Practical Winery
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phone:415/479-5819 · fax:415/492-9325
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September/October 2001

By M. Carmo Vasconcelos PhD. and Steve Castagnoli


Achieving consistent yields of high quality grapes in cool climates is challenging. Yields tend to fluctuate from year to year, and optimum maturity may not be reached every season. A short growing season, cool weather, and unfavorable precipitation patterns are factors that may affect the yield and quality of the vintage.

The success of winegrape production in cool climates can often be improved through proper canopy management. Canopy management provides a set of tools that allows grapegrowers to improve the canopy structure and microclimate.

The purpose of the study detailed below was to determine how different canopy management practices and combinations of these practices affect yield, fruit composition, vegetative growth, and carbohydrate reserves in the permanent vine structure.
Ultimately, the goal was to provide growers with tools to optimize winegrape production using these practices.

One aspect of canopy structure that should not be underestimated is age distribution of the leaf population. Grapevine leaves are net importers of carbohydrates until they reach 50% to 80% of their final size [18,36]. The photosynthetic rate increases until leaves attain full size (approximately 40 days after unfolding) and decreases steadily thereafter [22,23]. The most efficient leaves in the canopy, therefore, are those that are recently expanded (youngest full-grown leaves). The age of the vine canopy can be manipulated with selective leaf removal and shoot tipping at appropriate growth stages.

Removing shoot tips promotes lateral shoot growth at the nodes closer to the excised tip [13,37]. Lateral shoots developed during the period of active shoot growth will provide additional photo-assimilating surface during fruit ripening.

Lateral shoots become net exporters of carbohydrates as soon as they have two fully expanded leaves [12]. They provide assimilates to support their own growth and export the surplus to the main shoot, contributing to fruit ripening [19]. The most efficient leaves during ripening are located at the top of the canopy and those arising from lateral shoots [6].

Whether to retain, hedge, or remove lateral shoots in grapevine canopies has been a matter of controversy in many winegrape production areas in both the Old World and New World.

Lateral shoots are undesirable in vigorous vineyards because they lead to crowded canopies, with excessive shading and humidity and poor air circulation resulting in an imbalance favoring vegetative growth over fruit production and increased disease incidence [9,11, 30,31].

In moderate vigor vineyards, lateral leaves improve fruit quality and are the most important contributors to both sugar accumulation in the fruit during ripening and to starch accumulation in the parent vine [4].

Materials and Methods
Experimental design: The experiment was carried out on 180 own-rooted, 17-year-old Pinot Noir grapevines during two consecutive seasons. Vines were spaced 1.83 m x 2.74 m and were cane-pruned to four buds/m2
(11 buds/m vine-row) in the first season. They were balance-pruned to 28 buds/kg of one-year-old pruning wood in the second season. The following treatments were applied in factorial combinations:
Shoot tipping: removal of three to four apical leaves at full bloom, or no shoot tipping.
Lateral shoot length:
1. No laterals (laterals removed weekly as they arose, starting at full bloom);
2. short laterals (laterals cut back to four leaves at full bloom, and subsequent lateral growth removed weekly);
3. long laterals (laterals allowed to grow undisturbed).
Leaf removal in the cluster zone four weeks after bloom or no leaf removal: This treatment consisted of removing leaves and lateral shoots opposite the clusters in addition to one leaf immediately above and below the cluster. Each treatment-combination was replicated five times with three vines per plot in a completely randomized design.

Fruit set: Prior to bloom, one inflorescence per vine was enclosed in a mesh bag to retain all shed flowers. The bags were removed at the end of July, four weeks after full bloom, and all abscised flowers and fruitlets counted. At harvest, these clusters were picked separately, frozen at –20†C, and the number of berries was later counted. The number of flowers was calculated as the sum of shed flowers and berries. Percent fruit set was calculated as the quotient of the number of berries at harvest and the total number of flowers per inflorescence.

Yield and yield components: The crop was harvested October 1, 1995, and October 17, 1996. The number of clusters per vine was recorded. One hundred berries from each plot were chosen randomly to determine mean berry weight. Cluster weight was obtained by dividing total yield by the number of clusters. The number of berries per cluster was calculated by dividing cluster weight by the mean berry weight.

Fruit composition: A sample of 25 clusters per experimental unit was crushed for determination of soluble solids, pH, and titratable acidity.

Skin anthocyanin content was determined on a 100-berry sample from each experimental unit as described by Candolfi-Vasconcelos and Koblet [4]. An extinction coefficient of E 1% = 380 was used in the calculations [29].

Canopy development and vine vigor: Trunk volume (V) was estimated during pruning in February 1996 and 1997. For this purpose, the trunks were divided into a varying (n) number of sections that were approximately cylindrical in shape and the following formula was used:


The weight of one-year-old prunings, including woody laterals, was recorded in 1996 and 1997. Cane weight was obtained by dividing pruning weight by the number of canes.

Three shoots per replicate were collected September 9, 1996, for growth analysis. The number of main and lateral leaves were counted. Shoot length and diameter, and primary and lateral leaf area were measured.

The Ravaz Index [27] was calculated by dividing total yield per vine by the pruning weight recorded during the winter following each season.
Wood carbohydrate reserves: During pruning, wood samples from the trunk were collected and carbohydrates were extracted and analyzed using the method described by Candolfi-Vasconcelos and Koblet [4].

Statistical analysis: The Statview statistical package was used for statistical analysis of data. Results were subjected to correlation analysis and to a four-way analysis of variance (shoot tipping X lateral length X leaf removal X season).

The Waller-Duncan k-ratio test was used to compare means. Interactions between factors were rare, and the contribution of interactions to the total variance was very small relative to the main effects. For this reason, we chose to present only the means of the main effects. For completeness, all significant interactions found are reported in the text. The effect of cluster zone leaf removal was omitted from the tables when there was no significant response (Tables I, IV, and V).

Results and Discussion
Yield and yield components: Shoot tipping at bloom improved fruit set by 25% (Table I). Additionally, fruit set benefited from lateral shoot removal with a trend toward higher percent fruit set in response to complete removal of lateral shoots. The positive effect of tipping on fruit set has been established in previous studies [4,8,17,34].

Actively growing shoot tips compete with the developing inflorescences for assimilates. During bloom, the leaves in the mid- and upper-shoot section export carbohydrates to the shoot tip [12,18,26]. After hedging, the direction of translocation is reversed; instead of moving up to the shoot tip, assimilates are diverted downward [26] and made available to the developing inflorescences. This is thought to improve fruit set.

During early stages of development, lateral shoots depend on assimilates provided by the main shoot for growth, competing with other vegetative and reproductive sinks [20]. Elimination of all competing vegetative growing tips, either on the main or lateral shoots, increases the pool of available carbohydrates for floral development, which may result in improved fruit set.

Leaf removal in the cluster zone had no measurable effect on fruit set or any other yield component (data not shown). We chose to apply this treatment four weeks after full bloom based on prior research. Leaf removal in the cluster zone in the early stages of berry development can reduce fruit yield, because flower and fruitlet abscission may occur [4].

During bloom, shoots of V. vinifera have an average of 16 to 19 unfolded leaves [24]. Under non-stressing conditions at this stage of development, retranslocation of assimilates from the reserves stored in the permanent structure has ceased [35]. The basal leaves are fully expanded and are net exporters of carbohydrates [12,18].

Removal of basal leaves at full bloom equates to the elimination of a significant proportion of the primary source of photoassimilates. Four weeks after full bloom, the main shoot has 25 to 27 unfolded leaves (M.C. Vasconcelos, unpublished data, 1997). Elimination of basal leaves at this stage does not affect fruit set [3,4].

The final number of berries per cluster, cluster weight, and yield per shoot were increased by shoot tip removal, but not by other treatment factors (Table I). These increases largely reflect those observed in fruit set with similar trends in response to lateral shoot removal.

There was no treatment effect on berry weight or bud fertility (clusters/shoot), but there was a significant effect of season (Table I). Removal of mature leaves during the two-week period following bloom reduces bud fertility in the following season [4]. Carbohydrate shortage during this period is critical for fruit production in both the current and following seasons.

The growing season affected yield per vine (Table I) mainly due to the number of buds left per vine after pruning, but also through increased bud fertility and heavier clusters (Table I). Shoot tip removal considerably reduced cane and pruning weights after the first season of implementation of the treatments (discussed below), affecting the number of buds left after balance pruning.

There was a significant interaction between the shoot tipping treatment and the season (p = 0.0017). Yields per vine were 2.7 and 3 kg/vine for the non-tipped and tipped vines in the first season, respectively. These differences were not significant. In the second season, however, non-tipped vines had more shoots and clusters per vine which resulted in higher yields (5.7 and 4.1 kg per vine for non-tipped and tipped vines, respectively).

Across all treatments, total yield per vine was closely related to number of shoots per vine (r = 0.743, p < 0.0001), number of clusters per shoot (r = 0.673, p <0.0001), and berries per cluster (r = 0.457, p < 0.0001).

Percent fruit set was inversely related to the number of flowers per inflorescence (Fig. 1). This compensation mechanism is an interesting phenomenon. It seems to indicate that even after the number of clusters and flowers are determined, fruit set provides an additional opportunity to regulate the crop by adjusting it to available resources.

Fruit composition: Juice-soluble solids concentration was reduced by shoot tipping (Table II). Brix increased, however, with increasing lateral shootlength. These two responses can be explained if the corresponding leaf age distribution and photosynthetic activity of different aged leaves are considered.
Photosynthetic activity is higher in recently formed leaves, with the peak of photosynthesis occurring when leaves attain full size, followed by a gradual decrease with increasing leaf age [1,21,22,33].

Young leaves were present on non-tipped vines and those with lateral shoots. Thus, non-tipped vines or those with lateral shoots should have higher overall canopy photosynthesis, resulting in a larger pool of photoassimilates available for accumulation of sugars in the fruit. Furthermore, it has been shown that the presence of fully expanded young leaves is advantageous for sugar accumulation in the fruit [4].

Removing four basal leaves four weeks after bloom reduced juice soluble solids at harvest (Table II). It has to be noted that under western Oregon climatic conditions, vegetative growth stops relatively early compared to that of winegrape growing regions in central Europe that receive precipitation during the summer months.

In eastern Switzerland, where leaf growth does not stop until veraison, there was no significant decrease in juice soluble solids on vines where all the leaves on the primary shoot had been removed [3]. In that study, lateral shoots were able to reconstruct an adequate assimilating surface and compensate for the absence of main leaves.

In this experiment, basal leaf removal was not compensated for by increased lateral shoot growth. There was no interaction between leaf removal and lateral shoot length treatments for leaf area (Table III). Lower juice soluble solids in response to removal of basal leaves can be explained by reduction of the leaf tofruit ratio from 15 to 10 cm2/g fruit (Table III).

Growers should guard against excessive fruit exposure due to leaf removal, and they should evaluate this practice for each vineyard using historical data on canopy exposure and previous wine quality [14]. If foliage or fruit already receive adequate exposure, leaf removal may cause a reduction in berry weight and soluble solids, probably because too much leaf area has been removed [2].

There were no significant differences in titratable acidity among treatments, but juice pH responded similarly to juice-soluble solids, indicating that “younger” canopies hasten fruit ripening (Table II). In contrast with our results, it is generally accepted that increased cluster exposure to sunlight decreases juice acid content [15,16,28,31,32,37].

Skin anthocyanin content was not affected by increased exposure resulting from leaf removal in the cluster zone (Table II).
Reports on the effect of sun exposure on anthocyanin content are inconsistent. Increasing sun exposure of berries did not change anthocyanin content in Pinot Noir [25], but increased the color of Cabernet Franc [10] and Cabernet Sauvignon [7].

The presence of more lateral leaves improved skin anthocyanin content both per berry and per amount of fruit (Table II).
Canopies composed only of lateral leaves generate fruit with higher soluble solids and anthocyanin content as compared to non-defoliated controls [4]. Lateral leaves, being the youngest leaves in the canopy, may play a major role in metabolic processes occurring during fruit ripening.

Canopy development and vine vigor: Average leaf size (main and lateral leaves) increased with shoot tip removal but did not respond to other treatments (Table III). It has been shown that one of the compensation mechanisms for defoliation is the increase in size of the remaining leaves [4]. However, we did not observe this compensation in response to basal leaf removal and lateral shoot shortening.

Removal of shoot tips decreased total leaf area by 47%, primarily because of reduced main leaf area (Table III). Complete removal of lateral shoots decreased total leaf area by 43% and 45% as compared to treatments with trimmed laterals and long laterals, respectively (Table III).

Vines with laterals cut back to four leaves had 20% less lateral leaf area than vines with untrimmed lateral shoots, but there was no significant difference in total leaf area between these treatments. Total leaf area per vine was not significantly reduced by leaf removal in the cluster zone (Table III).

Shoot tip removal increased the proportion of leaf area arising from lateral shoots but leaf removal in the cluster zone did not change this ratio (Table III).
Shoot diameter during mid-ripening did not respond to shoot tip removal, even though the hedged shoots were 76% shorter (data not shown).

Trunk volume measured in the winter during dormancy was not affected by any treatments (Table IV) but it decreased after the second season. This could not be accounted for with changes in water content (data not shown). The decrease in trunk volume may be the result of increased yields in response to balance pruning in the winter prior to the second season.

The fruit is the primary sink for assimilates during the six weeks after veraison [5]. After that, the roots become the most powerful sink [5]. During ripening and under non-stressing conditions, leaf photosynthesis is the only source of carbohydrates for fruit development [5].

However, carbon reserves in the trunk and roots can complement current photosynthesis to support fruit maturation in cases of photosynthate shortage [5]. The larger crop load during the second season may have limited replenishment of carbohydrate reserves in the trunk (Table V) and caused the smaller trunk volumes.

Pruning weights were not affected by lateral shoot length (Table IV) or leaf removal (data not shown) but they decreased greatly with shoot tipping (Table IV). Pruning weights and average cane weight were less following the second season, possibly in response to balance pruning (Table IV).

There was a significant interaction between the shoot tipping treatment and the season: shoot tipping decreased cane weights from 73 g to 42 g after the first season and from 49 g to 43 g after the second season. Vines in balance should have canes between 30 g and 40 g, 40 g being preferred in cool climates (R.E. Smart, personal communication, 1995). Vines without lateral shoots had the lowest cane weights (Table IV).

The Ravaz Index represents the ratio of reproductive to vegetative growth. Balanced vines should remain between 5 and 7 [27]. Shoot tipping increased the Ravaz index from 4 to 6 (Table IV). Trimming or eliminating lateral shoots also increased the Ravaz index (Table IV). Leaf removal did not change this ratio and did not affect cane or pruning weight (data not shown).

The Ravaz Index increased more than two-fold from the first to the second season (Table IV). This is probably a response to balance pruning prior to the second season. This supports the general belief that appropriate pruning levels, matching vine capacity with cropping level, are extremely important in achieving a balance between vegetative and reproductive growth.

Carbohydrate reserves in the wood: Concentration and total amount of starch in the trunk during dormancy were not significantly affected by any canopy management treatments (Table V). There was a trend, however, toward increased concentration and total amount of starch in response to shoot tipping, lateral shoot length (Table V), and leaf removal (data not shown).

Sugar concentration and total amount per trunk decreased with shoot tipping (Table V). The total amount and concentration of non-structural carbohydrates were not significantly affected by shoot tipping (Table V) or leaf removal (data not shown). Total carbohydrate reserves stored in the trunk increased, however, with lateral shoot length, in agreement with prior research by Candolfi-Vasconcelos [3].

Starch content in the permanent vine frame was related to juice soluble solids during the preceding season (year 1: r = 0.326, p = 0.011; year 2: r = 0.411, p = 0.001), suggesting that sugar accumulation in the fruit and starch accumulation in the reserve organs occur simultaneously. This is in agreement with results reported previously [4,5].

 

Conclusions
Canopy management techniques used in this experiment were targeted at changing leaf age distribution of the vine canopy at critical times during the growing season. They should be implemented only with full understanding of their impact on carbohydrate translocation patterns and leaf photosynthetic response to aging.

Canopy management practices can be used to maximize the amount of carbohydrates partitioned to inflorescences during bloom to improve fruit set or to reduce partitioning to reduce fruit set.

Results obtained in this study indicate that eliminating immature leaves during bloom increases fruit set and promoting vegetative growth during bloom reduces fruit set. Therefore, by manipulating the number of competing sinks for carbohydrates during early stages of berry development, it is possible to increase fruit set in poor set varieties or decrease cluster compactness in varieties prone to bunch rot.

Canopy management can be used to maximize carbohydrate partitioning to fruit during ripening; this can be achieved by actively promoting the availability of young, fully expanded leaves during fruit ripening.

Retaining lateral shoots hastened fruit ripening, improved fruit color, and increased the level of carbohydrate reserves in the trunk. This is a valuable technique that can be used to improve fruit composition and vine survival in short-season winegrape regions.

Removal of leaves in the cluster zone at four weeks post-bloom did not affect yield or yield components, but it decreased juice-soluble solids and did not improve skin anthocyanins. Our experiment was conducted in a moderate vigor vineyard, and removal of three to four leaves in the cluster zone seemed to be excessive. This cultural practice should be reserved for vigorous vineyards with crowded canopies.

Edited text from American Journal of Enology & Viticulture, Vol. 51, No. 4, January/February 2001.
The 2001 Best Paper Award for viticulture was awarded by the American Society for Enology & Viticulture to the authors.
M. Carmo Vasconcelos, Associate Professor, and Steve Castagnoli, Faculty Senior Research Assistant, Department of Horticulture, Oregon State University, Corvallis, OR 97331.
Corresponding author [Fax: 541-737-3479; E-mail]
The research described in this text was done in the southern Willamette Valley, Alpine, OR, 1996/97.
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Acknowledgements: The authors thank the Oregon Wine Advisory Board for financial support of this project and colleagues Bernadine Strik and Les Fuchigami for critical review of the manuscript.