Direct absorption of water by hairy leaves of Phlomis fruticosa and its contribution to drought avoidance

Abstract

Water was sprayed on the adaxial surfaces of hairy and nonhairy leaves to study the possible significance of trichomes in dew or rainwater absorption. Plant species adapted to Mediterranean climate, experiencing periodic water shortage, were used. Water retention was higher and its duration significantly longer on hairy leaves, confirming visual observations in the field. Gravimetric measurements and fluorescence microscopy with the apoplasmic indicator Calcofluor showed that surface water quickly penetrated into the mesophyll of hairy but not nonhairy leaves. The trichome did not participate in the entrance pathway. Direct absorption of water increased the water potential of water-stressed cut leaves, improving their photosynthetic performance through decreased abaxial stomatal resistance. No such effects were found in nonhairy leaves. In a long-term experiment with whole seedlings of the hairy Phlomis fruticosa L., growth rate and photosynthetic pigment content in plants receiving water only through the leaves were intermediate between those of well-watered and water-stressed plants. We conclude that leaf hairs, besides other functions, may also improve leaf water status by entrapping and retaining surface water, thus assisting in its final absorption into the mesophyll. Their contribution to drought avoidance may be critical under some circumstances. Key words: dew, drought, Phlomis fruticosa L., trichome.