Determination of Foliar Uptake of Water Droplets on Waxy Leaves in a Controlled Environmental System

Abstract

<abstract> <b>.</b> More definitive techniques for determination of plant cuticle permeability are required to select proper doses of active ingredients and spray additives to improve pesticide application efficacy. A controlled environmental system with 100% relative humidity was developed for direct measurements of foliar uptake rates of sessile water droplets at various locations on waxy leaf surfaces during two diurnal periods between 10:30 and 13:00 (noon) and between 16:00 and 19:00 (evening) at ambient temperatures of 15°C and 20°C. Sessile droplets of 340 and 540 μm diameter were produced with a single-droplet generator. Stomatal densities and osmotic pressures on leaves were also measured with a cold field emission scanning electronic microscope (CFESEM) and a vapor pressure osmometer, respectively. At 100% relative humidity inside the environmental chamber, droplets remained unchanged after they were deposited on a non-permeable glass, but they penetrated leaf tissues after they were deposited on leaves. Foliar uptake time varied with droplet size, droplet deposition location on leaves, and diurnal period, but varied little with <ext-link ext-link-type=uri xlink:href=app:ds:osmotic%20pressure>osmotic pressure</ext-link> and stomatal density. An average uptake rate of 540 μm diameter droplets on a leaf surface at 20°C and 100% relative humidity was 0.401 × 10^-3 ±0.063 × 10^-3 μL s^-1, which was 2.1 times the average uptake rate of 340 μm diameter droplets. The incorporation of the controlled environmental system with 100% relative humidity and the sessile droplet generator to measure foliar uptake rates of water droplets would provide a unique approach to elucidate the foliar uptake mechanism of spray droplets for efficient and effective control of target pests on specific plants.