Impaginato 427 Adv. Hort. Sci., 2023 37(4): 427­432 DOI: 10.36253/ahsc­13659 Use of the biostimulant Retard Cherry® as a strategy to delay blooming period in sweet cherry trees J.A. Yuri *, M. Palma, Á. Sepúlveda, J. Sánchez­Contreras, M. Moya Centro de Pomáceas, Facultad de Ciencias Agrarias, Universidad de Talca, Talca, Chile. Key words: Bloom, frost damage, fruit set, global warming, phenology, Prunus avium L. Abstract: Spring frosts are a limiting factor in sweet cherry production in cen­ tral­southern Chile. Sweet cherry trees cv. ‘Regina’ and ‘Sweetheart’ were stud­ ied to evaluate the effect of foliar application of a biostimulant (Retard Cherry®) prior to leaf fall on the bloom delay, fruit set, fruit drop, yield and quality. Data were compared to a non­product control. The study was conduct­ ed in the Maule Region, Chile. Results showed that the use of Retard Cherry® delayed full bloom by 6­8 days between cultivars compared to the control; however, there was no delay in the harvest date. The climatic conditions favored high fruit set (37%­49%) and low fruit drop (63%­70%) between culti­ vars in both treatments. Regarding fruit quality, no differences in size, soluble solids concentration and color were observed with the product, but a decrease in firmness were observed for ‘Regina’. These results show that Retard Cherry® is an effective tool in delaying bloom, providing trees with more favorable cli­ matic conditions for pollination and fruit set. 1. Introduction Sweet cherry (Prunus avium L.) is a fruit tree of temperate climate, native to Asia Minor (Iezzoni et al., 2017). Its cultivation has been widely distributed in Mediterranean climate countries such as Turkey, Italy and Spain, in the United States and in Middle Eastern countries such as Iran (Bujdosó and Hrotkó, 2017). In the southern hemisphere, Chile is the main exporter, with a volume of 350 thousand tons in the 2020/21 sea­ son in a cultivated area of 48,960 ha (iQonsulting, 2021; ODEPA, 2021). Emergence from recess and regulation of blooming time in deciduous fruit trees involves a combined process of winter cold and spring warmth accumulation (Fadón et al., 2020). In sweet cherry trees, temperature conditions during the cold accumulation phase are estimated to be the main driver of bloom (Fadón et al., 2021). In mild winter areas, insuffi­ cient cold accumulation can cause delayed bloom, floral malformations, and low fruit production in the trees. In contrast, when high cold accumu­ lation accompanied by spring warmth occurs in this period, bloom may be (*) Corresponding author: ayuri@utalca.cl Citation: YURI J.A., PALMA M., SEPÚLVEDA Á., SÁNCHEZ­ CONTRERAS J., MOYA M., 2023 ­ Use of the bio‐ stimulant Retard Cherry® as a strategy to delay blooming period in sweet cherry trees ­ Adv. Hort. Sci., 37(4): 427­432. Copyright: © 2023 Yuri J.A., Palma M., Sepúlveda Á., Sánchez­Contreras J., Moya M. This is an open access, peer reviewed article published by Firenze University Press (http://www.fupress.net/index.php/ahs/) and distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Competing Interests: The authors declare no competing interests. Received for publication 1 September 2022 Accepted for publication 12 September 2023 AHS Advances in Horticultural Science Short note https://doi.org/10.36253/ahsc-13659 https://dergipark.org.tr/en/search?q=%22+phenology%22 http://www.fupress.net/index.php/ahs/ http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ Adv. Hort. Sci., 2023 37(4): 427­432 428 advanced and flower opening concentrated, increas­ ing the risk of spring frost damage (Herrero et al., 2017), which can damage flowers and buds (Miranda et al., 2005) and generate a large decrease in crop production and profitability (Kaya et al., 2021). Moreover, low temperatures can affect blooming synchrony between variety and pollinizer, limiting pollinator activity, delaying pollen tube development and fruit set (Guo et al., 2015). Climate change in the near future is expected to generate warmer springs with greater thermal fluctu­ ations, which may alter plant phenology and increase the risk of spring frost damage in species of temper­ ate climate (Augspurger, 2013). Some tools used to reduce spring frost damage are overhead sprinkler irrigation, wind towers with heaters and heated macro­tunnels. However, these technologies have a high implementation cost and are not always effec­ tive enough in control (Yuri et al., 2017). A complementary strategy to avoid frost damage in fruit trees is to delay tree bloom, shifting it to a period with greater climatic stability, more favorable for pollination and fruit set (Liu and Sherif, 2019). Plant growth regulators evaluated in stone fruits can extend bud dormancy (Durner and Gianfagna, 1991), delay bloom (Ebel et al., 1999) or synchronize bloom with another variety, as well as delay harvest (Basak et al., 1998). In some cases, however, the application of these products can cause flower abscission, low fruit set and yield (Crisosto et al., 1990; Liu and Sherif, 2019). On the other hand, the use of foliar biostimulant, as sustainable alternatives to plant growth regulators, could increase flower bud resis­ tance to winter cold and delay bloom, without detri­ mental effects on fruit production. This study aimed to evaluate the effect of foliar application of the biostimulant Retard Cherry® on bloom delay before leaf fall as well as on fruit set, fruit drop and fruit quality in ‘Regina’ and ‘Sweetheart’ sweet cherry trees in central Chile. 2. Materials and Methods Plant material and experimental site The study was conducted during the 2018/19 growing season in two commercial orchards of sweet cherry (Prunus avium L.) located in San Clemente, Maule Region, Chile (35°32’ S, 71°27’ W, 230 m a.s.l.), at less than 5 km between them. In one orchard, ‘Regina’ sweet cherry trees on ‘Gisela­6’ rootstock were evaluated; they were planted at 4.0 × 1.8 m in 2015. In the other, ‘Sweetheart’ sweet cherry trees on ‘Colt’ rootstock were evaluated; they were plant­ ed at 5.0 × 2.5 m in 2010. Both cultivars were trained in Central Leader. Orchards management were car­ ried out according to commercial standards in the region. Seasonal environmental conditions are sum­ marized in Table 1. Climatic data was recorded by an automatic weather station Vantange 2 (Davis Instruments, Hayward, CA, USA) near the orchards. During the Table 1 ­ Environmental conditions 2018, San Clemente, Chile Note: Chill Hours: Weinberger, 1950. Chill Units (Utah): Richardson et al., 1974. GDH: Anderson and Seeley, 1992. GDD: Stanley et al., 2000). Variable Annual May 1­Jul 31 Aug 1­Dec 25 October Air temperature (°C) Mean 13.6 7.4 13.7 13.3 mean maximum 21.4 13.3 21.3 20.2 mean minimum 7.3 3.1 7.2 7.6 Maximum 35.6 25.2 35.4 26.4 Minimum ­5.1 ­5.1 ­5.1 1.5 Relative humidity (%) mean minimum 46.1 66.8 42.1 44.5 Precipitation (mm) 493 234 205 37.8 Solar radiation (MJ m­2) 5.718 573 2.677 829 Chill Hours 1.433 857 Chill Units (Utah) 478 1.308 GDH 72.507 30.59 6.65 GDD (base 10°C) 1.807 697 123 Yuri et al. ‐ Use of the biostimulant to delay blooming period 429 blooming period, environmental conditions favourable to bee activity were calculated, defined as the number of hours per day with air temperature higher than 15°C and solar radiation higher than 300 W m­2. Experimental design The experimental design was a randomized by complete block divided into two treatments (5,000 m2 per treatment). The treatments were: (1) control without product; (2) foliar application of Retard Cherry® (AM Ecological S.A., Chile). The product was applied twice at doses of 1.0 and 0.5 L/ha, prior to 50% leaf drop: for ‘Regina’ on March 26 and April 9; for ‘Sweetheart’ on March 15 and 30. Applications were made with a conventional hydro­pneumatic sprayer (Parada SpA, Santiago, Chile) with a spray volume of 1,200 L/ha. All measurements were made in 10 replicates, each consisting of two branches per replicate, trees per replicate, considering three edge rows per side. Bloom delay Blooming evolution (%) was determined weekly from stage ‘first white’ to ‘full bloom’, counting the number of open flowers per date. The full bloom date was defined when 80% had flowered. Bloom delay was determined by subtracting the days between the full bloom date of the control and that of the treatment. Fruit set, fruit drop and yield Fruit set and fruit drop were monitored on the same branches studied at bloom. Fruit set (%) was evaluated 20 days after the full bloom evaluation by counting the number of fruits formed in relation to the total number of flowers per branch. Fruit drop (%) was determined by the number of fruits that did not reach harvest in relation to the number of fruits formed. Yield (kg/tree) was also determined from individual trees. Fruit quality Harvest date was determined on the basis of fruit color. Evaluation of weight (g), diameter (mm), color, firmness, and soluble solids concentration (SSC) con­ sidered a sample of 50 fruits per treatment. Color was determined visually by scale (light red = 1, red = 2, mahogany red = 3, dark mahogany = 4 and black = 5). Firmness (g mm­1) was measured with a FirmTech II texturometer (BioWorks Inc, Wamego, USA). SSC (°Brix) was measured on the same fruits with a digital refractometer (Atago, PLAS­BX/ACID5, Japan). Statistical analysis The data obtained underwent an analysis of vari­ ance (ANOVA) and the means were compared with the Tukey test (P≤0.05). When necessary, a transfor­ mation of the data was carried out. Analysis was per­ formed with the Statgraphics Centurion XVI program (Warrenton, Virginia, USA) and the figures were gen­ erated using SigmaPlot 10 software (WPcubed GmbH, Germany). 3. Results Sweet cherry trees treated with Retard Cherry® showed a 6­8 day delay in the full bloom date with respect to the control, with a greater delay in the case of ‘Regina’ (Table 2, Fig. 1). Although the appli­ cation of Retard Cherry® delayed bloom, it had no effect on harvest date (Table 2). A similar level of fruit set was maintained Fig. 1 ­ Effect of foliar application of Retard Cherry® on blooming dynamic in sweet cherry trees cv. Regina. Table 2 ­ Effect of foliar application of Retard Cherry® on the date of phenological stage in sweet cherry trees ‘Regina’ and ‘Sweetheart’ Cultivar/treatments 80% full bloom Harvest ‘Regina’ Control 09­Oct 25­Dec Retard Cherry 17­Oct 25­Dec ‘Sweetheart’ Control 27­Sep 21­Dec Retard Cherry 02­Oct 21­Dec Adv. Hort. Sci., 2023 37(4): 427­432 430 between treatments with an average of 49% for ‘Regina’ and 37% for ‘Sweetheart’ (Table 3). Even though fruit drop was numerically higher in the treat­ ment with Retard Cherry®, it was not statistically sig­ nificant, which is reflected in tree yields, with a mean of 8.6 kg/tree for ‘Regina’ and 14 kg/tree for ‘Sweetheart’ (Table 3). The ‘Regina’ sweet cherry trees treated with Retard Cherry® showed 18% lower firmness than the control, with no change in fruit color, while ‘Sweetheart’ showed a higher incidence of fruit with lower color, although this was not noticeable to con­ sumers. Fruit weight, diameter, and SSC were not affected by treatment (Table 4). 4. Discussion and Conclusions The results of fruit set are in concordance with those of Raffo and Curetti (2021) who reported a delay of up to 10 days in leaf emergence and full bloom of several sweet cherry cultivars in Rio Negro, Argentina. A greater assimilation and subsequent transport of reserves to the plant, prior to leaf fall, would allow an adequate dormancy and a more homogeneous bloom, effect that could be favored by the foliar application of biostimulants in autumn. Fruit set in sweet cherry trees is normally low, and highly dependent on pollen availability and climatic conditions during and after pollination (Hedhly et al., 2007). In Chile, sweet cherry growers use the follow­ ing scale of fruit set intensity: high 34­40%; medium 15­20%; low 8­10% (C. Tapia, pers. comm, October 11, 2021). Sagredo et al. (2017) found that the effective pol­ lination period in ‘Regina’ sweet cherry cultivars was about 5 days post anthesis, with the highest fruit set levels occurring 2­3 days post anthesis. Similarly, Zhang et al. (2018) showed that the peak pollen ger­ mination and stigma receptivity of certain sweet cherry cultivars occurred 2­3 days post anthesis under three ambient temperature scenarios. In addi­ tion, the pollen tube required at least 48 h to reach the ovule. Regarding to bee activity, more than 100 bee vis­ its per minute were observed by Koumanov and Long (2017) with proper hive management, temperatures above 18°C and wind speed less than 16 km h­1. On the other hand, Vicens and Bosch (2000) indicated that bee activity (A. mellifera) was fully active with air temperature above 14°C and solar radiation greater than 300 W m­2. The high fruit set obtained in this study could have been favored, among others, by the prevalence of suitable climatic conditions for bee activity during blossom and for fruit set of that season (Table 5; Fig. 2). No frost was observed during blossom in both orchards. Post­bloom environment of the trees treat­ ed with Retard Cherry® was more stable, with an average daily mean air temperature about 1­2 °C higher than that measured in the control trees one week earlier. Table 5 shows the great difference in the condi­ tions for bee flight at blooming between the two cul­ tivars. The application of Retard Cherry, by delaying blooming, had a much greater effect on ‘Sweetheart’, as it distanced it from the riskiest date of low tem­ Table 3 ­ Effect of foliar application of Retard Cherry® on fruit set, fruit drop and fruit yield in ‘Regina’ and ‘Sweet­ heart’ sweet cherries Means in a column followed by the same letter do not differ sta­ tistically, according to Tukey test (P≤0.05). n= 10/treatment. ND= no detected. Table 4 ­ Effect of foliar application of Retard Cherry® on fruit quality in ‘Regina’ and ‘Sweetheart’ sweet cherries at harvest Means in a column followed by the same letter do not differ sta­ tistically, according to Tukey test (P≤0.05). n = 50/treatment. Cultivar/treatments Fruit set (%) Fruit drop (%) Yield (kg/tree) ‘Regina’ Control 50 a 65 a 8.6 a Retard Cherry 47 a 75 a 8.6 a P­value 0.45 0.16 ­ ‘Sweetheart’ Control 33 a ND 14.0 a Retard Cherry 41 a 63 14.0 a P‐value 0.41 ­ ­ Cultivar/ treatments Weight (g) Diameter (mm) Color (1­5) Flesh firmness (g mm­1) SSC (°Brix) ‘Regina’ Control 12 a 28 a 4.0 a 231 a 20 b Retard Cherry 12 a 28 a 4.4 a 189 b 21 a P‐value 0.41 0.31 0.09 0.00 0.01 ‘Sweetheart’ Control 13 a 29 a 4.2 a 274 a 21 a Retard Cherry 12 b 29 a 3.6 b 271 a 20 b P‐value 0.04 0.46 0.01 0.80 0.00 Yuri et al. ‐ Use of the biostimulant to delay blooming period 431 peratures and frost in early spring; something similar occurred in the case of ‘Regina’, although with less intensity, as it is a later blooming cultivar. Harvest date was not affected using Retard Cherry® on sweet cherry trees, although there was evidence of lower fruit firmness on Regina and slight­ ly less coloration on ‘Sweetheart’ (Tables 2 and 4). The fruit ripening stage is characterized by a rapid increase in weight and size, due to an increase in cell size, leading to a reduction in firmness. Sugar content increases, keeping acids relatively constant; however, color is the one that shows the greatest changes, being a relevant factor in determining the harvest date in sweet cherries (Tudela et al., 2005; Muskovics et al., 2006). In previous studies, Raffo and Curetti (2021) reported that the use of Retard Cherry® showed a marked delay in color development of ‘Sweetheart’ sweet cherries. The use of GA3 allows delay harvest in sweet cherry trees and has also proven to increase fruit size, firmness and SSC (Basak et al., 1998; Horvitz et al., 2003; Raffo and Curetti, 2021). Therefore, GA3 would be a complementary tool to the use of foliar biostimulant Retard Cherry®, to extend the harvest window, with good quality fruit and better prices. 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