**4. Discussion**

Data from this study clearly indicate that both of the rabbiteye blueberry cultivars responded rapidly to ethephon applications. The time taken for 50% of fruit to ripen was advanced by up to 3 days after ethephon treatment compared with the control (Figure 1). Ethephon also increased the number of ripe fruit; ripe fruit increased from 42% to 61% in Premier at 7 days after application and 46% to 83% in Powderblue 6 days after treatment, indicating that the application of this PGR can accelerate

the progression of ripening and reduce the time to harvest in blueberry. Several previous studies demonstrated that ethephon accelerates ripening and can reduce the number of required harvests in blueberry [19–21,49]. Results from the current study further expand these findings of acceleration of fruit ripening by ethephon to rabbiteye blueberry.

For a PGR to be effective as a ripening aid, in addition to accelerating ripening it should display minimal negative effects on postharvest fruit quality. Ethephon treatment resulted in a substantial decrease in the proportion of defective fruit after postharvest storage (29 days), at least in Premier. However, fruit texture was not affected by ethephon application in Premier. In Powderblue, compression force at 15 days after ethephon treatment was slightly higher but was not different by 29 days after storage, indicating that ethephon had minimal and temporary effects on fruit firmness characteristics during postharvest storage. These data are generally consistent with those of Dekazos [20] who used rabbiteye blueberry. However, Ban et al. [21] reported substantial reduction in firmness in another rabbiteye blueberry cultivar, Tifblue, in response to ethephon. In that study, fruit slices were used for analysis of firmness rather than intact fruit as used in the current study as well as several others [20,50,51], which may explain the different observations. In the current study, no change in fruit weight in response to ethephon treatment was observed, consistent with results from a study on Tifblue treated with 200 mg L−<sup>1</sup> ethephon [21], and in two highbush blueberry genotypes treated with similar doses of ethephon (240 mg <sup>L</sup>−1) [19]. With similar or comparable (500 mg <sup>L</sup>−1) doses of application, Eck [19] and Howell et al. [52] reported that ethephon did not affect TSS in highbush blueberry, consistent with results from the current study. In rabbiteye blueberry, Dekazos [20] reported no effect of ethephon on TSS even with repeated 500 mg L−<sup>1</sup> applications or a single 1000 mg L−<sup>1</sup> application. However, Ban et al. [21] reported an increase in TSS with ethephon applications at 8 days after treatment in Tifblue, although the effects of this application on postharvest storage were not evaluated in this study. Overall, it appears that ethephon applications do not generally alter TSS content in blueberry fruit during postharvest storage. Several studies have reported a decrease in TA after ethephon applications (ranging from 200 to 3840 mg <sup>L</sup>−1) in highbush and rabbiteye blueberry [19–21]. In the current study, TA levels were unaffected by ethephon in Premier and slightly increased in Powderblue during postharvest storage (15 days). It is possible that the genotypes used here responded differently for this ripening parameter. Juice pH was generally not affected by ethephon treatment as has been seen previously [19,20], except at 1 day after storage. Dekazos [20] reported changes in fruit color parameters in response to ethephon, in contrast to that reported here. As indicated above, Dekazos [20] used repeated and higher doses of ethephon, which may explain the different results observed. Overall, data from this study sugges<sup>t</sup> that ethephon application at 250 mg L−<sup>1</sup> may have minimal effects on rabbiteye blueberry fruit quality during postharvest storage.

The role of ethylene in regulating blueberry ripening and postharvest quality is not completely clear [50]. Although a peak in respiration and ethylene production has been observed in blueberry in some studies [6,17,18], this was not conclusive in others [22]. Treatment of mature fruit with the ethylene perception inhibitor 1-MCP enhanced ethylene production, accelerated loss of fruit firmness, and had little effect on fruit quality characteristics in rabbiteye blueberry cultivars [50], underlining the complex and unclear role of ethylene in regulating blueberry ripening. Recently, based on the analysis of transcriptomics data during various stages of fruit development in highbush blueberry, Gupta et al. [53] indicated that genes associated with ethylene biosynthesis were abundant during the initiation of ripening, suggesting that ethylene may in fact play specific role(s) in modulating the progression of this process in blueberry fruit. The data presented in the current study demonstrating the effect of an ethylene-releasing compound on the progression of ripening further support a potential role for ethylene in the regulation of the ripening program in rabbiteye blueberry. These data indicate that blueberry fruit are responsive to external ethylene. Further studies evaluating the climacteric/non-climacteric nature of blueberry ripening are needed to better understand the potential role of ethylene in the regulation of this fruit developmental process. This information will also be critical for fine-tuning the timing of ethephon application in relation to fruit development.

In the current study, ABA generally did not affect the rate of ripening in blueberry fruit, even when applied at a rate of 1000 mg <sup>L</sup>−1, although it increased the proportion of pink fruit. It may be likely that ABA (1000 mg <sup>L</sup>−1) was able to stimulate the synthesis of anthocyanin pigments associated with pink color in the fruit. Phytotoxicity symptoms were observed in leaves when ABA was applied at 1000 mg L−<sup>1</sup> (data not shown). Application of ABA did not consistently affect any of the fruit quality characteristics measured across the two cultivars except for compression in Premier at 29 days and TSS in Powderblue at 15 days after harvest. Furthermore, ABA applications appeared to increase the proportion of defective fruit at 29 days after storage in Premier. Overall, external ABA applications did not influence the progression of ripening in rabbiteye blueberry, in contrast to some previous reports with highbush blueberry and closely-related bilberry, where ABA concentration was found to increase during ripening leading to the hypothesis that it may regulate anthocyanin biosynthesis and other ripening related characteristics [35,36]. It may be that rabbiteye blueberry is less responsive to ABA or that the genotypes studied exhibited limited ABA responsiveness. Further analysis involving comparison of different blueberry genotypes and species may be needed to clarify any potential roles of ABA in blueberry ripening.

While MeJA applications have been noted to alter the progression of ripening in several fruits such as strawberry, raspberry, peach, apple, and tomato [14,39,43], in the current study MeJA application did not result in any consistent effects on the progression of ripening even when the application doses were at 1 mM. Further evaluation may be required to determine whether higher doses of MeJA can affect ripening in blueberry. However, previous studies have indicated that MeJA application at 10 mM and higher accelerate fruit detachment and result in extensive fruit drop in blueberry [54–56]. Hence, if higher doses of MeJA are successful at accelerating the progression of ripening, this needs to be optimized such that fruit detachment responses are not induced. Additionally, quantification of jasmonates during fruit development and specifically during ripening may help provide further insights into their potential roles in blueberry ripening.
