Fig. 1
A cross section of a dormant bud. The three buds within the compound bud can be seen.

Buds

A bud contains growing points that develop in the leaf axil, the area just above the point of connection between the petiole and shoot. The single bud that develops in this area is described in botanical terms as an axillary bud. It is important to understand that a bud develops in every leaf axil on grapevines, including the inconspicuous basal bracts (scale-like leaves). In viticulture terminology, we describe the two buds associated with a leaf – the lateral bud and the dormant bud (or latent bud). The lateral bud is the true axillary bud of the foliage leaf, and the dormant bud forms in the bract axil of the lateral bud. Because of their developmental association, the two buds are situated side-by-side in the main leaf axil.

Although the dormant bud (sometimes called an “eye”) looks like a simple structure, it is actually a compound bud consisting of three growing points, sometimes referred to as the primary, secondary, and tertiary buds within one bud. The distinction between secondary and tertiary buds is sometimes difficult to make when observing the bud visually and is often of little importance, so it is common to refer to both of the smaller buds as secondary buds. These three buds are packaged together within a group of external protective bud scales within the compound bud. As the bud develops, it follows the pattern of nomenclature as the buds on the shoot: the primary growing point is the axillary bud of the lateral bud; the secondary and tertiary growing points are the axillary buds of the first two bracts of the primary growing point.

The dormant bud is the focal point during dormant pruning, since it contains cluster primordia (the fruit-producing potential for the next season). It is called dormant to reflect the fact that it does not normally grow out in the same season in which it develops.

The dormant bud initiates the year prior to its growthGrapeExtOrg3.jpg as a shoot. During that prior season, it undergoes considerable development. The three growing points of the compound bud each produce a rudimentary shoot that ultimately will contain primordia (organs in their earliest stages of development) of the same basic components that comprise the current season’s fully grown shoot: leaves, tendrils, and in some cases flower clusters. The primary bud develops first; therefore it is the largest and most fully developed by the time the bud goes dormant. If it is produced under favorable environmental and growing conditions, it will contain flower cluster primordia before the end of the growing season. The flower cluster primordia thus represent the fruiting potential of the bud in the following season. Reflecting the sequence of development, the secondary and tertiary buds are progressively smaller and less developed. They generally will be less fruitful (have fewer and smaller clusters) than the primary bud. Bud fruitfulness (potential to produce fruit) is a function of the variety, environmental conditions, and vineyard production practices. Dormant buds that develop under unfavorable conditions (shade of a dense canopy, poor nutrition, etc.) produce fewer flower cluster primordia for the following season.

In most cases, only the primary bud grows, producing the primary shoot in the following season. The secondary bud can be thought of as a “backup system” for the vine; normally, it grows only when the primary bud or young shoot has been damaged, oftentimes from freeze or frost in spring. However, under some conditions such as severe pruning, destruction of part of the vine, or boron deficiency, it is possible for two or all three of the buds to produce shoots in spring (Winkler et al., 1974). Tertiary buds provide additional backup if both the primary and secondary buds are damaged, but they usually have no flower clusters and thus no fruit. If only the primary shoot grows, the secondary and tertiary buds remain alive, but dormant at the base of the shoot.

The lateral bud will grow in the current season, but growth may either cease soon after formation of the basal bract or it can continue, producing a lateral shoot (summer lateral) of variable length. Regardless of the extent of lateral bud development, a compound bud develops in the basal bract, forming the dormant bud. Long lateral shoots sometimes produce flower clusters and fruit, which is known as "second crop." However, because these develop later in the season than fruit on the primary shoot, “second crop” fruit does not fully mature in many areas of the country. If a lateral bud does not grow in the current season, it will die.

Flowers and Fruit

A fruitful shoot will usually produce one to three flower clusters (inflorescences) depending on variety. Flower clusters develop opposite the leaves typically at the third to sixth nodes from the base of the shoot, depending on the variety. If three flower clusters develop, two develop on adjacent nodes, the next node has none, and the following node has the third flower cluster. The number of flower clusters on a shoot is dependent upon the grape variety and the conditions of the previous season under which the dormant bud (that produced the primary shoot) developed. A cluster may contain several to many hundreds of individual flowers, depending on variety.




Fig. 2
Grape buds and flowers.
A compound bud with primary, secondary, and tertiary buds (T),
and flowers from formation to cap fall to pre-fertilization.

The grape flower does not have conspicuous petals, instead, the petals are fused into a green structure termed the calyptra, but commonly referred to as the cap. The cap encloses the reproductive organs and other tissues within the flower. A flower consists of a single pistil (female organ) and five stamens, each tipped with an anther (male organ). The pistil is roughly conical in shape, with the base disproportionately larger than the top, and the tip (called the stigma) slightly flared. The broad base of the pistil is the ovary, and it consists of two internal compartments, each having two ovules containing an embryo sac with a single egg. The anthers produce many yellow pollen grains, which contain the sperm. Wild grapevines, rootstocks (and a few cultivated varieties such as St. Pepin) have either pistillate (female) or staminate male flowers -- that is, the entire vine is either male or female. Vines with female, pistillate flowers need nearby vines with staminate or perfect flowers to produce fruit. The majority of commercial grapevine varieties have perfect flowers, that is, both male and female components.


Fig. 3
An individual grape flower is shown with floral parts labeled

Stages of Bloom

When the individual flowers on a grape inflorescence open, it looks different than the bloom of most flowers. The cap separates from the base of the flower, becomes dislodged and usually falls off, exposing the pistil and anthers. The anthers may release their pollen either before or after cap fall. Pollen grains randomly land upon the stigma of the pistil, allowing pollination. Multiple pollen grains can germinate, each growing a pollen tube down the pistil to the ovary and entering an ovule, where a sperm unites with an egg to form an embryo. The successful union is termed fertilization, and the subsequent growth of berries is called "fruit set." The berry develops from the tissues of the pistil, primarily the ovary. The ovule together with its enclosed embryo develops into the seed.


Fig. 4
A grape inflorescence with nearly 100% cap fall

Because there are four ovules per flower, there is a maximum potential of four seeds per berry. Unfavorable environmental conditions during bloom such as cool, rainy weather can reduce fruit set (number of berries) and seeds per berry, thereby affecting berry size. Berry size is related to the number of seeds within the berry, and very few seeds leads to smaller berries. However, berry size can also be influenced by environmental conditions, management practices, and water management. Some immature berries may be retained by a cluster without completing their normal growth and development, a phenomenon known as “ coulure” or “hens and chicks” (Mullins et al., 1992).

Reviewed by Tim Martinson, Cornell University and Patty Skinkis, Oregon State University

References:
Mullins, M. G., A. Bouquet, and L. E. Williams. 1992. Biology of the Grapevine. Cambridge University Press.