Finite dose diffusion studies: II. Effect of concentration and pH on NAA penetration through isolated tomato fruit cuticles

Abstract

The effects of NAA [2-(1-naphthyl)acetic acid] concentration and pH on penetration of NAA from aqueous droplets (5 μl) through isolated tomato (Lycopersicon esculentum) fruit cuticles were studied using a finite dose diffusion system. Penetration time-courses were characterized by a lag phase, which generally extended beyond the time of droplet drying. Initially penetration rates increased, reached a maximum penetration rate, remained constant for several hours, and then decreased with time. Penetration approached a plateau within 120h after droplet application. Increasing NAA concentration in the donor droplet increased NAA penetration in both the presence and absence of the citric acid buffer (20mM, pH 3.2). Maximum rates of penetration and the total amount of NAA that penetrated at 120 h were both linearly related to the initial concentration of the donor droplet (ranging from 0.001 to 0.1 mM NAA). The apparent first-order rate constants for the concentration dependence of NAA penetration rates were greater with buffer than without (0.94 x 10 -8 vs 0.50 × 10 -8 ms -1 , respectively). While pH of the receiver solution (pH 6.2 vs 2.2) did not affect penetration, decreasing donor pH from 6.2 to 2.2 increased NAA penetration at 120h.