Abstract
Minimal pruning (MP) is a technique used to reduce labor costs and produce high-quality winegrapes. To evaluate the effects of MP on grapes cultivated in warm-climate regions, a long-term study on Tempranillo (Vitis vinifera L.) was conducted in Badarán, La Rioja, Spain. For each vintage between 1999 and 2013, yield and total soluble solids (TSS) were evaluated in grapes from MP vines and from conventionally hand-pruned (CHP) vines. In 2014 and 2015, grapes were analyzed at 22 Brix to assess the effects of MP on fruit maturation and quality. The long-term study showed that MP increased yield by 56% and reduced TSS by 9% compared to CHP. Results from 2014 and 2015 demonstrated that MP delayed fruit maturity (22 Brix) by ~17 days. At the same TSS level (22 Brix), MP vines had 24% lower berry weight, 57% lower cluster weight, and 51% greater yield. Must from MP fruit had greater total anthocyanin concentrations compared to must from CHP fruit (+17% in 2014 and +21% in 2015); however, potential improvements in wine color were more likely due to smaller berry size than to greater anthocyanin synthesis per unit area of berry skin. These results indicate that MP can delay berry ripening and may help to improve wine color.
Climate change models predict that an average warming of 2°C will occur in global wine-producing regions by 2050 (Jones et al. 2005). Under this warming trend, problems faced by the wine industry include a decoupling of phenolic and technological maturity of grapes and excessively high alcohol contents in wine, especially in warm-climate regions such as Spain (Martínez de Toda et al. 2013).
Anthocyanins are an important quality component of red wine grapes. In general, factors that favor carbohydrate accumulation also contribute to anthocyanin synthesis, especially in the first five weeks after veraison (Pirie and Mullins 1977). However, high temperatures during berry development can delay the onset of anthocyanin accumulation, ultimately leading to low levels at harvest (Sadras and Moran 2012). During berry maturation, high temperatures can also inhibit the biosynthesis of key enzymes and lead to anthocyanin degradation (Mori et al. 2005, 2007). In addition, high temperatures can accelerate grapevine phenological stages (Keller 2010), leading to a decoupling of phenolic and technological maturity (i.e., sugar concentrations, titratable acidity, and juice pH). While sugar accumulation occurs earlier and more rapidly during warmer periods of the growing season, the accumulation of phenolic compounds is inhibited and berry anthocyanin concentrations may not reach desirable levels at harvest if temperatures are too high. The combination of high total soluble solids (TSS) and low acidity can lead to unbalanced wines with high alcohol content.
For an established vineyard, the negative effects of global warming on fruit maturation could be mitigated by adopting cultural techniques that delay maturation, such as shoot trimming (Martinez de Toda et al. 2013, Palliotti et al. 2014), postveraison distal leaf removal (Palliotti et al. 2013), late winter pruning (Palliotti et al. 2014), double pruning (Gu et al. 2012), and minimal pruning (MP). Thirty years of research in Australia showed that traditional severe pruning was unnecessary in some viticultural regions and could produce low-quality wines due to the development of shaded, tight clusters with large berries, which could lead to challenges in controlling pests and diseases (Clingeleffer 2010). In most cases, MP leads to higher yields compared to hand pruning (Morris and Cawthon 1981, Reynolds 1988, Martinez de Toda and Sancha 1998, Schultz and Weyand 2005), and improves canopy light and vine health by reducing vine vigor (Archer and van Schalkwyk 2007, Clingeleffer 2010). Low bud fruitfulness, small clusters, and low berry weight are yield components associated with MP (Bates and Walter-Peterson 2008). While MP can save labor and reduce management costs, it does not perform well for some late-ripening cultivars, especially those grown in cool climates and areas with high rainfall (Schwab 2005). Without crop adjustment, MP can lead to overcropping, which in turn leads to delayed or insufficient ripening (Morris and Cawthon 1981, Bates and Morris 2009); this effect can be reduced by trimming low-hanging fruiting canes or by applying mechanical crop thinning 20 to 30 days after bloom (Poni et al. 2000). On the other hand, by delaying maturation, MP could be an effective tool for counteracting the effects of climate warming by enhancing anthocyanin accumulation in berries and by maintaining acidity. Archer and van Schalkwyk (2007) found that MP resulted in better color in berry skins and in consistent alcohol levels in wine. Holt et al. (2008) found that grapes from mechanically pruned vines had consistently higher anthocyanin concentrations than those from cane- or spur-pruned vines. Based on 30 years of experience in Australia, Clingeleffer (2010) concluded that grapes from minimally pruned vines generally produce enhanced wine color. In contrast, Morris and Cawthon (1981) found that continuous mechanical pruning led to low TSS and poor color. Similarly, Rousseau et al. (2013) found lower color intensity in wines from MP vines compared to traditionally pruned vines.
The main goal of this study was to evaluate the effectiveness of MP for delaying grape maturity under the climate conditions of the La Rioja Valley in northern Spain. Another goal was to assess the effects of MP on fruit quality, including the relationship between berry anthocyanins and TSS.
Materials and Methods
Plant material and growth conditions
The study was conducted in a commercial Vitis vinifera cv. Tempranillo vineyard in Badarán (lat. 42°22′4.4″N; long. 2°48′33.2″W, 620 m asl), La Rioja, northern Spain. The vineyard was planted in 1986 on 41-B rootstock. Spacing was 1.1 × 2.6 m (vine × row) in north–south oriented rows with a density of 3500 vines/ha. The MP treatment was applied to vines that originally had a spur-pruned free-horizontal cordon (without shoot positioning) at a height of 150 cm, but had not been pruned since 1996. Every three or four years, from 1996 to 2015, MP vines were trimmed to maintain a regular shape by mechanical trimming to prevent shoots from contacting the ground and to prevent excessive extension. The most recent trimming was carried out in summer 2015. The control vines underwent conventional hand pruning (CHP) and were trained in the traditional goblet shape (two to three arms per vine) and pruned to 12 buds per vine. The vineyard was maintained using common regional viticultural practices (e.g., water shoot removal, trimming in summer, and cluster thinning, if necessary). Climatic data were obtained from the nearest meteorological station, in Villar de Torre. Mean monthly temperatures from 2005 to 2013 were calculated and considered as normal average monthly temperatures.
Experimental design and measurement of variables
The experiment was conducted in two rows using a completely randomized design consisting of three replicate plots of 10 vines each for each of the two pruning treatments (CHP and MP). From 1999 (3 yr after MP was established), grapes from CHP and MP vines were harvested at the same time in each vintage. Yield and TSS in juice were measured each year.
In 2014 and 2015, all grapes were analyzed at the same TSS level (22 Brix). The veraison date was recorded as the date on which 50% of the berries began to show color. Maturity was monitored throughout the ripening phase. To estimate leaf area per shoot (cm2), leaf disc sampling was used (Smart and Robinson 1991) on 15 shoots per treatment. For each shoot, the weight of all leaves (without petioles) was divided by the weight of 100 discs (3.80-cm2 each) and multiplied by 380. Fruit was harvested when TSS averaged 22 Brix. Yield, clusters per vine, and shoots per vine were determined on five vines per plot (15 vines per treatment). Cluster weight was measured on five clusters per treatment replicate. Berry weight was measured on 200 berries per replicate, randomly sampled from the harvested fruit. After weighing, each 200-berry sample was manually crushed to obtain juice for chemical analysis. TSS, pH, titratable acidity (TA), tartaric acid, and malic acid were analyzed by standard methods (OIV 2013). Total anthocyanins were determined at 22 Brix according to Iland et al. (2004) and were expressed on a concentration (mg/g berry fresh mass) and density (mg anthocyanins/cm2 grape skin surface) basis. Anthocyanin concentration indicates the potential wine color, while anthocyanin density reflects the anthocyanin synthesis capacity of grape skins.
Yield and TSS data from the long-term trial were analyzed with a paired-samples t-test (p ≤ 0.05). Data from 2014 and 2015 were tested for homogeneity of variance using Levene’s test and were then subjected to two-way (pruning method × year) analysis of variance (ANOVA), using the general linear model and F-test; since interactions between treatment and year were observed for some parameters, pruning systems were also analyzed by year using one-way ANOVA. The statistical analysis was performed using SPSS 16.0 for Windows (SPSS, Inc.).
Results and Discussion
Long-term observations
From 1999 to 2013, yield was higher in MP than in CHP vines (15,300 kg/ha versus 9800 kg/ha), which was in agreement with a 10-year MP experiment with Riesling in Geisenheim, Germany, in which MP led to 25 to 75% higher yield (Schultz and Weyand 2005). Grapes from MP vines had lower TSS (average Brix = 18.4) than grapes from CHP vines (average Brix = 20.2); 18.4 Brix would produce a wine with a potential alcohol content of 10.8%. This level would have been too low for most winemakers until recently, but it has become acceptable given the growing demand for low-alcohol wines. This long-term observation is mostly consistent with a previous study performed in the same region with Grenache (Martinez de Toda and Sancha 1998). It can be concluded that MP is a viable, labor-saving technique for certain cultivars in the Rioja wine region.
Weather conditions
In 2014, temperatures were unusually warm in September and October, when grapes reached maturity (Figure 1). In comparison, 2015 was unusually hot from May through July, but September and October were relatively cool.
Mean monthly temperature in the study area (Villar de Torre, La Rioja, Spain) during the growing season.
Yield components
MP delayed veraison by one to two weeks (Table 1). In 2014, MP increased yield by 77% compared to CHP, whereas yield per vine was not affected by pruning treatment in 2015. MP vines had 10 to 11 times more shoots, but only 20 to 40% of the shoots bore fruit, compared to 100% of shoots on CHP vines. Berry weight was 12 to 35% lower, and the number of berries per cluster was 47 to 53% lower, in MP vines compared to CHP vines. These effects of MP on yield components are consistent with previous studies (Poni et al. 2000, Schultz and Weyand 2005, Bates and Walter-Peterson 2008).
Effects of minimal pruning (MP) and conventional hand pruning (CHP) on yield components, vine leaf area, and leaf area-to-production ratio in Tempranillo vines in 2014 and 2015.
The ratio of leaf area to fruit production is often used to assess potential berry maturation and quality; during maturation, this ratio should generally range from 0.8 to 1.2 m2/kg (Kliewer and Dokoozlian 2005). According to these values, MP vines had adequate leaf area to support fruit ripening in both years. However, most of the expected effects of MP (delayed veraison and TSS accumulation, lower berry weight, fewer berries per cluster, etc.) occurred. Champagnol (1984) found that clusters are mainly supported by leaves that occur on the same shoot, although nutrients can be transferred from other shoots during maturation. In this study, as is typical, MP vines had many nonfruiting shoots, the leaves of which could contribute only indirectly to berry composition. In addition, because they retain more buds and experience earlier bud-burst, MP vines develop a canopy more quickly than do conventionally pruned vines (Lakso 1993). In this study, shoots on MP vines had fewer leaves (average = 10) than shoots on CHP vines (average >15). Final canopy size was attained earlier by MP vines than CHP vines, which continued to generate new leaves and lateral shoots. Poni et al. (1994) reported that leaves normally reached maximum photosynthetic capacity at 30 to 35 days of age. After ~50 days, photosynthetic capacity began to decline, and 4-mo-old leaves retained 45% of maximum photosynthetic capacity (Poni et al. 1994). Hence, it can be inferred that during the ripening phase, the source of photosynthate in MP vines is “old leaves,” while CHP vines retain younger leaves with greater photosynthetic capacity. All of these factors contribute to a generally low “source-to-fruit” ratio in MP vines.
Must composition
All grapes were analyzed at the same TSS level in both years (Table 2). MP delayed maturation by ~17 days. In 2014, grapes from MP vines had higher TA and better organic acid composition (i.e., higher tartaric acid and lower malic acid concentration), which was consistent with Clingeleffer (2010). Berry pH was surprisingly high considering the high TA content. In 2015, berry TA and pH were lower in MP than in CHP vines. These results indicate that further study of the effects of MP on berry acidity is warranted.
Effects of minimal pruning (MP) and conventional hand pruning (CHP) on must composition and berry anthocyanins in Tempranillo in 2014 and 2015.
At 22 Brix, MP vines produced grapes with higher total anthocyanin concentration (mg/g) compared to CHP vines in both years. There was no difference in anthocyanin synthesis capacity (mg anthocyanins per cm2 skin surface) between the pruning treatments, which suggests that wines made from MP grapes would be more intensely colored, mainly as a result of smaller berry size rather than because of enhanced anthocyanin synthesis per unit area of berry skin. The lack of response of skin anthocyanin content to pruning treatment was surprising given that light pruning, which increases fruit exposure, can enhance anthocyanin biosynthesis independent of berry size (Holt et al. 2008), and given that the canopy of MP vines can be more porous, thus enhancing fruit exposure (Reynolds 1988, Lakso 1993). Our results may indicate that there was no effect of pruning on fruit exposure, but we did not evaluate treatment effects on canopy density and fruit exposure. Ambient temperatures may also have influenced our results. Mori et al. (2005, 2007) observed that higher temperatures during the day or night led to decreased anthocyanin accumulation. However, their experiments were conducted under greenhouse conditions, and the difference in temperature between control and high-temperature treatments was substantial (Δ T = 10°C and 15°C). In this experiment, from veraison to harvest maturity (22 Brix), mean daily air temperatures for CHP and MP vines were 17.8 and 16.9°C in 2014, and 17.6 and 17.1°C in 2015, respectively. These differences were not likely to affect anthocyanin synthesis. Martínez de Toda et al. (2014) reported an increase in the anthocyanin-to-sugar ratio in Grenache in response to severe trimming and with a mean temperature difference during maturation of −2.3°C relative to the control. Therefore, delaying maturation by creating cooler conditions during ripening might be an effective way to restore the anthocyanin-to-sugar ratio, but the difference in temperature should be greater than obtained in our experiment.
Conclusions
MP vines produced moderately higher yields and delayed berry development under the climatic conditions of the La Rioja Valley in northern Spain. Berry ripening was achieved under MP, and the higher anthocyanin concentrations in fruit from MP compared to CHP vines was a result of smaller berries rather than of anthocyanin synthesis capacity. The slightly cooler ripening conditions caused by MP did not appear to enhance anthocyanin accumulation. Further studies should be done using other varieties and under different climatic conditions to confirm and evaluate the delayed maturation caused by MP and to assess the effects of this pruning strategy on fruit quality.
Acknowledgment
This research is partially funded by the China Scholarship Council.
- Received April 2016.
- Revision received June 2016.
- Revision received August 2016.
- Revision received September 2016.
- Accepted September 2016.
- ©2017 by the American Society for Enology and Viticulture
Literature Cited
Vol 68 Issue 1
