Demonstrating the Role of Plant Epicuticular Waxes in Water Retention on Leaf Surfaces Using Brassica Leafy Vegetables.

As the outermost barrier on plant surfaces, the epicuticular waxes create a hydrophobic surface that plays crucial roles in many important plant interactions with the environment, such as providing plants with tolerance to drought and resistance to pathogens and insects. Epicuticular waxes also affect surface retention of agricultural sprays like herbicides, pesticides, and plant growth regulators, and play an important role in postharvest quality of fruits and vegetables. We report here the development of an inexpensive and engaging laboratory-based activity that can help students learn about the role of these important plant waxes. After spraying water on leaf surfaces, this exercise demonstrates that much more water is retained on the leaf after epicuticular waxes are removed mechanically, than on a leaf having normal waxes. To provide more in-depth understanding, this exercise demonstrates the impact of these waxes on the leaf surface contact angles and shape of the water droplets themselves. Leaf surfaces of collard greens (Brassica oleraceae L. Acephala group) are sprayed with water and then a digital camera is used to record contact angles and droplet shape, which are then quantified using ImageJ software, both before and after mechanical wax removal. The results demonstrate that droplets are broader, spreading to cover more surface area, and their contact angles are significantly reduced after epicuticular wax removal. As a supplement to this classroom exercise, images of surface wax crystals produced using a scanning electron microscope, and wax chemical composition determined using a gas-chromatograph/mass-spectrometer, for surfaces both with and without wax removed is presented to enhance student learning about how these epicuticular waxes shed leaf surface water, and to provide a foundation for instruction about the importance of the plant’s outermost hydrophobic barrier in protecting plants from environmental stress,  and its affect on the efficacy of common sprays applied to control plant pests, kill weeds, and regulate plant growth.