Foliar Uptake of Water by Wet Leaves of Sloanea woollsii, an Australian Subtropical Rainforest Tree

Abstract

At leaf water potentials (ψ) of around -2.5 MPa, detached leaves of Sloanea woollsii F.Muell., an Australian subtropical rainforest tree, were able to absorb small amounts of water vapour from a saturated atmosphere, but absorbed considerably more liquid water if their surfaces remained wet. When leaves attached to small branches exhibiting a ψ of -2 MPa were sprayed with water and maintained in a saturated atmosphere, leaf ψ returned to saturation values within about 6 h. In a further experiment, a ψ of -2 MPa was imposed on detached, forked branches. Branches were then exposed to a saturated atmosphere and leaves on one half of the fork were kept wet whilst the rest remained dry. Leaf ψ was measured periodically for both dry and wet leaves and in both cases was found to increase with time. This indicated that leaf surface water was imbibed by wet leaves and transported into the branch resulting in alleviation of low water potentials in the dry leaves. In the submontane rainforests in which S. woollsii occurs, extended periods with little or no rainfall occur regularly. Throughout the year, foliage is wet by fog or dew alone for about 25% of the time. It is suggested that the presence of leaf surface water during rainless periods when leaf Psi is low may be important for the survival of S. woollsii. The actual mechanism of foliar absorption is uncertain, but is likely to be direct imbibition through the cuticle.