COS 50-5
Drinking the night away: Frequent nocturnal foliar absorption and reverse sap flow in a temperate mangrove fores
Foliar water uptake has been shown to be an important contribution to plant water balance in an increasing number of taxa. Leaf absorbed water may not only benefit the leaf, but also be translocated to other parts of the plant. This process requires an inversion of the soil-plant-air gradient in water potentials. Since the decline in water potential with increasing salinity should reduce the threshold at which the soil-plant-air gradient is inverted, we hypothesized that more negative osmotic potentials at the roots would favor greater foliar uptake and reverse sap flow under higher salinity conditions. We examined the prevalence of foliar absorption and sap flow in response to environmental conditions in the mangrove Avicennia marina growing naturally in seawater and hypersaline salinities.
Results/Conclusions
Foliar uptake of water by mangrove leaves occurred regularly, with leaf water potentials exceeding those of soil water after humid nights. Furthermore, this foliar absorption was accompanied by significant reversals of sap flow after rain or heavy dewfall, reaching a significant proportion of total transpiration the next day (mean = 7.4%, s.d. = 9.6, n = 12), with some individuals exceeding 20%. Sap flow responded very rapidly to wetting events. Imposition of a watering treatment led to an immediate (<10 min) and large reduction in sapflow velocity (mean= 85%, s.d. = 6%, n=12) . This effect was consistent across both sites, two days and two different watering levels. Contrary to expectations, we found no salinity-associated differences in foliar absorption or reverse sap flow, possibly reflecting high salinities at both study sites. These results imply that foliar absorption and reverse sap flow may be important factors for consideration in mangrove ecology and hydrology.