X-Ray Microanalysis of Apples Treated with Kaolin Indicates Wax- Embedded Particulate in the Cuticle

Gala' apples trees were treated with kaolin (Surround) in mid-July, and fruit were sampled 1 and 7 days after treatment. Apples were gently rinsed with cold deionized water to remove loose surface material. Small sections of cuticle tissue were shaved from the fruit surface with a razor blade and flash frozen on an aluminum block held at -190oC with liquid N2. The tissue was freeze-dried, coated with a thin film of carbon, and examined using a field emission scanning electron microscope coupled with an energy dispersive x-ray microanalysis system capable of light element detection. Qualitative secondary electron imaging combined with quantitative elemental compositional analyses indicated kaolin particles may become embedded in the cuticle between individual wax platelets. INTRODUCTION Sun-induced injury (solar injury, SI) can be a costly problem in some years for tree fruit growers, especially in hot, arid regions. Although the injury to the fruit appears to be a combination of heat and solar radiation exposure (Lipton and O’Grady, 1980), synergistic thresholds for temperature, and radiation wavelength and intensity have not been clearly defined. To reduce this type of injury, therefore, either temperature or radiation, or both, must be reduced or regulated. Lipton and Matoba (1971) reduced the surface temperature of ‘Crenshaw’ melons (Cucumis melo, L.) by as much as 8 °C by applying a mineral whitewash material to the fruit surface, and thus reduced SI damage. Lipton (1972) also demonstrated that mineral whitewash applied to the melon fruit surface reduced internal tissue temperature and, as a result, improved fruit quality. (Mineral whitewash contains kaolinite, bentonite, attapulgite, as well as spreading and sticking agents). More recently, Glenn et al. (2002) reported kaolin alone reduced solar injury to apple fruit. Kaolin, also known as kaolinite, is a clay mineral with the chemical composition Al 4(OH)5O2Si2. The basic building blocks are layers of silica tetrahedra and layers of alumina octahedra in a 1:1 relationship. The unit cell repeats in the x and y direction to form extensive sheets in the xy plane, and in the z direction to form stacks of sheets (Barak and Nater, 2002). When placed in water the platelets shift apart, each plate attracting water to its negative face. Kaolin particulate may absorb 7 to 10 times its own weight, consequently swelling up to 18 times its dry volume. Because of these characteristics, we wanted to know first, whether small particles would lodge in the micro-cracks of the fruit cuticle surface, second, whether the particles would become an integral part of the cuticle as the micro-cracks continued to ‘heal’ with new growth of wax platelet, and third, because of the marketability of appearance, whether the integration of clay particles within the cuticle matrix would alter the properties of the cuticle itself. MATERIALS AND METHODS Mature 'Gala'/EMLA MM.106 apples trees in northcentral Washington were treated with a single application of kaolin (Surround) in mid-July. Fruit were sampled 1 and 7 days after treatment, placed in a cooler held at 34oC and transported to the Proc. XXVI IHC – Deciduous Fruit and Nut Trees