Uptake via the leaf surface allows plants to perceive volatiles with closed stomata

Plants can detect threats by perceiving airborne chemical signals, but how these volatiles are taken up by the leaves is not well understood. Stomata are generally considered the major uptake pathway. However, as stomata close at night or during drought, this may severely limit the capacity of plants to sense volatiles. Using the crassulacean Kalanchoë laxiflora, which possesses crassulacean acid metabolism (CAM) leaves that open their stomata at night and C3 leaves that open their stomata during the day, we uncover that the leaf surface functions as an alternative volatile uptake pathway that unlocks volatile perception with closed stomata. We find that 80% of the wound-induced green leaf volatile (Z)-3-hexenyl acetate is taken up via the stomata when they are open, while 20% is taken up through an alternative pathway. Over the course of the day, non-stomatal uptake accounts for 30% of total volatile uptake. Neither residual stomatal conductance nor hydathodes can account for the latter. The non-stomatal pathway shows a preference for lipophilic volatiles and is modulated by cuticular thickness and epidermal/sub-epidermal structure. The leaf surface thus emerges as the most parsimonious non-stomatal volatile uptake pathway. Uptake via the leaf surface is highest when stomata are closed and is selectively enhanced through phytohormone application, suggesting active regulation. Volatile uptake via this pathway is sufficient to activate anti-herbivore defenses and resistance. Together, our findings reveal that plants can take up volatiles via the leaf surface when their stomata are closed, which allows them to perceive volatiles under variable environmental conditions.