High relative air humidity and continuous light reduce stomata functionality by affecting the ABA regulation in rose leaves

Plants developed under high (90%) relative air humidity (RH) have previously been shown to have large, malfunctioning stomata, which results in high water loss during desiccation and reduced dark induced closure. Stomatal movement is to a large extent regulated by abscisic acid (ABA). It has therefore been proposed that low ABA levels contribute to the development of malfunctioning stomata. In this study, we investigated the regulation of ABA content in rose leaves, through hormone analysis and beta-glucosidase quantification. Compared with high RH, rose plants developed in moderate RH (60%) and 20 h photoperiod contained higher levels of ABA and beta-glucosidase activity. Also, the amount of ABA increased during darkness simultaneously as the ABA-glucose ester (GE) levels decreased. In contrast, plants developed under high RH with 20 h photoperiod showed no increase in ABA levels during darkness, and had low beta-glucosidase activity converting ABA-GE to ABA. Continuous lighting (24 h) resulted in low levels of beta-glucosidase activity irrespective of RH, indicating that a dark period is essential to activate beta-glucosidase. Our results provide new insight into the regulation of ABA under different humidities and photoperiods, and clearly show that beta-glucosidase is a key enzyme regulating the ABA pool in rose plants.