Differential effects of elevated air humidity on stomatal closing ability of Kalanchoë blossfeldiana between the C 3 and CAM states

Abstract High relative air humidity (RH ≥ 85%) impairs stomatal functionality, attenuating plant capacity to cope with abiotic stress. Previous studies were limited to C 3 species, so the RH effect on stomatal physiology of CAM plants remains unexplored. We addressed the topic through comparisons of C 3 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 Kalanchoe 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 C 3 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. Kalanchoe 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 C 3 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 C 3 state in Kalanchoe .