Another disadvantage of growing table grapes in subtropical areas is that high temperatures during ripening can inhibit anthocyanin biosynthesis in the berries from V. labrusca and hybrid cultivars . This results in inadequate fruit color, and thereby a decrease in market acceptance and the potential economic value of the commodity . The seedless table grape Selection 21, a new hybrid of V. vinifera × V. labrusca recently developed by the Grape Genetic Breeding Program of Embrapa Grape and Wine, Brazil, obtained from the cross of [Arkansas 1976 × ] × “BRS Linda,” is a clear example of a cultivar that lacks red color development when grown in subtropical regions. The plant growth regulator ethephon, an ethylene-releasing agent, has long been known to improve berry color when applied at véraison . More recently, the application of –cis-abscisic acid has also been shown to stimulate anthocyanin accumulation and thereby improve berry color . S-ABA appears to be more effective than ethephon in enhancing grape color and it has other potential benefits compared to ethephon,large pot with drainage including a shorter postharvest interval, and an exemption from tolerance in most countries.
The introduction of S-ABA as an active ingredient in a commercial plant growth regulator prompted many studies on V. vinifera cultivars under temperate climate conditions. Such studies have shown that the efficacy of S-ABA varies with the cultivar , the S-ABA concentration , the time of application and the environmental conditions . Abscisic acid is an important regulator of ripening and anthocyanin biosynthesis in grape berries . Studies have shown that exogenous application of S-ABA can significantly increase the activity of a wide range of genes involved in anthocyanin biosynthesis . Most of these studies tested the effects of a single application of S-ABA before or during véraison. However, studies of the effects of S-ABA several applications at different concentrations and timings following véraison are still needed to optimize the use of this plant growth regulator in table grape cultivation . In grapes, the anthocyanin biosynthesis pathway involves multiple steps that are controlled by MYB transcription factors, such as VvMYBA1 and VvMYBA2 . In red varieties, the VvMYBA1 gene is only expressed after véraison. Both VvMYBA1 and VvMYBA2 regulate anthocyanin biosynthesis during ripening by strictly controlling the expression of the canonical UDP-glucose:flavonoid 3-Oglucosyltransferase gene . Determining how long grape berries are competent to induce the expression of anthocyanin biosynthetic genes may help determine the optimal time, number, and frequency of S-ABA applications. Currently, little is known about the potential benefits of multiple applications, which may be desirable if a single application results in an insufficient response.
The aim of the present study was to determine the effects of S-ABA applications at different concentrations and times on the quality and biochemical properties of berries from the seedless grape Selection 21 hybrid during three growing seasons in the region of Paraná, Brazil. We evaluated a variety of parameters including: grape color development, berry phenolic profiles, and gene expression of transcriptional regulators and biosynthetic enzymes of the anthocyanin pathway after treatments with S-ABA. The results of this report indicate that two S-ABA applications during and after véraison extend the competency of grape berries to respond to ABA and induce the accumulation of anthocyanins, resulting in higher grape berry coloration. The study was conducted during three consecutive seasons in a commercial vineyard located in Marialva, state of Paraná , Brazil , using 4-year-old vines of hybrid seedless grape Selection 21 grafted onto IAC 766 Campinas root stock. According to the Köppen classification, the climate of the region is Cfa , with an average temperature below 18◦C in the coldest month and above 22◦C in the hottest month and an average annual rainfall of 1,596 mm. The region’s soil is classified as dystroferric red latosol . The vines were trained using a bilateral overhead trellis system, where vines were spaced at 2.5 m × 2.5 m , and each vine had 6.25 m2 total canopy area. Cane pruning was performed during the 2013, 2014, and 2015 seasons and was followed by application of 3% hydrogenated cyanamide to the two apical buds to induce and standardize sprouting. The number of canes per vine was evenly adjusted to 40 and the number of shoots per vine was also established to 40 .
Considering that a grape bunch of the Selection 21 weighs on average 460 g, the load per vine is 18.40 kg, which represents an estimated yield of 29.44 tons/ha. Furthermore, to avoid drifting, a non-treated vine was left as side border between two treated vines, which almost duplicated the experimental area. In each plot, all grape bunches were treated , and the bunch samples were collected from random positions at each side of the canopy to account for intracanopy variability. Control plants were not subjected to any treatment, instead, they were sprayedwith water at the same time and following the same procedures as the S-ABA treatments. The effects of applying S-ABA isomer at different concentrations and times were evaluated in terms of berry quality traits. ProTone R, the commercial growth regulator used in this study, has an active ingredient concentration of 100 g/L S-ABA. As shown in Figure 1, the initial treatments tested in the 2013 and 2014 seasons corresponded to: control or water spray, 200 mg/L S-ABA application at 7 days after véraison , 400 mg/L S-ABA application at 7 DAV, 200 mg/L S-ABA application at 7 DAV plus an additional application at 21 DAV, and 400 mg/L S-ABA at 7 DAV plus an additional application at 21 DAV. In the 2015 season, only the control and treatments of 400 mg/L S-ABA with one or two applications were performed and analyzed. Berry samples from the 2015 season were collected from each treatment at four different times: 7 DAV , 14 DAV, 28 DAV, and 35 DAV for further targeted analyses . For all seasons, a randomized complete block experimental design was used, with five treatments and three to four replicates, and with each plot consisting of one vine . Véraison was determined by measuring soluble solid content and firmness of grape berries randomly sampled in the experimental vineyard. At véraison, the mean grape SSC concentration was 9◦Bx, and 20% of the berries in more than 50% of the grape clusters presented softening . The berries presented a mean of 11◦Bx at 7 DAV, the time of the first S-ABA application, and a mean of 13◦Bx at 21 DAV, the time of the second S-ABA application. Ten grape clusters representative of each plot were marked prior to treatment application. For treatment applications, grape clusters were sprayed in the morning using a knapsack sprayer at a pressure of 568.93 psi with JA1 hollow cone nozzle tips at a volume of 800 L/ha to provide complete and uniform coverage. In addition, 0.3 mL/L of Break-Thru R a non-ionic surfactant was added to all treatments. During the trials, the standard regional cultivation practices with regard to nutrition, weed control,drainage collection pot and pest and disease management were used. Clusters of each plot were manually harvested when SSC stabilized . The clusters were cleaned, and damaged berries were discarded. Color coverage of the bunches was determined using 10 grape clusters per plot by visually rating the clusters on a scale of 1–5 using the following scale: 0–20%, 21–40%, 41–60%, 61–80%, and 81–100% coverage . The same grape clusters used for evaluating color coverage measurements were used for berry sampling. For physicochemical analyses, two berries were collected from the upper, middle, and lower portion of each grape bunch, yielding a total of 70 berries per plot. Total anthocyanins and color index of red grapes were determined in berry samples from all seasons. The following variables were analyzed only for the 2013 and 2014 seasons: color coverage, total polyphenols, and berry firmness. All physiological analyzes were performed in the Laboratory of Fruit Analysis of the Agricultural Research Center, Londrina State University, Brazil.
The total anthocyanin concentration of the berries was determined using 30 berries per plot, which were frozen and stored at −20◦C. The berry skins were removed using tweezers, taking care to remove only the skin, without pulp. The skins were washed once with water, followed by washing in deionized water and drying with absorbent paper. A 5-g skin sample was then placed in a polystyrene tube containing 50 mL of acidified methanol and stored in the dark for 48 h at room temperature. The tubes were then removed from the dark and stirred manually for 5 s. Absorbance was determined using a Genesys 10S spectrophotometer at 520 nm with the solvent as blank. In the 2015 season, three grape clusters of uniform size and at the same phenological stage were identified in each plot, and two berries were collected randomly from each bunch at each sampling time. The berry skins were removed, frozen, kept at −80◦C, and transferred to the Department of Viticulture and Enology of the University of California, Davis, CA, United States, for further analyses. The skins were then placed in liquid nitrogen and ground using a TissueLyser II . RNA was extracted using 0.5 g of ground tissue based on the protocol described by Blanco-Ulate et al. . RNA concentration and purity were determined using a NanoDrop 2000c Spectrophotometer , and RNA integrity was checked by electrophoresis on 1.5% agarose gel. Reverse transcription was performed using 1 µg of RNA and M-MLV Reverse Transcriptase according to the manufacturer’s instructions. qRT-PCRs were performed using the SYBRRGreen PCR Master Mix kit . The PCR program consisted of 70◦C for 10 min, 36 cycles at 42◦C for 2 s, and 37◦C for 50 min. VvActin was used as the reference gene and processed in parallel with the genes of interest. Gene sequences used for primer design were obtained from the GenBank of the National Center for Biotechnology Information using Primer-BLAST software . The relative levels of target gene expression were calculated using the formula 2 . The linearized values correspond to the relative gene expression within a given sample and are comparable across genes. Four biological replicates of S-ABA treated and control grape berries were used to obtain the relative gene expression data. Application of abscisic acid increased the total anthocyanin concentration in berry skins of the seedless grapeSelection 21 during the 2013 and 2014 seasons, regardless of the S-ABA concentration and time of application . However, berries that received 400 mg/L of S-ABA at 7 and 21 DAV had significantly higher, almost two to three times more, anthocyanin concentrations than other treatments. According to the CIRG, berries from control treatments had a green to a yellow color in both seasons . In 2013, berries treated with one or two applications of 200 mg/L S-ABA or one application of 400 mg/L S-ABA at 7 DAV, and those in the 2014 season that were treated with one application of 200 mg/L S-ABA developed a pink color . Remarkably, berries of the 2013 season treated with two applications of 400 mg/L S-ABA and berries of the 2014 season treated with one or two 400 mg/L S-ABA applications, developed a stronger red color . For both the 2013 and 2014 seasons, color coverage was lowest in control grapes and highest in grapes treated with two applications of 400 mg/L S-ABA. Increase in total polyphenols was evident in grapes subjected to two 400 mg/L S-ABA applications during the 2013 and 2014 seasons. These berries also presented the lowest mean berry firmness . Importantly, the increased softening due to S-ABA application did not result in higher frequency of cracked berries in any of the studied seasons. Qualitative assessment of berry cracking was performed visually. Further analyses of the effect of 400 mg/L S-ABA treatments on CIRG, total and individual anthocyanins concentrations, and gene expression of transcription factors and biosynthetic enzymes were performed with grape berries collected from the 2015 trial. Measurements of CIRG confirmed previous results obtained during the 2013 and 2014 seasons, at the time of harvest , grapes treated with two S-ABA applications had the highest CIRG values . Grape bunches from the control treatment presented pink berries , whereas those treated with one or two applications of S-ABA had red berries . As determined in the previous seasons, berries treated with 400 mg/L S-ABA also presented higher total anthocyanin content than the control at 14 and 28 DAV .