Night-time water relations and gas exchange in cut shoots of five boreal dwarf shrub species: impact of soil water availability.

Recent findings suggest that drought may affect plants' daytime and night-time stomatal regulation differently. However, knowledge of night-time stomatal behaviour in dwarf shrubs growing in boreal ecosystems is lacking. We sampled cut shoots from dwarf shrub species to elucidate their capacity to transpire at night and the effect of drought on stomatal regulation. The shoots' water relations and gas exchange were measured under controlled conditions in a growth chamber. The studied species demonstrated considerable differences in their diurnal water use. The night-time water use percentage of daytime water use (NWU) reached up to 90% in and . In , and , the NWU was 62, 27 and 26%, respectively. The shoots of showed a significant increase ( < 0.001) in the transpiration rate () during the night. However, in , a decrease ( < 0.05) in nightly was observed. The shoot conductance () at the end of the night was lower than daytime in all studied species, but the difference was not significant for . Across the species, NWU was negatively related ( < 0.001) to the soil volumetric water content (SWC) in the plant habitat. However, daytime and were positively related ( < 0.05) to the habitat SWC. Only in was night-time higher ( < 0.05) in dry conditions than in wet conditions. Our results demonstrate high variability in diurnal water relations in dwarf shrubs, which can keep stomata open in the dark even when drought limits daytime and .