Ethylene regulates many aspects of fruit development including maturation, senescence, and abscission . Grape is considered non climacteric but an ethylene peak detected at veraison, the onset of ripening, may be higher than the physiological threshold for metabolic activities , and Giovannoni reported some aspects of non-climacteric ripening are probably associated with ethylene responses. Likewise, Chervin et al. reported that ethylene seems required for the increase in berry diameter, decrease in berry acidity and anthocyanins accumulation that occurs after veraison. Regardless of the endogenous role of ethylene in grape berry development, ethephon has well-established commercial uses in viticulture to promote fruit maturationrelated processes, including the synthesis and accumulation of anthocyanins in berries and the accumulation of soluble solids , and grape berries, which generally don’t abscise naturally, can be induced to abscise with exogenous application of ethephon or other compounds that stimulate ethylene production by grape berries . The potential for ethephon as an abscission agent for table grapes is a relatively new concept that has been little studied . If ethephon is to ever be registered for that use, the potential for excessive residues will have to be considered. This is especially important since relatively high rates of ethephon are needed to stimulate grape berry abscission, square plant pots the process occurs quickly , and berries are consumed whole, without peeling.
Therefore, the present study aimed to verify the effects of ethephon on the abscission of grape berries of two globally important seedless table grape Vitis vinifera cultivars, and on the residual concentration of ethephon in the berry in order to evaluate its potential for aiding in the production of fresh-cut fruit.Experiments were carried out in 2012 in Thompson Seedless and Crimson Seedless table grape vineyards located in the countryside of Adelfia and Francavilla Fontana , respectively. Both Thompson Seedless and Crimson Seedless were grafted onto 140 Ru and trained to an overhead trellis system , with the first spaced 2.8 m between rows and 2.5 m within rows and the latter 3.0 m between rows and 2.5 m within rows. Grapevines were cane pruned with 12–14 nodes per cane. Vines were drip irrigated from May to September . Soil water potential was kept below −300 kPa, and leaf water potentials were maintained at values < −0.6 MPa. Fertilizer addition, pest control, and other vineyard operations were conducted according to local practices. A randomized block design was used with three blocks and three treatments, and each treatment in the block consisted of six grapevines selected with a uniform number of clusters. Each treatment consisted of: control, ethephon at 1445 mg/L, ethephon at 2890 mg/L. The concentrations used in this trial were established on results obtained in preliminary studies . Ethephon was dispersed in water with 0.1% of a surfactant and applied directly to the clusters of vines selected for abscission treatments. Clusters from control vines were treated with water containing the surfactant only. The ethephon or control solutions were applied with a handheld sprayer until run-off when the fruits reached sufficient soluble solids for harvest . After the berries dried, each cluster was enclosed in a mesh bag to collect any berries that may abscise.Berries were sampled before treatment, 2 h after treatment and in successive days, as reported in Table 1.
Measurements of FDF, berry skin color, and firmness were as described previously. In brief, FDF was determined as the force required to detach the berry from the rachis as measured with a mechanical gauge . Berry skin color was measured with a chroma meter that reports color in terms of lightness , chroma , and hue , where L ∗ refers to the lightness of a color, from black = 0, to white = 100, C ∗ refers to the intensity of a color, with 0 being achromatic, and h◦ is the position on the color wheel where 0◦ = red, 90◦ = yellow, 180◦ = green, and 270◦ = blue . Berry firmness was measured with a 2-mm needle digital penetrometer in accordance to previously described procedures. A hand-held, temperature compensating digital refractometer and an automatic titrator were used for the following determinations: soluble solids content , pH, titratable acidity . For all these measurements, 10 clusters from each vine were selected and three berries from each cluster were sampled to measure the FDF and three berries for the other measurements. Pre-harvest abscission was determined by counting any abscised berries that had collected in the mesh bags on observation days . Abscised berries were placed in plastic bags and stored in a portable ice box for transport to the laboratory where the integrity of the berry, including the presence/absence of a pedicel, and a wet or dry stem scar was observed with the aid of a binocular microscope at 30× . Berries that abscised pre-harvest and those that fell during harvest, handling or after light shaking constituted the total percentage of dropped berries. The abscised berry percentage was calculated as [/ × 100].Ethephon residues were determined according to the method proposed by Takenaka . For each treatment 30 berries were randomly collected from 10 clusters, stored in a portable FIGURE 1 | Mesh bags to prevent pre-harvest berry loss of Thompson Seedless and Crimson Seedless table grapes. ice box, and carried to the laboratory for analysis. Cartridges SPE NH2 500 mg of Phenomenex activated as suggested by manufacturer were used in the purification step. The purified samples were evaporated to dryness with a rotavapor at 40◦C, taken up with 1 ml of methanol and subjected to derivatization. One hundred microliters of reconstituted samples were transferred to 1.5 mL eppendorf, diluted with 500 µL of acetone and derivatized by adding 10 µL of trimethylsilyldiazomethane . The reaction vials were maintained at 50◦C for 30 min, then 10 µL of 1 M acetic acid in methanol were added in order to stop the reaction.
After centrifugation, 2 µL of the clear upper phase were injected in the GC-MS system.Analysis of variance was performed with the software XLSTAT-Pro , the level of significance was set at 0.01. The assumptions of variance were verified with the Levene test and the Lillefors test . The mean values obtained for the different treatments were statistically separated by using the REGWQ test. Crimson Seedless berries may vary in hue from red-yellow to red-purple. Such colors result in a range of h◦ that bracket red, which has a hue of 0. Most h◦ -values were between 0 and 18◦ but values of a few measurements were between 315 and 360◦ , indicating a red-purple berry. Hue angles between 360 and 315◦ were transformed into negative numbers by subtracting 360, thereby establishing a continuous range of h◦ -values from which the average h◦ could be correctly calculated. As regard the analysis of residuals, we used the Kinfit package—Routines for fitting kinetic model to chemical degradation data—in R 3.1.2 to compute DT50 and DT90 values.Ethephon application did not affect berry color of Thompson Seedless until 14 days after treatment . At that time, ethephon-treated fruit was darker in color , and had lower C ∗ and a greater h◦ , indicating the fruit were somewhat more yellow colored than non-treated fruit and generally had a more mature appearance. These findings are consistent with other reports that ethephon affects berry skin color by stimulating the accumulation of phenolic compounds . Ethephon treatments clearly reduced FDF because most of the berries on treated clusters were so loosely attached that they abscised before harvest or during handling . However, plastic pots for planting the few remaining berries on treated clusters were just as tightly held as the berries on non-treated clusters, so no treatment effects on FDF could be measured . A similar result was reported for Thompson Seedless treated with methyl jasmonate, another abscission agent . FDF may decline within a few days of treatment with abscission agents , so timely harvest may be needed when reductions in FDF are large. Abscission agents did not reduce fruit firmness, but FDF and berry firmness decreased from the time of ethephon application whether the clusters were treated or not . As suggested earlier, ethephon at either concentration tested stimulated an almost complete berry abscission from the rachis . The effects of the two concentrations were similar, with only a few berries still attached to the rachis by harvest time , and the abscised berries generally had dry stem scars . Dry stem scars could be desirable for fresh-cut fruit since the scars help prevent juice leakage and minimize the exposure of interior berry tissues to the atmosphere and to pathogens that might reduce shelf-life or berry quality. However, pre-harvest berry abscission could lead to significant yield losses , though yield loss might be minimized byearlier harvest or the use of catch systems, i.e., nets under the canopy. Ethephon did not affect SSC, pH, or TA . Few studies have examined the effect of abscission agents on grape berry composition, but our results generally agree with Uzquiza et al. who reported few and minor treatment effects on winegrapes. Even though a registered use of ethephon on grape is the promotion of fruit maturity, effects on grape composition are often variable, and ethephon applications to promote fruit maturity are made at veraison, a much earlier stage of fruit development .
Abscission agents are applied to mature fruit, so there is less opportunity to affect fruit composition. Moreover, abscission agents quickly initiate the development of an abscission layer between the pedicel and berry . The rapid action of abscission agents necessitates a short time period between application and harvest, further limiting the potential for differences in composition to develop.Treatment with either concentration of ethephon stimulated significant pre-harvest abscission , both >40% and almost 55% at the dose of 2890 mg/L . A similar effect on Crimson Seedless has been recently reported . The treatments tested were less effective at inducing abscission of Crimson Seedless than they were at inducing abscission of Thompson Seedless. Differences among varieties in responsiveness to abscission agents has been previously reported in grape , and it has also been observed that some table grape varieties are more susceptible than others to “shatter,” or “dry drop,” a post-harvest disorder characterized by the development of an abscission layer between the pedicel and berry . The physiological basis for varietal differences in responsiveness to abscission agents is uncertain, but the application of very high rates of ethephon can induce abscission in varieties that are otherwisen on-responsive , suggesting that the less responsive varieties may be less sensitive to ethylene. As observed with Thompson Seedless, SSC, pH, and TA of Crimson Seedless were not affected by abscission agents .The lack of compositional effects are probably due to similar reasons identified and discussed earlier for Thompson Seedless.Microbial associations have been shown to be critical in the development and functioning of plant and animal host organisms . For plants, there exists a wealth of data on how root and soil-associated microbial communities can shape plant growth, competition with neighbors, disease resistance,and nutrient uptake . For example, in Arabidopsis the composition of their soil-based communities influences the plant’s resistance to drought stress , while community diversity has been linked to nutrient uptake, with positive consequences for plant growth . In contrast to the well-defined role of below ground plant-associated communities, less is known about the importance of bacteria inhabiting the above-ground portion of the plant, the phyllosphere. The phyllosphere microbiome is inhabited by a diverse consortium of bacteria, with densities ranging from 106 to 107 cells per square centimeter , as well as fungi, archaea, viruses . These epiphytic microbial communities are subject to a hostile environment, often encountering high levels of UV radiation, temperature fluctuations, and desiccation . Thus far, the investigation of phyllosphere microbiome impacts on plant health has generally been limited to their role in protection from disease, such as in cases of pear fire blight , tobacco wildfire disease , as well as Pseudomonas syringae associated diseases . Although some evidence suggests that microbial communities can have key functions beyond disease resistance , for example through nitrogen fixation or the production of growth-regulating signals , there remains limited direct evidence for their role in plant fitness more broadly, such as through growth, flower, fruit, or seed production .