586 Studies on the Mode of Action of NH4NO3 as a Spray Additive on Penetration of 14C-NAA through Isolated Tomato Fruit Cuticle

Abstract

NAA, a weak organic acid plant growth regulator (pKa 4.2), penetrates the plant cuticle preferentially as an undissociated molecule (<10% dissociated at pH 3.2). We have reported, using a finite dose diffusion system, that NH4NO3 (AN, 8 mM) at pH 5.2 (>90% dissociated) enhanced the penetration of 14C-NAA through isolated tomato fruit cuticular membranes (CM). AN appears to preferentially enhance penetration of the dissociated NAA molecule over the nondissociated form. A possible mode of action is that AN affects the cuticle matrix, allowing for greater NAA penetration. Acid treatment (4 N HCl) of the cuticle, which alters the cuticle's ionic characteristics, resulted in a 10% reduction in NAA penetration from droplets in the presence of AN. When AN (80 mM) was included in the receiver solution of the diffusion cell in an effort to infuse the cuticle matrix, NAA penetration was not increased compared to when AN was present in the applied droplet. AN (8 mM) also increased NAA penetration through dewaxed tomato cuticular membranes (DCM; +252% for DCM vs. +190% for CM in 120 h). Since AN only enhances NAA penetration when included with the NAA in the treatment droplet, the AN effect may be related to the role of the droplet/deposit (droplet residue) as a donor. This conclusion is further supported with sorption studies, where AN over a 100-fold concentration range (0.8–80 mM) did not increase NAA sorption by tomato fruit CM and where no deposit is present. The role of the physicochemical nature of the deposit, including the chemical/ionic characteristics of any additive (i.e., AN) and active ingredient will be discussed.