Effects of pH, Temperature, Humidity, and Rewetting on Cuticular Penetration of ABA

Penetration of C-14-S-abscisic acid (ABA) through isolated tomato fruit cuticular membranes (CM) was investigated using a finite dose diffusion system. In this system, penetration is monitored by non-destructive sampling from a drying donor droplet through the CM into a deionized water receiver. Penetration was rapid and highly variable during the liquid phase of the droplet and averaged about 2.5% of the amount applied during the first hour. Penetration continued to occur at a decreasing rate from the dried down droplet deposit and averaged 4.5% of the amount applied by 312 h. ABA penetration decreased as pH of the donor solution increased. The penetration variability during the liquid phase of the droplet was likely related to penetration through broken trichomes in this batch of CM because transpiration, gas flow, and penetration of acridine orange were all linearly related to trichome density. Once the donor droplet dried, trichomes did not apparently further contribute to penetration. ABA penetration increased with increasing temperature or addition of the Brij 98 or Agral 90 surfactants or CaCl2 salt. ABA penetration also markedly increased by increasing relative humidity from 50% to 100% above the deposit or rewetting of the deposit. Upon rewetting, ABA penetration during the liquid phase was initially rapid, but again decreased as droplet drying proceeded. Our data indicate that ABA penetration is substantially limited by the drying of the donor droplet and is increased by deposit hydration.