The persistence of humic acid droplets on leaf surface

The aim of the present laboratory assay was to study the influence of a mixture of humic acids on physical characteristics of aqueous drops and their persistence on the leaf. The bioassay was set in a controlled growth room using Vigna sinensis L. seedlings with two primary leaves. The treatments were: deionized water; surfactant (Etravon 0.05%); mineral solution (12.6 mg/L of N as ammonium nitrate and 0.2 g/Lof micronutrients); and humic acids ("Zymo" at 0.18 g/L on dry weight basis with a total N content of 12.6 mg/L). The 20-p,l drops were applied on the upper leaf surface using a manual micro pipette and drop persistence was estimated using a color digital camera. The assay was repeated on detached and in vivo leaves, and on leaves with increasing age. Another trial was carried out using leaves which had been previously acclimated by spraying the same solutions before the experiment. Thermo graphical analysis of the drops on the leaf surface was also performed. The drop life was shorter in detached leaves than in vivo leaves without any difference due to acclimation, which was likely too short to induce a positive effect. The addition of surfactant reduced drop life. All other treatments did not induce significant differences compared to control. As a consequence the drop life induced by humic acids was slightly longer than that due to surfactant, even though the drops were similar in shape, and thus more flat and extended than those with minerals. It can be hypothesized that humic acids reduce the speed of droplet drying while their wetting action may enhance nutrient absorption. This combination of factors may account for the effectiveness of these materials.