Differential effects of elevated air humidity on stomatal closing ability of Kalanchoë blossfeldiana between the C3 and CAM states

•The high RH effect on stomatal physiology of C3 and CAM state plants was assessed.•One week exposure to high RH led to increased stomatal conductance.•High RH during or following growth impairs stomatal functioning in C3 state plants.•No effect of high RH on stomatal response to desiccation was noted in leaves detached from plants in CAM-state. High relative air humidity (RH≥85%) impairs stomatal functionality, attenuating plant capacity to cope with abiotic stress. Previous studies were limited to C3 species, so the RH effect on stomatal physiology of CAM plants remains unexplored. We addressed the topic through comparisons of C3 and CAM states in a facultative CAM species. These states were validated by diel measurements of net assimilation rate and malate level. In the first two experiments, three Kalanchoë interspecific hybrid cultivars were exposed to moderate (60%) or high (90%) RH. Both leaves that expanded at high RH and leaves that had expanded at moderate RH and were subsequently exposed to high RH (for nine days) showed increased stomatal conductance. In the third experiment, both C3 and CAM state plants of one K. blossfeldiana cultivar were exposed to low (40%), moderate (60%) or high (90%) RH. Plant transpiration during night-time was inversely related to ambient RH in either state, whereas during day-time a significant effect was only noted at 90% RH. Kalanchoë leaves showed a very effective control of water loss upon water deprivation, especially in the CAM state. Following a single week exposure to 90% RH, detached leaves showed increased rates of water loss during desiccation in C3 state plants. No effect of high RH on stomatal response to desiccation was noted in leaves detached from plants in CAM-state. It is concluded that the negative effect of either growth or one-week exposure to high RH is restricted to the C3 state in Kalanchoë.