CUTICULAR WATER PERMEABILITY AND ITS PHYSIOLOGICAL SIGNIFICANCE

Cuticles act as solution-diffusion membranes for water transport. Diff usion in pores does not contribute to cuticular transpiration. An exte nsive literature survey of cuticular permeances (P) and minimum leaf c onductances (g(min)) to water is presented. The two variables cannot b e distinguished with most experimental techniques. Results from differ ent experiments are in good agreement with each other for some species , for example, Fagus sylvatica L., but not for others, such as Picea a bies (L.) Karst. In a data set of 313 values of P or g(min) from 200 s pecies, distributions of results obtained with different techniques we re found to differ significantly. Likely reasons include water loss fr om incompletely closed or incompletely sealed stomata, and the depende nce of P on moisture content of the cuticle and on storage time of iso lated cuticles. Contrasting evidence for an interaction between cuticu lar transpiration and stomatal sensitivity to air humidity is presente d. The occurrence of unusually high g(min) in trees growing at the alp ine treeline and its physiological significance are discussed. It is s hown that g(min) is of little value as a predictor for drought resista nce of crops, with the possible exception of Sorghum bicolor L. Moench . Possible water uptake from fog or dew across cuticles is considered briefly.