Abstract
The influence of solar radiation and temperature on the accumulation of berry skin anthocyanins was evaluated in Vitis vinifera L. cv. Nebbiolo during ripening. In 2006 and 2007, five fruit-zone light exposure regimes were established using plastic netting or leaf removal. Shading was applied in three periods: from fruit set to veraison, from fruit set to harvest, and from veraison to harvest. Fruit-zone shading had no impact on yield per vine and cluster weight, but early fruit-zone shading caused a slight delay in berry development, although it did not lead to lower berry size at harvest. Fruit-zone shading reduced total soluble solids and anthocyanin accumulation. Generally, fruit shading decreased the 3′-hydroxylated anthocyanin concentration and increased the 3′,5′-hydroxylated anthocyanin concentration. Fruit-zone leaf removal caused a delay of berry development, but did not affect berry size, cluster weight, or yield at harvest. Excessive sunlight exposure caused sunburn damage and did not increase total soluble solids or anthocyanin accumulation.
Solar radiation is essential for grape berry ripening as it drives photosynthesis and sugar accumulation and influences secondary metabolism. Moreover, it determines berry temperature. While it has been demonstrated that sun leaves have greater photosynthetic activity than shade leaves (Cartechini and Palliotti 1995, De Cortazar et al. 2005), cluster shading investigations have led to separate conclusions. According to several studies, cluster shading has little or no effect on fruit weight (Reynolds et al. 1986, Morrison and Noble 1990, Spayd et al. 2002, Downey et al. 2004, Jeong et al. 2004, Cortell and Kennedy 2006), although one report found fruit shading can decrease fruit weight (Dokoozlian and Kliewer 1996). Previous studies reported little or no impact of shading on berry total soluble solid (TSS) at harvest (Morrison and Noble 1990, Haselgrove et al. 2000, Spayd et al. 2002, Downey et al. 2004, Jeong et al. 2004, Cortell and Kennedy 2006, Ristic et al. 2007) or decreased TSS content (Reynolds et al. 1986, Rojas-Lara and Morrison 1989, Dokoozlian and Kliewer 1996, Bergqvist et al. 2001). Many authors have demonstrated that cluster shading reduces anthocyanin accumulation (Rojas-Lara and Morrison 1989, Morrison and Noble 1990, Gao and Cahoon 1994, Dokoozlian and Kliewer 1996, Haselgrove et al. 2000, Bergqvist et al. 2001, Spayd et al. 2002, Jeong et al. 2004). Light exposure is a limiting factor during the early stages of grape ripening (Haselgrove et al. 2000, Downey et al. 2004) and its level may be critical even before veraison for maximum pigment production (Dokoozlian and Kliewer 1996).
The relationship between sunlight exposure and berry temperature is important for berry composition and metabolism (Spayd et al. 2002). The effects of light on fruit composition are heavily dependent upon the extent to which berry temperature is elevated as a result of increased sunlight exposure, because high berry temperature can inhibit color development (Bergqvist et al. 2001, Yamane et al. 2006, Tarara et al. 2008).
In northwest Italy, its region of origin, cv. Nebbiolo is used for the production of high-quality wines. Nebbiolo is sensitive to terroir and characterized by elevated vigor and reduced berry skin color (Guidoni et al. 2008). Its typical berry skin anthocyanin profile is distinguished by the prevalent presence of peonidin-3-glucoside followed by malvidin-3-glucoside (Guidoni et al. 2008). The low total anthocyanin concentration of Nebbiolo could result in wines with insufficient color. Growers use several canopy management practices such as leaf removal to improve berry color.
This study investigated the impact of altered berry temperature and sunlight microclimatic conditions on berry development and composition of Nebbiolo grapes, emphasizing anthocyanin accumulation and profile. The microclimate was altered by basal leaf removal to maximize cluster exposure to sunlight and by artificially shading the fruit zone during different stages of ripening, with the goal of characterizing the effects of different light exposure levels on anthocyanin accumulation.
Materials and Methods
In 2006, the study was conducted in a commercial vineyard of Vitis vinifera cv. Nebbiolo located in Vignane (northwest Italy), which had been planted in 1996 on a hillside with ~20 degree slope and a western exposure. Severe cutworm (Noctua spp.) infestation in spring 2007 caused the study to be moved to a nearby commercial vineyard located in Vezza. This vineyard was planted in 1999, on a hillside with a ~45 degree slope and a western exposure. In both vineyards vine rows were north–south oriented, vines were pruned to a single 10-bud cane, and shoots were vertically trained. The vineyards were managed according to standard practices for this cultivar and the region.
In both years, five fruit-zone light exposure levels were established in the vines by artificial shading or leaf removal: naturally occurring shade (experimental control, C); shaded fruit-zone vines from fruit set to veraison (FV); shaded fruit-zone vines from veraison to harvest (VH); shaded fruit-zone vines from fruit set to harvest (FH); and fruit-zone leaf removal at fruit set (LR).
Fruit-zone shading was established by using 2-m wide 2220 WO Iride Due anti-hail, run-resistant, double-thread nets (Arrigoni Spa, Como, Italy). The UV-stabilized HDPE nets had a 16% light screening factor. They were placed vertically along the fruit zone, folded in half, thus forming a 1-m wide double layer. For the LR treatment, all leaves were removed at the fruit-zone level—that is, 5 to 6 basal leaves from the main shoots and 2 to 3 basal lateral shoots—to obtain maximum cluster light exposure. All five treatments were arranged within three complete randomized blocks with 15-vine plots. Observation and sampling were performed on the 10 central vines.
In order to monitor and evaluate light conditions in the C, LR, and FH treatments, photosynthetically active radiation (PAR) sensors (model S-LIA-M003; Onset Computer Corporation, Pocasset, MA) were positioned at fruit-zone level and connected to data loggers (Hobo H 21-002 Microstation; Onset). Berry temperature on each side of the canopy was monitored by thermocouples (model TC6-T; Onset) inserted just beneath the skin of the berries and connected to data loggers (Hobo H12 Type T thermocouple). Air temperature at the cluster level was monitored by sensors (Hobo H08-032-08) placed at the fruit zone.
PAR and berry temperature sensors were applied in two replicates of the C, LR, and FH treatments. Both PAR and temperature were measured and recorded once every 20 min throughout fruit growth and ripening. Mean diurnal PAR (0500 to 2100 hr) and temperature (00 to 24 hr) patterns were generated from hourly means calculated from the raw data.
From veraison, defined as BBCH code 81 (BBCH 2001), every two weeks 250 berries were sampled from the three replicates of each treatment by cutting the pedicels with scissors. For each treatment, six groups of 10 berries were randomly sampled and weighed. Three groups were then used as triplicates for further analysis. Berry skins were removed from the pulp, blotted dry by paper towel, and placed in 40 mL acidified methanol (1% HCl, v/v) (Revilla et al. 1998). The samples were placed in an oven at 30°C for 72 hr. The skins were then removed from the extracting solution and the latter was stored at −20°C until analyzed. The pulp was homogenized, centrifuged, and TSS was determined using a temperature-compensated PR-1 digital refractometer (Atago, Tokyo, Japan). Anthocyanin concentration and profile analyses were performed by HPLC (series 200 Diode Array Detector; Perkin-Elmer, Waltham, MA) (Di Stefano et al. 1991). For this purpose, samples were prepared as follows: 1 mL extracting solution was vacuum-evaporated at 30°C with a Laborota 4000 efficient rotary evaporator (Heidolph Instruments, Schwabach, Germany), diluted with 1 mL 10:50:40 v/v/v formic acid:methanol:water solution, and filtered into clear vials (Agilent Technologies, Santa Clara, CA) using 0.2 μm GHP membrane filters (GHP Acrodisc 13 mm syringe filter; Pall Life Sciences, Ann Arbor, MI). Solvent A was formic acid:water (10:90, v/v) and solvent B was formic acid:methanol:water (10:50:40, v/v/v). Individual anthocyanins were identified by comparing the retention time of each chromatographic peak with available data in the literature (Di Stefano et al. 1995). The concentration of individual anthocyanins was expressed as mg kg−1 berry fresh weight using malvidin-3-O-glucoside chloride (Extrasynthèse, Genay, France) as an external standard. Total anthocyanin was calculated as the sum of the concentrations of the free and derivative anthocyanin forms. All chemical reagents used were purchased from Merck (Darmstadt, Germany) and filtered through a 0.2 μm filter before analysis.
Cluster weight, yield per vine, bunch rot incidence and cluster sunburn damage were evaluated at harvest. Data were subjected to one-way analysis of variance (ANOVA) using SAS software (SAS Institute, Cary, NC).
Results
Climatic conditions.
According to the meteorological data provided by the Ufficio Agrometeorologico Regione Piemonte, 2007 was warmer than 2006, especially in spring (Figure 1⇓). The average maximum monthly temperature was 18.9°C in 2006 and 20.2°C in 2007. The average monthly temperature was 13.3°C in 2006 and 13.7°C in 2007. As a consequence of the high spring temperature, all phenological phases were more advanced in 2007 than in 2006: budburst, fruit set, veraison, and harvest occurred in 2007 on 6 April, 31 May, 3 Aug, and 19 Sept and in 2006 on 24 April, 15 June, 9 Aug, and 2 Oct, respectively.
PAR and temperature.
Solar radiation, from veraison to harvest, was higher in 2007 than in 2006 (Figure 2⇓) (maximum values 1125 μmol m−2 s−1 in 2006 and 1308 μmol m−2 s−1 in 2007). In both years, leaf removal increased cluster light exposure, as evidenced by the higher PAR values recorded on LR vines than on C vines (157 μmol m−2 s−1 in 2006 and 231 μmol m−2 s−1 in 2007). Shading nets caused a reduction of PAR incidence on the fruit zone and consequently on the clusters (11 μmol m−2 s−1 in 2006 and 54 μmol m−2 s−1 in 2007).
Daytime berry temperature in all treatments was higher than cluster-level air temperature. Conversely, berry and air temperature were similar during the night. In 2006, average hourly berry temperature during the study ranged between 15.5 and 30.5°C. LR and FH berry temperature was similar to C berry temperature. In 2007, the average berry temperature of all treatments from veraison to harvest was higher than the year before. In 2006, the average berry temperature never exceeded 30.5°C; in 2007, it ranged from 31 to 37°C during the afternoon for all treatments. The warmer temperature conditions of 2007 increased berry temperature differences among treatments, which, during the afternoon, varied up to 2.5°C between C and LR vines and 3°C between C and shaded vines (Figure 3⇓).
Berry development and composition.
In 2006, the incidence of bunch rot tended to be lower for LR vines (14%) and higher for shaded vines (average 36%) compared with C (25%), but the differences were not statistically significant. There was no bunch rot incidence in 2007. In 2007, possibly because of the combination of high temperature and high winds, serious sunburn cluster damage occurred on LR vines (37.5% berry damage) and VH vines (21%); C, FV, and FH vines were less affected (8.7%, 0.6%, and 1% damage, respectively). During the final stages of ripening, unusually persistent winds (data not measured) caused partial berry dehydration in the treatments not protected at the time by netting (C, LR, and FV), with loss of berry weight during the final two weeks reaching 12.7%, 10.4%, and 4.2%, respectively (Figure 4⇓). FH and VH berries were shaded and did not lose weight.
There were no differences among treatments in yield (2.38 kg/vine) and cluster weight (308 g/cluster) in 2006. In 2007, because of sunburn, yield (1 kg/vine) and cluster weight (164 g/cluster) in LR vines were lower than in the other treatments (1.9 kg/vine and 296 g/cluster). Despite a delay in berry development in LR, FH, and FV vines at veraison, there were no differences in berry size among the treatments in 2006 at harvest (Figure 4⇑). As mentioned above, in 2007 C, LR, and FV berries lost weight during the last two weeks before harvest, while VH and FH berries continued gaining weight, leading to lower fruit weight for C and LR vines than for VH vines. There were no differences among treatments during ripening otherwise.
Vine leaf area (LA) (3.14 m2/vine) and LA/yield (1.32 m2/kg) did not differ among treatments in 2006. In 2007 LR vine leaf area was less than the average of the other treatments (2.1 m2 versus 3.3 m2) but, due to the reduced yield, the LA/yield ratio was similar to that of the other treatments (1.8 m2/kg). Thus, it seems possible that the differences in berry composition derived from different microclimate conditions.
There were no differences in TSS content among treatments at harvest in 2006 (Figure 4⇑). TSS accumulation was similar for C and LR vines throughout ripening. This was also true for VH vines except for one single date (28 dpv), when VH vines showed a delay. Compared to C vines, TSS accumulation was slower and much lower in FV and FH vines during the early stage of ripening; however, that did not result in lower TSS at harvest. In 2007, TSS accumulation during berry ripening and TSS content at harvest were not affected by leaf removal. Vines covered with nets from fruit set to veraison (FV and FH) reached the lowest TSS concentrations at veraison. After net removal, TSS in FV vines increased more than in FH vines, but to levels below those in C vines at harvest. TSS accumulation in VH vines after the application of nets declined slightly compared with C vines.
Anthocyanin accumulation.
All treatments had higher anthocyanin concentrations at harvest in 2006 (Figure 5⇓, Supplemental Table 1) than in 2007 (Figure 6⇓, Supplemental Table 2). No differences among treatments were observed in total anthocyanin concentration throughout ripening in 2006. Only VH vines showed a temporary delay in anthocyanin accumulation after net application, but total anthocyanin concentration was not different than in C vines at harvest.
Except for FV, the 3′-hydroxylated/3′,5′-hydroxylated anthocyanin ratio of all treatments in 2006 was lower than C at harvest (Figure 7⇓) because of higher 3′,5′-hydroxylated anthocyanin and lower 3′-hydroxylated anthocyanin concentrations. However, the differences were significant only for certain compounds and in certain treatments (Figure 5⇑, Supplemental Table 1). As observed for TSS concentration, postveraison shading (VH) temporarily halted anthocyanin accumulation at 28 dpv, specifically for peonidin-3-glucoside and cyanidin-3-glucoside, the biosynthesis of which resumed thereafter at a higher rate than the other treatments.
In 2007, cluster shading reduced anthocyanin accumulation (Figure 6⇑, Supplemental Table 2). Leaf removal caused a temporary acceleration of anthocyanin accumulation, although it did not result in higher anthocyanin concentration at harvest. Nonetheless, and in contrast to 2006, it caused an increase of the 3′-hydroxylated/3′,5′-hydroxylated anthocyanin ratio compared to C (significant differences only at 32 dpv) because of the decrease of 3′,5′-hydroxylated anthocyanin and the increase of 3′-hydroxylated anthocyanin (Figure 7⇑), although there were few treatment differences. Even though early shading did not exert a significant effect in the 3′-hydroxylated/3′,5′-hydroxylated anthocyanin ratio compared with C (exception at 5 and 20 dpv), it led to a downward trend in its value. Early shading also depressed cyanidin-3-glucoside and peonidin-3-glucoside biosynthesis as in the previous year. Even though the proportion of malvidin-3-glucoside of FV and FH vines at harvest was higher than in 2006, the concentration of this compound was lower and not different than both C and LR vines (Figure 6⇑, Supplemental Table 2). Late shading (VH) also depressed anthocyanin biosynthesis to a greater extent than early shading, but it did not alter anthocyanin profile respect to the C vines.
The proportion of acylated forms of anthocyanins was very low for all treatments and it was noticeably higher in 2007, ranging from 6.80 to 7.99% in 2006 and from 10.50 to 12.25% in 2007 (Table 1⇓). Late net shading caused a slight shift toward the synthesis of acylated anthocyanins in 2006 and to a lesser extent in 2007 and shifted the proportion of acylated anthocyanins in favor of 3-coumaroyl-glucosides in both years, particularly in vines shaded from veraison to harvest (VH and FH). The differences found in the concentration of the acylated anthocyanins were small and not consistent between the two years (Table 1⇓). Neither shading nor leaf removal seemed to cause a clear and consistent effect on the concentration of acylated anthocyanins found in the Nebbiolo berry skins.
Discussion
In FH vines, PAR was limited by the netting, thus clusters were exposed to the diffuse solar radiation. In LR vines, clusters were directly exposed to the incident solar radiation. Notwithstanding this, LR and FH vine berry temperature was similar in both years, likely because of the different air circulation conditions. Leaf removal allowed more air movement whereas the net in FH vines prevented it, giving rise to a decrease and an increase, respectively, of the expected berry temperature. In both years, the presence of net or leaf removal, under the conditions of this study, reduced berry temperature with respect to natural shading.
Berry development and composition.
The shading treatments had no impact on vine yield and cluster weight. Neither did leaf removal at fruit set in 2006, in agreement with one study (Main and Morris 2004), although other results have indicated that leaf removal had a negative effect on yield components, especially when performed at a very early stage (Petrie et al. 2003, Poni et al. 2006). In 2007, cluster sunburn damage on LR vines resulted in a heavy decrease in vine yield and cluster weight.
Early cluster shading and leaf removal caused a slight delay in berry development, but there was no effect on berry size at harvest (Figure 4⇑). This finding is in agreement with previous studies that showed little impact (Morrison and Noble 1990, Reynolds et al. 1986) or no impact (Spayd et al. 2002, Downey et al. 2004, Jeong et al. 2004, Cortell and Kennedy 2006) of fruit shading on berry development and fruit weight at harvest. Berry growth may be enhanced by increased exposure to indirect light, as long as fruit temperatures are not elevated beyond the optimum for development (Bergqvist et al. 2001). Moreover, under similar temperature conditions, berries grown without light during the initial stages of fruit growth had lower weight and diameter than berries exposed to light during the same period (Dokoozlian and Kliewer 1996), probably because of light-mediated effects on cell division and/or cell enlargement. It was also shown that sunlight exclusion in the clusters before flowering reduced berry size (Ristic et al. 2007).
Artificial fruit zone shading had a negative effect on TSS accumulation in 2007, in agreement with previous studies in which cluster shading lowered TSS concentration (Reynolds et al. 1986, Rojas-Lara and Morrison 1989, Bergqvist et al. 2001), regardless of the berry development phase in which shading was applied (Dokoozlian and Kliewer 1996). Leaf removal had no influence on TSS accumulation and content at harvest, in agreement with one study (Percival et al. 1994), although other studies suggested that leaf removal around fruit set reduced TSS concentration (Vanden Heuvel et al. 2005, Joscelyne et al. 2007, Poni et al. 2006), probably as a result of reduced whole-vine photosynthesis (Petrie et al. 2003). In our study, despite the impact of leaf removal on vine leaf area, especially in 2007, no difference was found between LR and C vine LA/yield ratio and TSS concentration in both years. TSS concentration was generally higher in 2007 than in 2006, possibly because of the more suitable value of LA/yield ratio in the former (1.8 and 1.3 m2/kg, respectively) that should improve photosynthetic activity and vine performances (Kliewer and Dokoozlian 2005), as also observed in Nebbiolo (Guidoni et al. 2008). Furthermore, it has been suggested that the effect of leaf removal on berry TSS concentration is year-dependent (Main and Morris 2004).
Anthocyanin accumulation.
Many metabolic processes stop or slow down at ~30°C in the grapevine (Coombe 1987) and with temperatures higher than 30°C skin anthocyanin accumulation can be inhibited (Spayd et al. 2002, Yamane et al. 2006). These limits are not yet well known and may depend on cultivar (Tarara et al. 2008). The lower anthocyanin accumulation observed in all treatments in 2007 compared to 2006 could have been the result of the higher berry temperature, exceeding 30°C for several hours during the day.
Although several studies have found that light exposure has a positive effect on cluster anthocyanin concentration (Rojas-Lara and Morrison 1989, Morrison and Noble 1990, Gao and Cahoon 1994, Dokoozlian and Kliewer 1996, Haselgrove et al. 2000, Bergqvist et al. 2001, Spayd et al. 2002, Jeong et al. 2004), other studies suggest that anthocyanin biosynthesis is not readily affected by sunlight (Downey et al. 2004). In the present study, leaf removal increased cluster sunlight exposure but did not alter anthocyanin concentration compared to the control. This could be the result of the elevated berry skin temperature that may have overridden the positive effects of cluster light exposure, strongly suggesting that berry skin temperature has more influence on anthocyanin accumulation than light (Spayd et al. 2002, Mori et al. 2005, Tarara et al. 2008) and that the effect of temperature can vary greatly along developmental stages (Yamane et al. 2006). At 100 μmol m−2 s−1 incident solar irradiation, the effects of light on anthocyanin biosynthesis are indeed heavily dependent on the extent to which berry temperature is elevated as a result of increased sunlight exposure (Bergqvist et al. 2001). In the present study, PAR of both C and LR reached or exceeded 100 μmol m−2 s−1, especially in 2007 (Figure 2⇑), and the temperature exceeded the threshold discussed above for several hours during the day (Figure 3⇑).
In 2006, PAR values in control and shaded vines were lower than those recorded in 2007: ~100 μmol m−2 s−1 for C vines and lower for shaded vines (Figure 2⇑). At the same time, berry skin temperature rarely exceeded 30°C. Considering that anthocyanin concentration of shaded vines was similar to that of C and LR vines, it could be assumed that, although seemingly limiting, light and temperature conditions were sufficient for anthocyanin biosynthesis in shaded and C vines. Moreover, anthocyanin concentration was higher in 2006 than in 2007, suggesting that the microclimatic conditions during that season were more favorable for the achievement of higher berry color.
Net shading in 2007 depressed anthocyanin biosynthesis beginning at the initial stages of observation and regardless of the phenological phase at which shading was applied. This result agrees with previous studies showing that reduced sunlight could be a limiting factor for anthocyanin accumulation (Rojas-Lara and Morrison 1989, Morrison and Noble 1990, Gao and Cahoon 1994, Dokoozlian and Kliewer 1996, Haselgrove et al. 2000, Bergqvist et al. 2001, Spayd et al. 2002, Jeong et al. 2004). However, in 2006, with lower fruit-zone PAR values, higher anthocyanin concentrations were reached compared to 2007, confirming that high sunlight may not be an absolute necessity for anthocyanin biosynthesis in grape berries (Downey et al. 2004). Moreover, the limiting effect of low sunlight exposure could become evident when berry temperature is also below the optimum level for anthocyanin biosynthesis. It has been observed that when shading does not alter berry temperature, it may have no impact on anthocyanin accumulation (Cortell and Kennedy 2006, Ristic et al. 2007), and an increase of anthocyanin biosynthesis can occur under low temperature conditions (20°C rather than 30°C) particularly following veraison, independently from light level (Yamane et al 2006). Low sunlight combined with high berry temperature may negatively affect anthocyanin biosynthesis (Tarara et al. 2008), but results of the present study suggest that high sunlight combined with high temperature does not necessarily decrease anthocyanin biosynthesis.
In 2007, the anthocyanin concentration of LR and C vines was higher than that of artificially shaded vines even with higher berry temperature and sunlight exposure (LR vines). Because of the imperfect set of treatments, interaction between light and temperature was not statistically testable, and therefore further studies are warranted. In both experimental years, net shading caused a decrease of 3′-hydroxylated anthocyanin concentration and an increase of malvidin-3-glucoside, especially in vines shaded from fruit set to harvest (Figures 5⇑ and 6⇑, Supplemental Table 1).
These results disagree with those found in some studies (Spayd et al. 2002, Downey et al. 2004, Cortell and Kennedy 2006, Ristic et al. 2007), but agree with others that concluded, under similar temperature conditions, berries from shaded vines show an anthocyanin profile with lower 3′-hydroxylated anthocyanin concentrations (Tarara et al. 2008). Under lower daytime temperature conditions, anthocyanin composition may have a higher proportion of 3′-hydroxylated anthocyanins (Cohen et al. 2008). In general, these studies suggest that solar radiation and temperature especially affect the biosynthesis of 3′-hydroxylated anthocyanins. The higher sensitivity to temperature of the enzyme catalyzing 3′-hydroxylated anthocyanin biosynthesis (F3′H) has been suggested in another study on Nebbiolo grapes, in which leaf removal during a warm year increased 3′-hydroxylated anthocyanin concentration compared to a cooler year, when a higher concentration was reached (Guidoni et al. 2008). This is in agreement with the results of the present study. Therefore, the various interactions between sunlight and temperature cause an alteration of anthocyanin composition, namely the proportion of anthocyanidins and the acylation mechanism. The latter in particular may be influenced by high temperatures impacting gene expression and enzyme activity in anthocyanin acyltransferase, responsible for the acylation reaction. This has also been suggested by the altering effect of years with high temperatures on this variable (Spayd et al. 2002, Downey et al. 2004).
The effect of elevated temperature and cluster shading on anthocyanin accumulation may depend, among other factors, on grape cultivar (Kliewer and Torres 1972, Price et al. 1995, Spayd et al. 2002, Downey et al. 2004, 2006). The most sensitive cultivars may be those with a high proportion of 3′-hydroxylated anthocyanins.
Net shading seems to have favored the synthesis of 3-coumaroyl-glucosides in this study, especially when nets were affixed after veraison (Table 1⇑). In Nebbiolo, the concentration in the skins of 3-coumaroyl-glucosides and of acylated anthocyanins was generally very low compared to nonacylated anthocyanins.
Conclusions
During two years, the attempt to modify the fruit-zone microclimate by netting or leaf removal has had only limited success. The shading nets should have decreased sunlight and berry temperature, but only the former was accomplished as the netting increased air temperature by reducing air circulation in the fruit zone. This did not cause any particular problem in the cooler year (2006), but in the warmer year (2007) resulted in elevated berry temperature underneath the shading nets, causing an unfavorable microclimate for anthocyanin biosynthesis.
Fruit-zone shading with nets delayed berry development without affecting yield components at harvest. In 2006, all treatments reached the optimum anthocyanin concentration for Nebbiolo, but in 2007 anthocyanin concentration was much lower for all treatments even though sunlight conditions were potentially more favorable. In 2007, artificial shading not only reduced total anthocyanin accumulation but also restricted 3′-hydroxylated anthocyanin biosynthesis and enhanced acylation with p-coumaroyl acid.
Fruit-zone leaf removal increased sunlight exposure but did not increase anthocyanin accumulation. It caused excessive sunburn in exposed clusters and therefore qualitative and quantitative crop damage, rendering this practice inappropriate for areas with comparable climate conditions. Under these conditions, natural shading was more favorable for anthocyanin accumulation during both years of the study.
These results confirm the importance of temperature on anthocyanin concentration, but interaction with sunlight can also influence anthocyanin profile. Further studies are necessary to improve knowledge of the interactions between sunlight and berry temperature. From a practical point of view, fruit quality in Nebbiolo will benefit from conditions under which berry temperature does not rise over 30°C, regardless of sunlight exposure.
Footnotes
Acknowledgments: This study has been financially supported by MIUR Project of National interest (PRIN). The authors thank the Azienda Agricola Sandrone Luciano in Barolo, where the trial took place, and Luis Sanchez for the critical review of the study and for his support.
Supplemental data is freely available with the online version of this article at www.ajevonline.org.
- Received January 2009.
- Revision received June 2009.
- Accepted September 2009.
- Published online March 2010
- Copyright © 2010 by the American Society for Enology and Viticulture