STOMATAL PLUGS OF DRIMYS WINTERI (WINTERACEAE) PROTECT LEAVES FROM MIST BUT NOT DROUGHT

Two outstanding features of the flowering plant family Winteraceae are the occlusion of their stomatal pores by cutin plugs and the absence of water-conducting xylem vessels. An adaptive relationship between th ese two unusual features has been suggested whereby stomatal plugs res trict gas exchange to compensate for the presumed poor conductivity of their vesselless wood. This hypothesized connection fueled evolutiona ry arguments that the vesselless condition is ancestral in angiosperms . Here we show that in Drimys,winteri, a tree common to wet forests, t hese stomatal occlusions pose only a small fixed resistance to water l oss. In addition, they modify the humidity response of guard cells suc h that under high evaporative demand, leaves with plugs lose water at a faster rate than leaves from which the plugs have been experimentall y removed. Instead of being adaptations for drought, we present eviden ce that these cuticular structures function to maintain photosynthetic activity under conditions of excess water on the leaf surface. Stomat al plugs decrease leaf wettability by preventing the formation of a co ntinuous water film that would impede diffusion of CO2 into the leaf. Misting of leaves had no effect on photosynthetic rate of leaves with plugs, but resulted in a marked decrease (approximate to 40%) in leave s from which the plugs had been removed. These findings do not support a functional association between stomatal plugs and hydraulic compete nce and provide a new perspective on debates surrounding the evolution of vessels in angiosperms.