Estimation of plasmalemma conductivity to ascorbic acid in intact leaves exposed to ozone

To establish the capacity of the leaf mesophyll plasmalemma of Phaseolus vulgaris L, to supply ascorbate (ASC) into the cell wall by simple diffusion, a method for calculating plasmalemma diffusional conductivity to ascorbic acid (AA) in intact leaves was evaluated. The core of the approach is that in the presence of a sink for ascorbate in the cell wall, cell wall total ascorbic acid concentration \ TAA \(ew) (=\ ASC \(ew) + \ AA \(ew)) reaches zero at some positive whole-leaf total ascorbic acid concentration \ TAA \(l). It is shown that \ TAA \(l) at \ TAA \(ew) = 0 is proportional to the sink for ASC in the cell wall and the reciprocal of plasmalemma conductivity. The predicted proportional relationship between \ TAA \(ew) and \ TAA \(l) was confirmed by decreasing TAA levels in leaves through predarkening. Furthermore, increasing the sink intensity for ASC in the cell wall by the acute exposure of leaves to 450 nmol ozone mol(-1) during re-illumination, \ TAA \(ew) reached zero at 2.7-fold higher \ TAA \(l) than without ozone, and the slope of the relationship increased twofold. Plasmalemma diffusional conductivities to AA of 2.9 x 10(-6) and 1.8 x 10(-6) m s(-1), needed to maintain \ TAA \(ew) at the observed level, were calculated from the increase in \ TAA \(l) at \ TAA \(ew) = 0 and from the two different estimates of the sink for ASC, A value of 1.3 x 10(-6) m s(-1) was calculated on the basis of the oil-water distribution coefficient for TAA. It is concluded that the demand for ASC in the mesophyll cell wall of the investigated leaves could he met by simple diffusion of AA through the plasmalemma. From the measured increase in the slope of the relationship \ TAA \(ew) versus \ TAA \(l), an increase in the cell wall pH of 0.3 units was estimated under the influence of ozone.