Night-time transpiration in barley (Hordeum vulgare) facilitates respiratory carbon dioxide release and is regulated during salt stress.

Background and Aims: Night-time transpiration accounts for a considerable amount of water loss in crop plants. Despite this, there remain many questions concerning night-time transpiration - its biological function, regulation and response to stresses such as salinity. The aim of the present study was to address these questions on 14- to 18-d-old, hydroponically grown barley plants.

Methods: Plants were either stressed for the last 4-7 d prior to, and during subsequent continuous (24 h), diurnal gravimetric transpiration analyses; or subjected to salt stress just before analyses; or stressed for 4-7 d and then transferred to control medium before analyses. The idea behind this experimental setup was to distinguish between a longer- (cuticle, stomata) and shorter-term (stomata) response of transpiration to treatments. Cuticular conductance was assessed through residual transpiration measurements in detached leaves. Cuticle wax load and dark respiration rate of leaves were determined. Leaf conductance to CO2 was calculated.

Key Results: Night-time and daytime transpiration rates were highly, and positively, correlated with each other, across all treatments. Night-time transpiration rates accounted for 9-17 % of daytime rates (average: 13.8 %). Despite minor changes in the ratio of night- to daytime transpiration rates, the contribution of cuticular and stomatal conductance to leaf (epidermal) conductance to water vapour differed considerably between treatments. Salt stress did not affect cuticle wax load. The conductance for CO2 of the cuticle was insufficient to support rates of dark respiratory CO2 release.

Conclusions: The main biological function of night-time transpiration is the release of respiratory CO2 from leaves. Night-time transpiration is regulated in the short and long term, also under salt stress. Stomata play a key role in this process. We propose to refer, in analogy to water use efficiency (WUE) during the day, to a CO2 release efficiency ('CORE') during the night.