If abscission zone activation and development could both occur postharvest, it may be possible to treat the fruit in a packing house which should enable more efficient application of active ingredients, in addition to preventing crop loss due to preharvest abscission. The results of the second experiment also demonstrate a benefit in applying ACC with JA, as previously shown with MeJA . The co-application of 500 ppm ACC was sufficient to make 4 mM JA as effective as 8 mM JA in promoting dry stem scars, one of the most important treatment effects with respect to the quality of detached grapes. Because 500 ppm was the lowest dosage of ACC tested, it is unknown if a lower dosage might be as effective, but this should be tested in future research since the 500 ppm and 1000 ppm ACC treatments had similar interaction effects with JA. Likewise, a lower dosage of JA could be effective, especially if combined with ACC. Thus, additional work should be carried out to determine if combination treatments could enable lower dosages of JA and ACC to be reliably effective.
In conclusion, the exogenous application of JA activates the pedicel-fruit abscission zone of Thompson Seedless grapes, rapidly reducing FDF, increasing the proportion of berries with dry stem scars after detachment, and leading to significant preharvest abscission within 2 days. Treatment effects require more than 2 mM and less than 8 mM JA if applied alone, hydroponic bucket but possibly less if co-applied with ACC. Additional work is needed to determine if harvest within 2 days after treatment is sufficient to reduce FDF and increase dry stem scar incidence while reducing preharvest abscission. Work should also be carried out to determine if preharvest or postharvest treatments are effective at inducing abscission postharvest.The experiments were conducted in September 2020 with own-rooted Vitis vinifera cv. Thompson Seedless grapevines and supported by an overhead-arbor trellis in a vineyard at the University of California Kearney Agricultural Research and Extension Center, in Parlier, CA. The vines were planted in 1995, trained to quadrilateral-cordons, and canepruned, leaving approximately 6 canes per vine, and 15 nodes per cane. Vines were spaced approximately 1.83 m within rows, and 3.65 m between rows which were oriented east to west. All vines were subjected to cultural practices considered normal and ordinary for dry-on-vine raisin grapes in the San Joaquin Valley, except that the canes were not severed, and raisins were not made. Each of the two experiments used some methods comparable to those employed in a previous study.
Clusters on individual vines were considered treatment replicates and each vine was considered a block. Since there were two experiments, each replicated six times, two groups of six adjacent vines were identified, with one group assigned to the first experiment, and the second group to the second experiment. There were seven treatments in the first experiment: an untreated control; 2 mM, 4 mM, and 8 mM MeJA ; and 2 mM, 4 mM, and 8 mM JA . Unique tags were made for each replicate and treatment and placed in different bags, according to their block. Available clusters were randomly assigned to each treatment by pulling the tags out of a bag and tying them to the peduncle of each cluster. The morning after labelling each cluster, solutions were prepared with the proper amount of MeJA or JA in water, with 0.05% of Latron-B1956 spreader-sticker . Control treatments consisted of water with Latron-B1956. Each solution was placed into a spray bottle , agitated well, and then applied to an appropriately labelled cluster until runoff occurred. After the clusters dried, they were enclosed in polypropylene mesh bags to catch any berries at risk of abscission. The bags had a re-sealable flap at the bottom from which abscised berries could be collected and weighed. In the first experiment the berries were collected once, 3 days after treatment.
The abscised berries were weighed and discarded, and then each cluster was harvested and taken to a laboratory in their mesh bags where FDF and dry stem scar measurements were made. At the laboratory, clusters were gently removed from their bags, and small shears were used to sever 10 berries from the top, middle, and bottom part of each cluster, retaining the pedicel and a short section of rachis with each berry. Each berry was then placed in a jig attached to a force gauge , and force parallel to the fruit axis was applied to the rachis until it detached from the berry and peak force was recorded. After each berry was detached, the stem end was observed and assigned to one of two classes: dry or wet stem scar. The proportion of berries in each sample with a dry stem scar was then calculated. The remaining berries on each cluster were then removed, combined with the ten used for FDF and dry stem scar measurements, and weighed. The total weight of each cluster was determined by adding the weight of any berries that had abscised prior to harvest, and the percentage of preharvest fruit drop was calculated based on the cumulative weight of the abscised berries and the weight of the berries remaining at harvest. The second experiment had nine treatment combinations: three levels of JA in a factorial design. The ACC was provided by Valent BioSciences . Cluster selection, solution preparation , treatment application, and bagging, were similar tothe procedures employed in the first experiment. In the second experiment, cumulative preharvest abscission was determined daily, by weight. After 3 DAT, all the clusters were taken to the laboratory for determination of FDF, dry stem scar development, and cluster weight determination. All data were subjected to analysis of variance using the GLM procedure of SAS . In the first experiment, a randomized complete block design was used, with seven treatments. Means were compared by Duncan’s new multiple range test . In the second experiment, a factorial design was employed with one factor being the dosage of JA, and the second being the dosage of ACC. If JA and ACC did not interact to affect a variable, then only the main effects of JA and ACC were considered. If JA and ACC interacted to affect a variable, then the nature of the interaction was determined, and the data were summarized in two-way tables. Fruit ripening behavior has been classically defined as either climacteric or non-climacteric based on the existence or absence of a burst in respiration rate, respectively . An increase of autocatalytic ethylene biosynthesis accompanies climacteric ripening . Climacteric fruits, such as tomato, banana, and most stone fruits, have the capacity for ripening if detached from the tree at the mature stage; while non-climacteric fruits, including strawberry, grape, and citrus fruits, cannot proceed to maturity post-harvest . Both ripening types can be found in the same species, as reported in melon . Japanese plum cultivars, categorized as climacteric, have been reported to differ in their ripening patterns . While the cultivar Santa Rosa presents a climacteric behavior , its bud sport mutant, Sweet Miriam, displays a non-climacteric behavior . The occurrence of both climacteric and nonclimacteric types in fruits with the same genetic background offers an ideal experimental system for the study of the mechanisms controlling ethylene-dependent and ethyleneindependent fruit maturation. In stone-fruits, including Japanese plums, fruit growth follows a double-sigmoid pattern, defined by four distinct stages . The first exponential growth phase involves cell division and elongation; pit hardening is characterized by endocarp hardening and almost no increase in fruit size. In the second exponential growth phase , cell division is resumed and the fruit reaches its final size. The fruit ripening stage is further divided into S4-I, where commercial harvest takes place, and S4-II, where the fruit reaches its full ripeness. Although Santa Rosa and Sweet Miriam were similar in fruit weight and size at the fully ripe stage, Sweet Miriam displayed longer S2, S3, and S4 stages and needed ~100 more days to reach S4-II as compared with Santa Rosa . Sugars are synthesized in leaves and translocated to fruits .
They provide energy and carbon structural sources during fruit growth and contribute to overall fruit taste, stackable planters as their content and composition largely determine fruit sweetness . In Japanese plums, as in other members of the Rosaceae family, the sugar-alcohol sorbitol is translocated to the fruit along with sucrose . Sor synthesis is catalyzed by the enzyme sorbitol-6-phosphate-dehydrogenase that mediates the reduction of glucose-6-phosphate to sorbitol- 6-phosphate . Sor breakdown is mediated by the activities of NAD+-dependent sorbitol dehydrogenase and sorbitol oxidase , which catabolize Sor into fructose and glucose , respectively . Previously, we reported that at the fully ripe stage, Sweet Miriam fruits displayed higher Sor contents, with significantly higher and lower specific activities of S6PDH and NAD+-SDH, respectively . Moreover, Sweet Miriam fruits showed lower Glu and Fru contents that were associated with increased Suc catabolism at the fruit S4-II stage . In addition to the major sugars Suc, Sor, Glu, and Fru, fruits also contain sugars that are present in significantly lower concentrations, including galactose , galactinol , raffinose , myo-inositol , and trehalose , among others. We hypothesized that differences in ripening behavior between the two cultivars could be associated with modifications in sugar metabolism and that the characterization of key sugar metabolic pathways could contribute to a better understanding of the changes operating in the bud sport mutant. Here, we used a systems biology approach to identify and characterize differences in Sor accumulation as well as changes in other major and minor sugars in fruits displaying contrasting ripening behaviors, a typical climacteric Japanese plum Santa Rosa and its non-climacteric bud sport mutant Sweet Miriam. We integrated gene expression profiles to identify key nodes in gene networks associated with the sugar metabolism reprogramming in the non-climacteric fruits. The expression patterns of these genes were further validated based on transcript levels, and the functions of gene products were assessed enzymatically and by metabolite analyses in fruits and leaves.RNA-Seq analysis enabled the identification of 29 096 unique genes . Subsequent calculation of normalized read counts for each gene resulted in a total of 17 721 genes used for statistical analysis. PCA of the expression values of the 17 721 genes showed that the first two principal components could explain 60.3% of the total transcript expression level variance in the score plot . Each assessed cultivar–stage combination was separated in an independent cluster, indicating that there were differences in their expression patterns. Nonetheless, within each cultivar–stage combination cluster, the three biological replicates displayed relatively low variation , demonstrating the reproducibility and reliability of the gene expression data sets. A cluster dendogram, built using the mean gene expression level of the three biological replicates in each cultivar–stage combination for the 17 721 genes, showed that SRS4-II was relatively distinct from the other cultivar–stage combinations . These results suggested that the main differences in gene expression among the cultivars occurred during the ripening phase ; while, within cultivars, changes in expression throughout development showed less variation for Sweet Miriam as compared with Santa Rosa . Differential expression gene profiling of the RNA-Seq data sets, using a 2-fold cut-off value in at least one pairwise comparison of the four cultivar–stage combinations , identified a total number of 5727 DEGs. When comparing both cultivars at the same developmental stage, the results indicated that during pit hardening there were 307 and 234 up-regulated and down-regulated genes, respectively; while at the fully ripe stage there were 989 and 714 up-regulated and down-regulated genes, respectively . In addition, when examining the fold change in expression levels, as shown on the heat map , less variability was detected in SMS2/SRS2, as compared with SMS4-II/SRS4-II, consistent with the pronounced transcriptomic changes that occurred during ripening of both cultivars . On the other hand, when comparing developmental stages within the same cultivar, Santa Rosa displayed 616 and 625 up-regulated and down-regulated genes, respectively; while Sweet Miriam exhibited 162 and 309 upregulated and down-regulated genes, respectively . In addition, lower variability within Sweet Miriam fruits was detected during development , as compared with Santa Rosa . Overall, changes in transcriptomic data, both for the entire gene sets and for DEGs between the two cultivars, correlated well with the differences in fruit ripening behavior; the relatively similar ethylene-independent S2 stage , followed by ethylene-dependent ripening stages of Santa Rosa fruit, as opposed to ethylene independent ripening of Sweet Miriam fruits .