TUESDAY, MAY 28, 2024. BY STAN GRANT, VITICULTURIST.

Grapevine Shoot Growth Balance: The Superstructure of Fruit Yield

If grapevine growth capacity is the voltage and growth vigor the current, the growth of shoots and roots is the work of the appliance. Fruit yield depends on the quantity of both, but of particular importance are the portions of seasonal grapevine growth directed into leaves and berries.

Shoot growth balance is the viticultural term commonly used to describe the ratio of leaves to berries in fully developed canopies. Shoot growth is said to be in balance when a sufficient, but not an excessive number of healthy, sunlit leaves are present for unimpaired ripening of fruit and canes. Given numerous variables can influence leaf exposure and function, balanced shoot growth extends over a range of leaf-to-fruit ratios for any given grapevine.

Over the course of a growing season, shoot growth balance develops in two overlapping stages, which are mainly vegetative (leaves) prior to fruit set and mainly reproductive (berries) after (Figure 9). As such, shoot growth balance is dynamic during the growing season and with regard to the yield of ripe fruit, is most consequential during the ripening period when, as mentioned above, canopies are fully developed.

Figure 9. Seasonal shoot and berry growth rates and shoot growth balance milestones. (Source: Progressive Viticulture, LLC©).

Leaf-to-fruit ratios outside of the balanced range are detrimental to fruit yield. Such shoot growth imbalances can involve too few leaves relative to berries (overcropping) or too many leaves relative to berries (undercropping) for favorable ripening (Figure 10).

Figure 10. Overcropped vines (A) and under-cropped vines (B). (Photo Source: Progressive Viticulture, LLC©).

Importantly, shoot growth imbalances not only limit the yield of ripe fruit in the current season but can have carry-over effects impacting future yield. The degree of a shoot growth imbalance determines the magnitude of both its near-term and long-term effects. Of greatest concern are imbalances leading to declining vine health and production capacity. Clearly, such shoot growth imbalances are viticulturally inefficient.

The Ravez index or yield to pruning weight ratio is a commonly referenced, but imprecise measure of shoot growth balance obtained after the growing season. During the growing season, a count of 8 to 12 leaves per shoot with two clusters at bloom is a positive indicator for a final count of 14 to 20 leaves after shoots stop elongating, which is the range associated with shoot growth balance. The most meaningful, but more difficult to obtain measure of shoot growth balance is the crop load or leaf area to fruit ratio.

Berry weight (and yield) increases as the crop load increases (Figure 11). The rate of increase, however, is not constant. Rather berry weight initially increases rapidly at low crop loads before increasing slowly at higher crop loads. In the berry weight versus crop load curve, the area near the inflection point where the rate of berry weight changes from rapid to slow indicates shoot growth balance. At the extremes near the ends of the curve, vines either overcropped or undercropped.

Figure 11. Tokay berry weights increase when the leaf area per unit berry increases.

In addition to larger berries, increasing crop loads increase soluble solids (Figure 12). These data substantiate what is well-known in the winegrape industry, small crops (large crop loads) mature faster than big crops (small crop loads). Naturally, crop loads well below the balance point run the risk of late harvest and late fruit delivery to the winery.

Figure 12. Tokay fruit soluble solids increase when the leaf area per unit berry mass increases.

Besides increasing berry size and soluble solids, larger crop loads normally increase average leaf weight, stem (cane) weight, and root weight. Further, they favor ample carbohydrate storage in woody tissues for use early in the next growing season. For these reasons, larger crop loads usually contribute to increases in grapevine growth capacity.

Vineyard Management for Balanced Crop Loads and Consistent Fruit Yields

Balanced shoot growth and crop loads are observable or at least, implied in yield responses to vineyard management inputs. Dormant season pruning is an important example. Below the region near the inflection point in the response, fruit yields increase rapidly with increasing bud number, and above it, they increase slowly (Figure 13). These responses suggest an underutilized capacity for fruit production (high crop loads) below the balance zone and an increasingly taxed capacity (low crop loads) above it.

Figure 13. The inflection points in yield response curves for varying buds per vine indicate shoot growth balance points, which vary according to variety, rootstock, and vineyard design.

(Please note, that the data trends in yield response curves are opposite of those in the crop load curves (Figures 11 and 12). Low yields are associated with high crop loads (large leaf area per unit fruit) and high yields are associated with low crop loads (small leaf area per unit fruit). Thus, yield response curves emphasize changes in fruit while crop load curves emphasize changes in leaves supporting fruit.)

The two curves in Figure 13 also show fruit yield responses to pruning depend upon the prevailing vine capacity within a vineyard, with zones of shoot growth balance being higher for horizontally divided canopy (quadrilateral cordon) vines than single canopy (bilateral cordon) vines. These data are consistent with published balanced crop loads for bilateral and quadrilateral cordon-trained vines (4 to 6 square feet of leaf area per pound and 2.5 to 4 square feet per pound of fruit, respectively).

Like bud numbers, increasing water applications increases fruit yields. When irrigations are applied throughout the growing season, the fruit yield inflection zone occurs at about 80% of the full water requirement, at least for the two varieties and vineyards presented in Figure 14. Below that quantity of applied water yields increase rapidly and above it, they increase slowly and eventually level off.

Figure14. Yield responses to the amount of irrigation water applied through the growing season indicate shoot growth balance.

Unlike increasing bud number and water applications, which are yield enlarging, cluster thinning is yield diminishing. Cluster thinning of a very fruitful variety before veraison causes a slightly disproportionate initial yield drop above an inflection point and below that point yield decline moderates (Figure 15). This moderation suggests a rebalancing of shoot growth involving a capacity redirection towards berries; berry size being the most changeable post-bloom yield component. These events coincide with the slowing or arresting of shoot elongation as canopies become fully developed or nearly so, such that the draw of vegetative growth on vine capacity is minimal.

Figure 15. Burger grapevines appear to redirect growth into berries when the number of clusters removed prior to veraison exceeds the balance point value.

In summary, yield responses to applied inputs, including labor and material resources, imply varying ratios of leaves to berries. A favorable balance between leaves and berries occurs around inflection points where yields change from rapid to less rapid. Such shoot growth balance is not a static characteristic, but a variable among vineyards and to some degree, among seasons for any given vineyard. For consistent high yields, vines ought to be pruned, irrigated, and otherwise managed to ensure balanced crop loads.

Conclusions

Grapevine growth capacity and crop load balance are the vineyard factors we can influence on the journey to fruit yield. Grapevine growth capacity, as directed through vineyard design, determines potential fruit yield, while vineyard management determines the relative partitioning of growth capacity between ripening capability (leaves) and yield (berries).

Based on these factors, we can achieve high yields of excellent-quality fruit over the life of a vineyard. The requirements to do so are as follows. First is a vineyard design appropriate for the site, plant material growth capacity, and variety of growth habits. In much of California, such designs include cordon training and spur pruning, high fruit zones, canopy division, and rows spaced as close as practical. Second, timely and complete canopy development and minimal foliage damage due to pests, diseases, and stresses to attain and maintain a favorable balance between leaves and berries.

High fruit yields maintained over the long term within the limits of vine capacity are, in fact, a component of sustainable winegrowing. This is so because high-yielding vines use resources, including water, more efficiently than low-yielding vines. In addition, high-yielding vineyards give a greater return on investment for most inputs compared to low-yielding vineyards. To be truly sustainable, the pursuit of long-term high yields must always respect the limits of vine growth capacity and strive for balanced crop loads.

This article is based on a presentation delivered at the 2021 Mid Valley Agricultural Services Grape Grower Meeting. The author dedicates this article to Maxwell Norton, UC Cooperative Extension Farm Advisor Emeritus, his partner in many a viticultural research project in pursuit of an understanding of winegrape yield.

Continued from IN PURSUIT OF YIELD: GRAPEVINE CAPACITY, BALANCE, & CROP LOAD – PART ONE

IN PURSUIT OF YIELD: GRAPEVINE CAPACITY, BALANCE, & CROP LOAD – PART TWO

Grapevine Shoot Growth Balance: The Superstructure of Fruit Yield

Vineyard Management for Balanced Crop Loads and Consistent Fruit Yields

Conclusions

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