This is the second part of the series on grapevine anatomy that began in the article here.
Shoots and Canes
This structure, called a shoot or a cane depending on its stage of maturity, generally develops on one of the vine’s arms, or at the top of the trunk in head-trained vines. It grows each year from a bud on the previous year’s cane. It carries all of the plant’s vegetative growth and fruiting structures for that year: leaves, inflorescences, tendrils and buds.
To clarify the naming: it is called a shoot while it is growing and its stem is green. Once “lignification” begins, as the tissues turn woody, it is conventionally called a cane.
At the base of the cane or shoot, the end closest to the trunk (in head-trained vines) or to the arms, are usually the inflorescences, the plant’s reproductive structures, which develop into clusters once pollinated. At the apical end of the cane, that is, at its tip, is the shoot’s growing tip, which is responsible for its growth in length over the season. The rest of the cane generally bears leaves, which carry out photosynthesis; buds, which will produce the following season’s growth; and some modified inflorescences called tendrils. In midsummer the shoot begins to mature as its tissues start to lignify. In late summer or early autumn the cane begins to drop its leaves, leaving only the buds, which go dormant over the winter and produce a new shoot the following spring.
Leaves and tendrils
Grapevine leaves can be divided into three parts:
- The petiole, which attaches the leaf to the cane
- The bracts, scale-like structures that grow at the base of the petiole
- The blade, or lamina
Five main veins radiate from the point where the petiole joins the blade, giving the vine leaf its characteristic five-lobed shape (the lobes may be more or less pronounced depending on the variety). The leaf is one of the most important parts of the cane, since it converts solar energy through photosynthesis. It also plays a key role in the plant’s respiration and in regulating its temperature. Tendrils originated as modified inflorescences. They wrap themselves around any object within reach, so their job is to hold the above-ground part of the plant in place.
Buds
Buds are the plant’s above-ground growth structures. In late winter or early spring, each bud gives rise to a new shoot with its own leaves, tendrils and inflorescences.
Buds normally contain three partially developed shoots: one primary and two secondary. These partially developed shoots may carry only partly formed leaves, in which case they are called vegetative shoots, or they may also carry partly formed inflorescences, in which case they are called fruitful shoots.
Under normal growing conditions only the primary shoot emerges, but when the plant is nutritionally out of balance, especially with excess vigor or severe pruning, or when part of the plant is destroyed, all three shoots may emerge. Buds are usually found at each node of the current year’s cane, but they sometimes also occur on the arms and trunk, where they grow into suckers.
When a bud on the current year’s cane breaks during the same season, it is known as a prompt bud (because it broke ahead of time) and produces what is known as a lateral shoot, a premature shoot that should have grown the following season.
Inflorescences and Clusters
The grapevine does not have a single flower but an inflorescence, which is essentially a series of individual flowers grouped in a bunch or cluster. This inflorescence is already partly formed inside the bud before budbreak. In early to mid-spring the inflorescence opens, each of its individual flowers is pollinated, and, if the weather is favorable, the cluster begins to develop. Certain conditions, such as insufficient winter chill, disease, mineral deficiencies, drought or excessive heat, can interfere with pollination and result in incomplete clusters.
The cultivated grapevine is hermaphroditic (both sexes in the same individual), which is why pollination is usually self-pollination. Once the flower has been pollinated, the berries begin to develop with the help of growth hormones released by the newly formed embryo inside the seed.
In the first stage of cluster growth, the cells that make up each berry multiply rapidly. In the second stage, growth comes from cell enlargement, that is, the cells formed in the first stage absorb water and increase in size.
Once ripe, the cluster formed from the inflorescence has two types of tissue. The first is the berries, or fruit, which have only primary cell walls. The second is the stem, or rachis, the plant skeleton that holds each of the berries; at advanced stages of maturity it may develop secondary cell walls and produce lignin.