Foliar water uptake improves the leaf tolerance to drought in tropical cloud forest trees
Tropical montane cloud forests (TMCF) are characterized climatically by recurrent fog events. These events represent an important hydrological input to the vegetation both by recharging soil water reserves through fog drip and by being directly absorbed through foliar water uptake (FWU). Changes in earth surface temperature might affect the height of cloud formation in tropical regions, which should decrease the frequency of fog events in TMCF. In order to understand how TMCF vegetation will respond to these possible climate scenarios, it is necessary to understand the ecological importance of FWU and the physiological mechanisms controlling it. We measured FWU in isolated leaves and branches of three TMCF tree species: Drimys brasiliensis Miers (Winteraceae), Myrsine umbellata Mart. (Primulaceae) and Eremanthus erythropappus (DC.) MacLeish (Asteraceae). We verified the occurrence of FWU in these species using apoplastic fluorescent tracers and deuterium labelling experiments. We estimated the degree of rehydration that FWU would provide to the leaves by measuring the increase of leaf water potential (Ψl) after immersing isolated leaves in water. We also conducted a drought experiment in a glasshouse to investigate the role of fog on the Ψlmaintenance.
We have found that the leaf rehydration dynamics through FWU (increase in Ψl after immersion) in all species was linearly related to the Ψl before the immersion (R2 = 0.81-0.93). During the glasshouse experiment, the species that possessed leaves with higher rehydration capacity could maintain their Ψl above the leaf turgor loss point (Ψtlp) for more time when they were exposed regularly to artificial fog. When D. brasiliensis was exposed to regular nebulization it took 13.5% longer to reach its Ψtlp (in relation to the control treatment that was not exposed to nebulization). M. umbellata took 8.4% longer to reach its Ψtlp, while E. erythropappus took only 3.5% longer. Our results suggest that FWU is a passive water flux through the leaf cuticle, stomata and possibly other specialized structures proportional to the leaf water status during the leaf wetting event. The rehydration caused by FWU might be important to maintain leaf physiological functioning during droughts. We also show that our simple method of estimating leaf rehydration capacity could be useful to assess the FWU importance to the leaf water relations and drought resistance of a broad range of species.