Definition - What does Leaf mean?
A leaf is an important part of a vine’s canopy, playing a vital role in the photosynthesis of the grape vine - the process by which the vine creates carbohydrates that it needs to grow and produce grape clusters via the sun's rays. Viticulturists use the physical appearance of the leaf and the ratio of leaf to fruit as guidelines to determine if the vine will be able to fully ripen the grape berries. The balance of leaf coverage needed for photosynthesis to number of grape clusters can help winemakers predict the quality of the grapes at harvest time. In addition, the appearance of the leaf - color, size, health - can provide advance notice of any diseases or infestation.
WineFrog explains Leaf
A grape vine’s leaves are produced at the apical meristem, also known as the top of the shoot. Each shoot first grows two or more bracts (small leaves) at the base. Then, it produces the first foliage leaf. These leaves grow from a larger area of the shoot called the node. Distances between the nodes indicate the rate of shoot growth, like a visual indication of the growth rates during the season.
A vine’s leaf has many parts:
- Blade: broad, flat part designed to absorb sunlight and CO2 and convert it into food.
- Lobes: the blade has five lobes - basal lobes, two closest to the stem; lateral lobes, middle ones on either side; apical lobe, singular lobe at the top.
- Petiole: stem that attached blade to the shoot.
- Lateral sinus: notch between the basal and lateral lobes
- Petiolar sinus: notch between the basal lobes and petiole.
- Stomata: thousands of microscopic pores (tiny holes) that absorb and diffuse CO2, O2 and water vapors. Stomata open up light and close in the dark.
Every varietal has a different leaf size and shape. Merlot, for example, has very large leaves, and Gewürztraminer has small leaves. In additional to the blade size, the shape of the teeth on the outer edge, the arrangement of the five lobes, and the angle and length of the veins all manifest differently depending on the grape.
The color of the leaf indicates the health and nutrition of the vine. Chlorophyll makes it green; when the vine stops photosynthesizing (in the winter), which breaks down the chlorophyll and changes the leaves’ color. But other reasons for a leaf color change is a deficiency in nitrogen or sulfur, making it turn prematurely yellow. Appearance of reddish spots or brown dead zones could be a sign of viral infection or contamination.