Contribution of leaf water uptake to transpiration and water status in arid ecosystems

Foliar water uptake (FWU) could be relevant for plants, allowing them to use alternative water sources than the primary one that is soil water uptake and thus prevent dehydration especially during the dry season in arid/semiarid ecosystems characterized by small and erratic water pulses. The main objective of this study was to evaluate the effects of FWU on leaf water status and its contribution to daily transpiration in an arid ecosystem in southern Argentina. Eight dominant species, including shrubs and grasses, with different rooting depth (<0.5, <1, <2 and >2 m) and thus different soil water access, were selected. We hypothesized that FWU is higher in species with shallow roots than in species with permanent water sources and thus FWU enhances plant water availability. We determined FWU capacity by changes in leaf mass after spraying with deionized water. The water potential (Ψ_Leaf) was determined in the field before and after small water experimental application. Leaf transpiration (E) was measured using a portable photosynthesis system. All study species exhibited leaf water uptake capacity, and was lower in species with deep roots than in the species with shallow roots during the growing season. However, the effects on Ψ_Leaf were different. During the dry season, only grasses had a substantial enhancement of water status after an experimental water application. A linear negative relationship between FWU and the changes in Ψ_Leaf after an experimental water pulse was observed during the spring season and an inverse relationship during summer. Species with higher FWU were those with higher transpiration rate suggesting that FWU contribute to plant water balance allowing stomata to remain open, thus enhancing carbon assimilation during the growing season.