A study of the spatiotemporal wettability of the adaxial surface of apple leaves

Abstract：【Objective】Change in the wettability of the adaxial or upper surface of apple leaves was studied at different growth periods during fruit development. Based on the result, the choice of pesticide formulations was recommended for apple in different growth periods. The study also provided a new idea for studying surface wettability of plant leaves.【Methods】The contact angle of water drop at different areas of the adaxial surface of apple leaves was analyzed at stages of flower withering, young fruit, fruit expansion and late fruit expansion period using horizontal and vertical grid average method.Then a contour map of contact angle distribution was constructed for apple leaf with interpolation method. The contact angles between the commonly used pesticides (beta-cypermethrin EC, tebuconazole SC and lambda-cyhalothrin EW) droplets and apple leaf were also measured at the recommended concentra-tions.【Results】The contact angles in different growth periods showed a defined pattern. At the stage of flower withering, the average contact angle of the whole leaf was (81.25±3.77) °. The contact angle at leaf margin was smaller than that in the middle area. The contact angle on the left side of the leaves was smaller than that on the right side. The largest contact angle was located in right middle area. At the stage of young fruit, the contact angle of whole leaf was (86.73±3.42) °. It increased to (89.54±3.50) ° at the stage of fruit expansion. The contact angle distribution in apple leaf during young fruit period and fruit expansion was similar to that at flower withering stage. During the late period of fruit expansion, the contact angle of whole leaf increased to (94.63±3.74) °. The contact angle was similar on both sides of the leaf margin, being smaller at the leaf apex than at the leaf base. Based on the hydrophobicity of the cuticle layer in apple leaf and the organic solvent or additives in the pesticide, the contact angle between pesticide formulations and apple leaf being lower than that between water and apple leaf could be judged preliminarily. Therefore, the requirement of wettability could be satisfied by choosing waterbased formulations for apple trees at flower withering and young fruit stages. However, at fruit expansion and late fruit expansion period, the wettability might be affected by water-based formulations and EC was a reliable choice to ensure the wettability. In order to justify the judgment, the contact angles between apple leaf and beta-cypermethrin EC, tebuconazole SC and lambda-cyhalothrin EW were measured in the late fruit expansion period. The average contact angles of beta-cypermethrin EC, tebuconazole SC, lambda-cyhalothrin EW in apple leaves were (67.50 ± 3.86) °, (85.43 ± 2.51) ° and (83.17 ±3.49) °, respectively, which are smaller than the contact angle of water and apple leaf.【Conclusion】The wettability of the adaxial surface of apple leaves became poorer with fruit development. The contact angle distribution of whole leaf was non axisymmetric. The contact angle at leaf margin was smaller than that in the middle part. With the apple trees growing stage, horizontally, the contact angle of leaf margin in the left was smaller than that in the right during blossom wither, young fruit stage and stage of fruit enlargement, but the contact angle of leaf margin in the left was similar with that in the right during late stage of fruit enlargement. Longitudinally, the contact angle of leaf apex was similar with that of leaf base during blossom wither, young fruit stage and stage of fruit enlargement, but the contact angle of leaf apex was smaller than that of leaf base during late stage of fruit enlargement. The contact angles in most areas of apple leaves were lower than 90° at flower withering and young fruit period, indicating the apple leaf surface was highly wettable. Therefore water-based formulations contained less organic solvent and additives could be chosen. But the contact angles in most areas of apple leaves were greater than 90° during fruit expansion and at the late fruit expansion period, indicating the apple leaf was poorly wettable. Therefore, the EC formulations containing high organic solvent and additive recommended for apple tree in these stages. It should be noted that the interpolation method could be used as a supplement to the horizontal and vertical grid average method, which was a feasible method to study the spatiotemporal distribution of the wettability in apple leaves. The distribution of contact angle of water on apple leaf surface could be used as a reference for the choice of pesticide formulations for apple tree. It is of significance to reduce the use of the organic solvent and the environmental impact of pesticides.