Do photosynthetic metabolism and habitat influence foliar water uptake in orchids?

Epiphytic and rupicolous plants inhabit environments with limited water resources. Those plants commonly express Crassulacean Acid Metabolism (CAM), a photosynthetic pathway that functions by accumulating organic acids in cell vacuoles at night. That accumulation reduces their leaf water potential and favors water absorption. Foliar water uptake (FWU) has been demonstrated in many species, and aids plant survival during drought events in environments with high water deficits. We hypothesized that FWU represents a strategy employed by epiphytic and rupicolous orchids for water acquisition and that CAM metabolism would favor greater rates of water absorption. We examined 6 epiphyte, 4 terrestrial, and 6 rupicolous orchids, evidencing C3 (n = 9) or CAM (n = 7) pathways. Five individuals per species were used to evaluate their FWU, their structural characteristics, and the water balances of their leaves. Rupicolous species with C3 metabolism were found to have FWU greater than the others. FWU (Cmax and k) may be related to succulence, specific leaf mass, and relative leaf water content. Those results indicated that the high orchid leaf densities favored FWU, as space for water storage increases with leaf density. Structural characteristics linked to water storage (such as high relative water content and succulence), on the other hand, can limit leaf water absorption rates by favoring high internal leaf water potentials. Epiphytic, rupicolous, and terrestrial orchids can show FWU. Rupicolous species showed high levels of FWU, probably through water absorption mainly from the mist. However, succulence in plants with CAM metabolism appears to mitigate FWU.