Leaves are the energy power plants of green plants and, next to the roots, the most important nutritional organs. In the vine, they form like the eyes (summer and winter eyes), tendrils and inflorescences (clusters or later grapes) as lateral outgrowths on the nodes of young growing shoots. As the shoot tip grows in length, new leaves continuously branch off, which develop in a variety-specific manner. With the help of leaf pigments such as chlorophylls, carotenoids and flavonoids, the light energy from the sun is absorbed by the leaves and converted into energy-rich glucose (grape sugar) and oxygen during photosynthesis using carbon dioxide and water. The carbon dioxide needed for this is absorbed from the air through the stomata (stomata ) mostly on the undersides of the leaves. The oxygen produced during the day escapes to the outside through these small stomata.
Due to the water vapour saturation deficit of the air, every plant cell constantly loses water, which evaporates outwards into the air. This complex process is called transpiration (evaporation of water on unvegetated or open land or water surfaces, on the other hand, is called evaporation). The constant loss is compensated for by the supply of water by the roots from the soil, so that there is a constant flow of water through the plant. This transpiration suction allows mineral nutrients in the soil solution to be transported upwards from the roots into the plant in the first place. 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 via active energy-consuming pumping movements in the roots (root pressure) and droplet-wise water release via special pores. This osmotic pressure is called guttation (excretion of excess water).
The leaf axil is the angle between the shoot axis and the leaf branching off from it. The point where the petiole attaches to the node is called the leaf base. The leaf blade (lamina = leaf surface) is located at the petiole. Depending on the grape variety, the leaf margin can be undivided, notched or convex. Stronger convexity results in three-, five- or seven-lobed leaves. The leaf blade is traversed by five main veins (ribs), which branch off into lateral veins and veins connected in a network. Through the latter, each cell of the leaf tissue is supplied with nutrients and water and the sugar produced is transported to the grapes. The leaves, along with the grapes, are a good indicator of the health of the vine. Diseases, lack of nutrients and pest attacks show up as galls and necroses, as well as discolouration or yellowing (see also under leaf diseases).
There is one leaf at each node (nodes) of a shoot. The leaves are alternate, i.e. arranged alternately on the left and right. A summer eye and a winter eye form in each leaf axil. The summer eye already shoots during the summer and forms a lateral shoot. The winter eye, however, does not emerge until the following spring and then forms the new summer shoot. On two successive 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, some tendrils are replaced by inflorescences (clusters). The tendrils and the clusters 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 identifying grape varieties. Leaf size (hand to plate circumference), leaf shape, number of lobes, depth of the bight, opening width of the stem and leaf side bights, type and density of serration, density of pubescence and bristling are important identification characteristics within the framework of ampelography. The use of ampelometric criteria such as the opening width of the leaf angles between the main veins and other measurable characteristics was first suggested by the ampelographer Hermann Goethe (1837-1911) in 1876. Before DNA analyses, the visual determination of grapevine varieties on the basis of morphological varietal characteristics was the only, yet sufficient, and to this day the fastest and cheapest method. The annual vegetative cycle of the vine ends with the leaf fall in late autumn. Leaf production is regulated with various foliage care measures. See also a list under Vine.
Graphic: taken from Bauer/Regner/Schildberger, Weinbau,
ISBN: 978-3-70402284-4, Cadmos Verlag GmbH
Autumn leaves: by Wildfeuer - Self-photographed, CC BY 2.5, Link
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Markus J. Eser
Weinakademiker und Herausgeber „Der Weinkalender“