Time for bud break? and How a Bud Becomes a Bunch

What is going on with this bud? Notice anything significant? This photograph was taken today by NMSU Viticulture graduate student Ms. Jacqueline Cormier.  

Of Buds and Bunches

Adapted from: “What causes a bud to produce a bunch… or not?”  by Devin Carroll, Advanced Agricultural Services, Inc. Hanford, CA., Practical Winery and Vineyard, Nov-Dec. 2009.

This short piece, adapted from the more detailed article cited above covers some vine processes and factors that affect bud formation and fruitfulness. The original article also covers the phenomenon of plant mediated bunch necrosis or PMBN. We will not cover that here.

Fruitfulness depends on the percentage of buds on a grapevine that produce flower clusters. Flower initiation, formation, bloom and fruit set, fruit development and ripening require a two-year cycle. The developing buds contain tissues that form primordial flower clusters and require a favorable environment to flower and fruit. There are several steps to achieve a desired level of fruitfulness: flower initiation and minimal bud necrosis. Bud necrosis is caused when the vine “decides” that to carry the bud to fruitfulness is not worth the metabolic costs. It takes more energy than the vine can afford and the bud is aborted. In this case, secondary buds within the compound bud will grow and allow the vine to survive but not bear fruit, or bear much less fruit. Injury to the vine can also cause bud necrosis…pathogens, heat, water stress or frost can damage or kill a bud. Bud mites, can infest buds and move from old to new buds during the early weeks after bud break and multiply from there. Hundreds of these mites can inhabit one bud and damage the inflorescence (flower cluster).

Inflorescence initiation

Grape flowers and fruit clusters are borne on new shoots that come from dormant buds.  Each dormant bud is formed at each node and leaf axil on a shoot the previous season and is actually a compound bud that contains three separate buds. The primary bud within the compound bud, for most Vitis vinifera varieties, under most growing conditions, will produce 1 to 3 clusters with 2 clusters being typical. However, this shoot may remain vegetative on young vines and the number of inflorescences is also variety dependent. For instance, Chambourcin (a hybrid) and Petit Verdot (Vitis vinifera) often have more than two clusters per shoot. The secondary bud can have a fruit cluster but not always, and the third bud typically produces vegetative growth (shoots and leaves) only. Regardless of flower clusters, each simple bud contains a 6-12 node shoot that is compressed and complete with primordial leaves, flower clusters (if present) and tendrils. This complete shoot is within each bud. Compound buds develop from the base of the shoot towards to the shoot tip. This process takes from two to three months beginning as soon as the node appears in the spring. After formation, the bud stops growing and becomes dormant until it breaks and grows the following spring. Grapevines do not form terminal buds as many temperate zone fruit crops do. The shoot tip will die back in the winter to a fully ripened (lignified) portion of the shoot that has a mature dormant bud. Cluster initiation for next year begins in midsummer and are fully formed within the dormant bud by the end of the growing season. Final development of the flowers occurs as the shoot emerges the following spring. The flowers are fully formed by bloom time when the shoots are about 18-24 inches long. Grapes generally flower late… about 4 to 5 weeks after bud break, later than most tree fruit crops. Grapes are wind pollinated for the most part and warm, sunny conditions at bloom favor fruit set. Note that the most fruitful buds are located at bud number 4 to 10 on the cane. Thus, when spur pruning (leaving only the number 1 to 4 buds) some degree of fruitfulness or total possible crop is sacrificed. If spur pruning, the buds left on spurs are formed before bloom. These represent next year’s entire crop. If canes are used, some crop will come from buds closer to the shoot tip, and they are exposed to environmental conditions that existed later than the buds near the base of the shoot.

It is possible for two buds to form at the same node. This phenomenon is common in Pinot Noir, and has been observed in Cabernet Sauvignon, Syrah, Merlot and Chardonnay. Because these are fairly widely planted varieties, this double bud phenomenon may be due to more reporting, rather than actually something more common to these varieties in particular.

The rachis (specialized stem bearing flowers and fruit) forms branches before the bud goes dormant, but the flowers themselves are formed after winter, at bud break. Therefore, environmental conditions just after bud break influence the final number of flowers. But cluster size contingent on rachis branching is determined by conditions in the prior year.

A non-specific tissue called an “anlage” gives rise to either an inflorescence (flower cluster) or a tendril. The inflorescence can be seen within a bud ten nodes from the shoot tip when the bud is dissected and viewed with a microscope. The vine’s decision to form flower clusters comes much earlier and is correlated with temperature especially in the buds about three nodes from the shoot tip. Therefore, both vine physiology and temperatures the shoot is exposed to in the first weeks following bud break play a role in vine fruitfulness.

Several factors impact flower cluster formation: temperature (hot and cold), light and shading, carbohydrate reserves, water, nitrogen, mineral deficiencies, and plant growth hormones.

Temperatures held at 68^oF immediately following bud break on Muscat grapes greatly reduced the number of flower clusters. Similarly, at temperatures > 95^o F, flower cluster numbers were greatly reduced. However, cold temperatures are likely the most common cause for low fruitfulness in a given variety the following year because temperatures in that range are more common in the spring.

Solar radiation (light) and favorable temperature are two critical climatic factors. Shade can reduce fruitfulness, but shading due to canopy is not the usual case early in the season when leaves are small or not yet emerged. Thus, cloudy, cool days as the shoot emerges and grows in spring can reduce bud fruitfulness. The amount of light falling on a shoot’s leaves is correlated to the number of flower clusters eventually formed in the buds of that shoot. Accumulated light rather than just peak intensity is the important aspect. A short day-length or several hours or days of cloudiness can reduce bud fruitfulness.        

Stored carbohydrates influence the formation of flower clusters in young buds. Carbohydrates in roots, trunk cordons, canes and spurs were stored the year before the buds were differentiated. Thus, growing conditions and vine strength two years ago strongly influence the current year’s fruitfulness. Overall, buds are a weak sink for carbohydrates compared to actively growing shoot tips. Rapid shoot growth will draw carbohydrates from the buds and shading on such vigorously growing vines also reduces fruitfulness.

Excess water will reduce the number of flower clusters on a vine…while water deficit does not. Overwatering and excess growth that promotes canopy shading likely draws carbohydrates away from buds and lowers their fruitfulness.  

Mineral deficiencies that include potassium and phosphorus can reduce bud fruitfulness. Low phosphorous detected around the time of bud break can be linked to cold soil temperatures that prevents uptake by roots despite the mineral being present in the soil. In the case of most New Mexico soils with elevated pH (above 7), phosphorous can be in short supply due to reduced availability. Potassium and phosphorus can be added to the vine via foliar sprays. This should be done shortly after bud break to increase fruitfulness in the first several buds.

Plant growth regulator compounds or hormones, gibberellic acid (GA) and cytokinins interact to regulate inflorescences in grapevines. Early on, GA can favor analage formation that give rise to inflorescences, but that same GA decreases inflorescences by favoring tendril formation. Cytokinins, on the other hand can favor inflorescences over tendrils. Although some kelp-based sprays have been reported to contain cytokinins…it is not clear if such cytokinins are the correct types for grapes.