Moderate salinity improves stomatal functioning in rose plants grown at high relative air humidity

Plants grown at high relative air humidity (RH ≥ 85%) show hampered stomatal closure in response to closing stimuli. We hypothesized that a moderate salinity during growth could trigger a stress response and stimulate stomatal functioning due to an increased leaf abscisic acid concentration ([ABA]). Cut rose ‘Prophyta’ was grown at moderate (63%) or high (89%) RH combined with three electrical conductivities (EC) in the nutrient solution (2, 4 and 6 dS m−1; adding NaCl). High RH resulted in higher pore area per leaf area in intact leaves, and higher stomatal conductance (gs) both in leaves subjected to desiccation and to light/dark transition, as compared to moderate RH. Increasing EC in high RH-grown plants lead to higher stomatal density but it enhanced stomatal closure in response to leaflet desiccation. This enhanced stomatal functioning was associated with increased [ABA] and [ABA + metabolites]. Nonetheless, plants grown at EC6 showed a significantly lower chlorophyll content, total plant dry weight and total leaf area. This negative effect on plant growth is related to ionic stress as the sodium and chloride concentrations increased in plants grown at EC6 compared to EC2 (up to 111- and 14-fold, respectively). This is the first study on the interactive effects of RH and salinity on stomatal functioning and anatomy during leaf development. It is shown that, when these two environmental factors that influence stomatal responsiveness in an opposite way are combined, moderate EC is able to improve stomatal responsiveness to leaflet desiccation in high RH-grown plants due to increased leaf [ABA].