Resistance to Water Loss from the Mesophyll Cell Surface in Plant Leaves

A method is described for the accurate measurement of the apparentresistance to water loss from the mesophyll cell walls of plantleaves (r_w), and for studying the mechanism underlying thisresistance. The method for distinguishing possible mechanismsinvolves a comparison of the calculated values of r_w at differentrates of evaporation. The value of r_w remained below 50 s m^–1at relative water contents greater than 11 ± 3% and 7± 2% for Pelargonium hortorum Bailey and Vicia faba L.respectively. Therefore r_w is relatively insignificant at normalphysiological water contents in these species. When r_w did increaseit was not sensitive to evaporation rate, suggesting that alowering of the vapour pressure at the evaporating sites wasnot involved. This contrasted with the results for cellulosefilter paper, where r_w was more sensitive to evaporative flux.