Grape leaves are thin and flat. As is common among leaves in general, they are composed of different sets of specialized cells. Today, on average, sunlight reaching their surface is about 4% ultraviolet (UV) (<400 nm), 52% infrared (IR) (>750 nm) and 44% visible (VIS) radiation. Little of the UV and IR are used by plants. As with other leaves that are green, only the red and blue ends of the visible part of the electromagnetic spectrum are absorbed, thus leaving green available by reflection and transmission. On the surface of the leaf (Figure 8.1), the cells of the outermost layer (the epidermis) are designed to protect the inner cells where the workings needed for gathering the sunlight used for photosynthesis and other chemistry necessary to the life of the plant are found. That is, the more delicate cells, beneath the epidermis, are involved in production of carbohydrates as well as the movement of nutrients in and products out of the leaf. The epidermis, exposed to the atmosphere, has cells that are usually thicker and are covered by a waxy layer made up of long-chain carboxylic acids that have hydroxyl groups (–OH) at or near their termini. These so-called omega hydroxy acids can then form esters using the hydroxyl group of one and the carboxylic acid of the next. This yields long-chain polyester polymers called “cutin.” As indicated in the earlier discussion of cells and, in particular, regarding the fatty acids of cell walls, the fatty acids found in the epidermis generally consist of an even number of carbon atoms, and for cutin, the sixteen carbon (palmitic acid) family (Figure 8.2) and the eighteen carbon family (oleic acid bearing a double bond or the saturated analogue stearic acid) are common. While one terminal hydroxyl group is usual (e.g., 16-hydroxypalmitic acid, 18-hydroxyoleic acid, or its saturated analogue 18-hydroxystearic acid) more than one (allowing for cross-linking) is not uncommon (e.g., 10,16-dihydroxypalmitic and 9,10,18-trihydroxystearic acid).
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