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

The effects of NAA [2-(l-naphthyl)acetic acid] concentration and pH on pene tration of NAA from aqueous droplets (5 mul) through isolated tomato (Lycop ersicon esculentum) fruit cuticles were studied using a finite dose diffusi on system. Penetration time-courses were characterized by a lag phase, whic h generally extended beyond the time of droplet drying. Initially penetrati on rates increased, reached a maximum penetration rate, remained constant f or several hours, and then decreased with time. Penetration approached a pl ateau within 120h after droplet application. Increasing NAA concentration i n the donor droplet increased NAA penetration in both the presence and abse nce of the citric acid buffer (20 mM, pH 3.2). Maximum rates of penetration and the total amount of NAA that penetrated at 120h were both linearly rel ated to the initial concentration of the donor droplet (ranging from 0.001 to 0.1mM NAA). The apparent first-order rate constants for the concentratio n dependence of NAA penetration rates were greater with buffer than without (0.94 x 10(-8) vs 0.50 x 10(-8)ms(-1), respectively). While pH of the rece iver 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. (C) 2000 Society of C hemical Industry.