing tree species like hybrid aspen.