Cuticular water permeability and its physiological significance

Cuticles act as solution-diffusion membranes for water transport. Diffusion in pores does not contribute to cuticular transpiration. An extensive literature survey of cuticular permeances (P) and minimum leaf conductances (g(min)) to water is presented. The two variables cannot be distinguished with most experimental techniques. Results from different experiments are in good agreement with each other for some species, for example, Fagus sylvatica L., but not for others, such as Picea abies (L.) Karst. In a data set of 313 values of P or g(min) from 200 species, distributions of results obtained with different techniques were found to differ significantly. Likely reasons include water loss from incompletely closed or incompletely sealed stomata, and the dependence of P on moisture content of the cuticle and on storage time of isolated cuticles. Contrasting evidence for an interaction between cuticular transpiration and stomatal sensitivity to air humidity is presented. The occurrence of unusually high g(min) in trees growing at the alpine treeline and its physiological significance are discussed. It is shown that g(min) is of little value as a predictor for drought resistance of crops, with the possible exception of Sorghum bicolor L. Moench. Possible water uptake from fog or dew across cuticles is considered briefly.