Leaves are the energy power stations of green plants and, besides the roots, are the most important nutritional organs. Like the eyes (summer and winter eyes), tendrils and inflorescences (shingles or later grapes), they form as lateral outgrowths on the nodes of young growing shoots. As the shoot tip grows in length, new leaves branch off continuously, which develop according to a species- and variety-specific blueprint. With the help of leaf pigments such as chlorophylls, carotenoids and flavonoids, the light energy of the sun is absorbed by the leaves and converted into high-energy glucose (dextrose) and oxygen during photosynthesis using carbon dioxide and water. The carbon dioxide required for this is absorbed from the air through the stomata (stomata), usually on the undersides of the leaves. The oxygen produced during the day escapes through these small stomata
Due to the water vapour saturation deficit in the air, each plant cell constantly loses water, which evaporates outwards into the air. This complex process is called evapotranspiration. The constant loss is compensated by the supply of water from the soil by the roots, so that a constant flow of water through the plant takes place. This transpiration suction allows mineral nutrients in the soil solution to be transported from the roots upwards into the plant at all. At 100% relative humidity and water vapour saturation of the air, water is no longer physically evaporated. In such cases, a slow water flow can be maintained by active energy-consuming pumping movements in the roots (root pressure) and a droplet-wise release of water through special pores. This osmotic pressure is called guttation (elimination of excess water).
The leaf axis is the angle between the shoot axis and the leaf branching off from it. The point of attachment of the petiole at the node is called the leaf base. The leaf blade (lamina = leaf surface) is located on the petiole. Depending on the grape variety, the leaf edge can be undivided, notched or indented. More pronounced curvature results in leaves with three, five or seven lobes. The leaf blade is traversed by five main veins (ribs), which branch out into side veins and net-like connected veins. The latter supply each cell of the leaf tissue with nutrients and water and transport the sugar produced to the grapes. Along with the grapes, the leaves are an excellent indicator of the health of the vine. Diseases, nutrient deficiencies and pest attacks show up as bile and necroses, as well as through discoloration or yellowing (see also under leaf diseases).
There is one leaf at each node (node) of a shoot. The leaves are alternately arranged on the left and right. In each leaf axil a summer and a winter eye is formed. The summer eye already sprouts during the summer and forms a stinging shoot. The winter eye, however, does not sprout until the following spring and then forms the new summer shoot. On each of two consecutive leaves there is a tendril on the side opposite the leaf (and the two eyes), whereby the third leaf is usually free of tendrils. There are no tendrils on the lowest nodes of the shoot. In the upper part of the shoot, some tendrils are replaced by inflorescences (glumes). The vines and the inflorescences are very similar in structure, so that there can also be mixed forms of the two.
The variety-specific morphology of the leaf is an important criterion for the identification of grape varieties. Leaf size (hand to plate circumference), leaf shape, number of lobes, depth of the bay, opening width of the stem and leaf side bays, type and density of the serration, density of hairs and bristles are important determining characteristics in ampelography. The use of ampelometric criteria such as the opening width of the leaf angles between the main nerves and other measurable features was first suggested by the ampelographer Hermann Goethe (1837-1911) in 1876. Prior to DNA analyses, the visual determination of grape varieties on the basis of morphological variety characteristics was the only, but sufficient and still the fastest and cheapest method. The annual vegetation cycle of the vine ends with the leaf fall in late autumn. Leaf production is regulated with various leaf care measures. See also a list under vine.
Graphic: taken from Bauer/Regner/Schildberger, Viticulture, ISBN: 978-3-70402284-4, Cadmos Verlag GmbH