Improved Yield, Fruit Quality, and Shelf Life in ‘Flame Seedless’ Grapevine with Pre-Harvest Foliar Applications of Forchlorfenuron, Gibberellic Acid, and Abscisic Acid

Grapes (Vitis vinifera L.) are one of the most favorite and common fruits in the world for its desirable scent and high soluble solid content. Fruit quality is more influential than yield in the production of excellent table grapes (Peppi et al. 2006). Grape quality is remarkable for the producer, consumer, and the exporter. Important characteristics that participate in the quality of grapes include their size, texture, taste and color. These characters could be modified with the use of plant growth regulators (PGRs) if their use is permitted under national regulations as, for example, in the USA, Italy, Australia, and Chile (Ferrara et al. 2015). Previous studies presented that plant growth regulators play an important role in fruit production, such as increase in berry weight of grape (Casanova et al. 2009), enlargement of fruit size in grape (Xu et al. 2019), apple (Muchjajib et al. 2016) and mandarin (Duarte et al. 2006), improving fruit set in mango (El Gammal et al. 2015), reducing fruit drop in apple (Khandaker et al. 2015), enhancing yield and quality of pomegranate (Adi Reddy & Manohar Prasad 2012), and delaying ripening of grapes (Ponce et al. 2002). Forchlorfenuron (CPPU), a synthetic cytokinin, and gibberellic acid (GA₃) are commonly applied by table grapes producers to improve berry size and firmness (Du Plessis 2008; Zoffoli et al. 2009; Xu et al. 2019). GA₃ stimulates cell expansion in the berries, while CPPU causes cells division (Ben-Arie et al. 1998). However, Korkutal et al. (2008) reported that GAs applied at too early time with extremely high concentration decreases vine vigor, which negatively impacts the fruit yield. ‘Flame Seedless’ has a particularly durable response to CPPU, in particular berry size (Melillo 2005); however, it reacts with reduced color intensity.

CPPU and GA₃ increase berry size as a desirable effect, but they also inhibit color development, which restricts their use on colored grape cultivar. A dosage of 8 g per acre of CPPU, applied to ‘Flame Seedless’ at fruit set increased berry size but also reduced color and delayed maturity (Peppi & Fidelibus 2008). Ethephon was applied to table grapes to enhance berry color, but its impacts are inconsistent and can lead to berry softening (Szyjewicz et al. 1984; Peppi et al. 2006). Abscisic acid (ABA) was found as an efficient alternative to ethephon for enhancing the color of table grapes (Koyama et al. 2019; Lurie et al. 2010). It is easier for application due to less restrictive period of its use, and an exemption from the requirement to have a tolerance in most countries (Ferrara et al. 2015), but the high cost has prevented the development of its practical applications for viticulture (Peppi et al. 2006). However, recently, the introduction of ABA as an active ingredient of a commercial plant growth regulator (ProTone^®) motivated many studies on Vitis vinifera cultivars grown under temperate climate conditions. When ABA is applied to grape, the ripening stage is accelerated by reducing the chlorophyll quantity and improving the color change (Gény et al. 2005). Many investigations have shown that the application of ABA may significantly stimulate the activity of several genes involved in and related to anthocyanin biosynthesis (He et al. 2010). The physiological activity of ABA sprays was proved by determining the mRNA levels of the key anthocyanin gene – flavonoid glucosyltransferase (UFGT) from the skin of ‘Crimson Seedless’ grapes treated with ABA applied at veraison (change of color) stage. The level of UFGT mRNA increased considerably within one day after application (Olivares et al. 2017). ABA can increase the anthocyanins’ content and enhance the color of ‘Flame Seedless’ grapes, however, it likewise causes fruit softening, which is an undesirable effect (Peppi et al. 2006). The combined use of GA₃, CPPU and ABA might be advantageous if the result will cause large, firm berries of sufficient color. Thus, the aim of this study was to assess if the application of CPPU and GA₃ followed by ABA might increase the size and firmness of ‘Flame Seedless’ grapes without extremely inhibiting coloring. Moreover, the effects of GA₃, CPPU and ABA on the yield, fruit quality at harvest day and on the berry shelf life of ‘Flame Seedless’ grape was investigated.

Coinciding increase in the yield of the grapevines by different GA₃ and CPPU treatments was reported earlier (Zoffoli et al. 2009; Al-Obeed 2011; Dimovska et al. 2011; Abu-Zahra & Salameh 2012). Strydom (2013) showed an increase in the yield of grapevines ‘Flame Seedless’, ‘Redglobe’, and ‘Crimson Seedless’ by spraying with CPPU. The role of pre-harvest application of GA₃ + CPPU in increasing fruit yield was previously stated in mango and grapes (Notodimedjo 2000; Han & Lee 2004).

Plant growth regulators, especially GA₃ and CPPU are frequently applied at low concentrations to seedless table grape cultivars in order to increase berry and cluster weight (Zabadal & Bukovac 2006; Han & Lee 2004; Retamales et al. 1995; Notodimedjo 2000; Lee & Han 2001; Du Plessis 2008; Zoffoli et al. 2009; Gholami et al. 2010). Grapevines are commonly treated from flowering to fruit stage with GA₃ and CPPU to extend the flowering period and increase cluster weight and length (Li et al. 2015). The effect of GA₃ on stimulating cell division and cell expansion, and on increasing fruits size was previously reported (Liu et al. 2006; Valero 2010; Raath 2012). GA₃ is also reported to stimulate growth by promoting plasticity of the cell walls and the hydrolysis of starch into sugars that reduces the cells’ water potential, inducing the entry of water into the cells and causing elongation and expansion (Marini 2006). CPPU has been revealed to induce both division and elongation of cells, which increases berry size when applied after fruit set to berries (Dokoozlian 2000). The combined treatment of GA₃ and CPPU increased the clusters and berries’ weight more than GA₃ or CPPU alone, suggesting the synergistic effect between cytokinin and gibberellin. This result goes in line with those of Zoffoli et al. (2009), Ferrara et al. (2014), and Xu et al. (2019).

In this study, decrease in berry firmness was evident in grapes subjected to GA₃ + CPPU + ABA treatment in both seasons. ABA application may negatively affect berries’ firmness, a critical characteristic for the profitable postharvest handling of grapes for the fresh market because it affects shelf life and transportability (Batista et al. 2015). ABA application is well known to cause decrease and loosening rigidity of the cell wall, leading to fruit softening and berries’ cracking (Thomas et al. 2008; Gambetta et al. 2010). Treatment of grapes with ABA can result in the induction of cell wall degrading genes, as polygalacturonases that promote pectin depolymerization and solubilization (Koyama et al. 2010). Such an effect of ABA on grape firmness was also recorded in ‘Flame Seedless’, (Peppi et al. 2006) in ‘Red Globe’ (Peppi et al. 2007), and ‘Crimson Seedless’ (Lurie et al. 2010) grapes, where it induced softening similar to that caused by ethephon applications.

Our results concerning berry firmness are consistent with the results from the previous work on other cultivars. Ben-Arie et al. (1998) found that GA₃ in combination with CPPU increased berry firmness of ‘Superior Seedless’ grape. Melillo (2005) and Avenant and Avenant (2006) found that ‘Redglobe’ berry firmness increased when CPPU was applied at veraison stage, and Peppi and Fidelibus (2008) reported a linear increase in ‘Flame Seedless’ berry firmness with CPPU at various concentrations. The increase in berries firmness by GA₃ and CPPU application could be attributed to their influence on delaying or reducing different phases of ripening (softening). Moreover, application of GA₃ and CPPU leads to delay senescence (Abeles et al. 1992). Besides, the lowest TSS, increased firmness, and delay of color development may be a result of the role of GA₃ and CPPU in delaying berry maturation (Zabadal & Bukovac 2006). GA₃ application should determine a reduction of sugar accumulation because of the occurrence of larger berry size (Crupi et al. 2016). On the contrary, girdling either alone or in combination with GA₃ caused an increment of TSS in mature grapes as found by Abu-Zahra and Salameh (2012).

Fruit color is an important factor in determining fruit marketability, along with total soluble solids (TSS) and titratable acidity (TA). Lowered TSS content, increased TA (Zoffoli et al. 2009) and lowered anthocyanin concentration (Peppi & Fidelibus 2008) due to GA₃ and CPPU were confirmed in this study. Increased TA is related to a delay in harvest date. The reduction of sugar accumulation as a result of GA₃ application may be connected with the higher berry size (Crupi et al. 2016). However, according to the maturity indices of the UNECE (2011), a minimum value of 16°Brix is required for ‘Flame Seedless’ and, in this study, GA₃ and CPPU resulted in a TSS of 18°Brix and higher. GA₃ and CPPU delay fruit maturity because of lower TSS, pH and slower color accumulation (Ben-Arie et al. 1998 and Du Plessis 2008). For example, 3 mg·dm⁻³ CPPU, applied to ‘Crimson Seedless’ at 6 to 10 mm berry diameter, resulted in increased TA but lower anthocyanin concentrations (Strydom 2013). In this study, exogenous application of ABA enhances the color of grapes by improving the anthocyanin synthesis and accumulation in the berries’ skin (Cantín et al. 2007 and Koyama et al. 2018). Consistent with the results obtained for ‘Flame Seedless’ grapes, anthocyanin concentrations increased with the application of 300 mg·dm⁻³ ABA during veraison stage (Peppi et al. 2006). In addition, Koyama et al. (2018) reported that application of ABA 400 mg·dm⁻³ resulted in a higher accumulation of the individual anthocyanins and total anthocyanins. Moreover, in our experiment, anthocyanin content was considerably increased by GA₃ + CPPU + ABA treatment. Han and Lee (2004) reported that ABA added to GA₃ and CPPU increased anthocyanin content more than two-fold as compared to the control. The decrease of anthocyanin content by GA₃ and/or CPPU could be the result of their effect on phenylalanine ammonia-lyase (PAL) activity, which is considered as the key enzyme in the synthesis of anthocyanin (Lee et al. 1996). ABA plays an important role in stimulating the enzyme. In this study, the initial harvest date is generally delayed approximately by 10 days by GA₃, CPPU, and GA₃ + CPPU treatments. However, a sufficient color and TSS contents were developed (Retamales et al. 1995; Du Plessis 2008). Exogenous ABA application has the potential to decrease the time to harvest, which is advantageous for the marketing of grapes (Cantín et al. 2007; Ferrara et al. 2015).

The increase in the keeping quality of the berries during the shelf life obtained in the current study might be interpreted by the positive influence of the GA₃ and CPPU in increasing fruit firmness, reducing weight loss, and delaying ripening (El-Otmani & Coggins 1991; Ben-Arie et al. 1998; Tumminelli et al. 2005; Samara et al. 2007). The delay may also be the main reason for the greater firmness and decrease in the shattering percentage of the berries. Moreover, this result could be attributed to the alterations in cluster and berries’ anatomy. According to Ben-Arie et al. (1998), the increase in the rachis weight was a result of an increased thickness of the pedicels, subsequently reinforcing the attachment of the berries and lessening the percentage of shattering.

The combined use of GA₃ at 25 mg·dm⁻³ and CPPU at 10 mg·dm⁻³ was effective to improve the berry size and fruit quality of ‘Flame Seedless’ grape, although they had a negative impact on coloration. This problem may be overcome by using the application of exogenous ABA at 300 mg·dm⁻³ at BBCH-81 phase. Moreover, pre-harvest application of GA₃ and CPPU at the early stages of fruit enlargement had a positive influence on increasing the yield and improving the berry and cluster physical characteristics, as well as extending the shelf life.