DETERMINATION OF EPIDERMAL TRANSPIRATION IN 4 CULTIVARS OF NICOTIANA-TABACUM L. USING EPIDERMAL STRIPS IN A QUASI-STEADY-STATE SYSTEM

A quasi-steady state method is presented for quantifying epidermal tra nspiration of epidermal strips where simple relations between transmem brane fluxes and parameters of diffusibility of penetrating compounds hold. Contrary to most permeability studies, we did not use astomatous , enzymatically isolated, or dried cuticular membranes, because these procedures ape largely responsible for the problems cited in the liter ature. Instead, we used freshly harvested stomatous epidermal strips, thus avoiding the sorption of lipids by the cuticular membranes during enzymatic isolation. Our approach allowed estimation of amounts and c omposition of intracuticular soluble lipids. Diffusion coefficients (D -values) were calculated with smaller associated standard deviations a nd an order of magnitude lower than previously reported; the fresh mat erial sorption of the diffusing compound by the membrane and hydration of the cuticular pores was greatly reduced. In the present study the hold-up time (t(e)) ranged from 66.2 +/- 0.3 to 110.3 +/- 0.9sec, Furt hermore, 0.1 mu m thick membranes were used, contrary to previous stud ies of water permeability that used cuticles more than 2 mu m thick. B ecause a small but constant now of penetrant could be detected during the first half of the steady now to t(e), small holes probably did not influence the reported permeability. Permeability coefficients (P-d) in the order of 0.65 x 10(-9) ms(-1) were calculated. P-d values in th e order of 5.68 x 10(-3) ms(-1) were calculated when incomplete stomat al closure occurred, while when areas of mass now were detected, P-d v alues in the order of 1.26 x 10(-2) ms(-1) were calculated. The degree of contamination of the epidermal strips by cellular debris was quant ified and expressed as the total chlorphyll content per exposed surfac e area of the epidermal strip, and an average of 8.7% contamination wa s observed compared to the total leaf chlorophyll content. Leakage fro m the system was calculated to be approximately 0.18 x 10(-10) ms(-1), which represents an average 2.7% experimental variability. These resu lts are discussed in terms of the limitations associated with using co mposite membranes that are stomatous and have trichomes, for possible application in drought tolerance selection.