Resumen de Elicitation strategies in preharvest to increase quality of table grape and pomegranate fruit at harvest and during storage
Pomegranate and table grape are fruit with high acceptation by consumers due to their organoleptic quality and their content in both nutritive and bioactive compounds related to human health. However, the table grape cultivars, ‘Crimson’ and ‘Magenta’, and the ‘Mollar de Elche’ pomegranate show an important lack of red intense colour at harvest and some others alterations that determine their quality losses during postharvest storage. Fungal decay in table grape and in the case of pomegranate its sensitivity to develop chilling injury (CI) when they are stored under cold conditions are key factors which compromise fruit storage, transport and marketing.
In recent years, research has been performed aimed to find preharvest treatments with natural compounds to increase fruit quality at harvest and to maintain it during storage, due to consumers’ concerns and legal restrictions regarding the use of postharvest chemical treatments. In this sense, the application of naturally occuring plant compounds as preharvest treatments to delay ripening and senescence, preserving fruit and vegetable quality, has received considerable attention. Therefore, the aim of this PhD Thesis is to provide solutions to the pomegranate and table grape quality problems through preharvest treatments with methyl jasmonate (MeJa), salicylic acid derivatives; salicylic acid (SA), acetyl salicylic acid (ASA) and methyl salicylate (MeSa), and oxalic acid (OA) to mainly solve the colour problems of these cultivars at harvest as well as their quality losses during postharvest storage, such as the chilling injury incidence and fungal decay in pomegranate fruit and table grape, respectively, increasing their shelf-life.
Treatments were performed by foliar spray application of 1, 5 and 10 mM concentrations in 2016 for both crops, containing 0.5 % Tween 20 as surfactant. Control vines or pomegranate trees were sprayed with 0.5 % Tween 20 aqueous solution. In 2017 season, these concentrations were repeated for pomegranate crop and reduced to 0.01, 0.1 and 1 mM in the case of table grape, since in general, these treatments at 5 and 10 mM decreased vine yield and delayed on vine-ripening process, which could be hastened by lower doses. However, no negative effects caused by OA treatments at these concentrations on table grape crop yield or berry ripening process were observed. Thus, these doses were not reduced in the second season for this treatment. In 2018, the concentrations of treatments were selected for both crops based on these previous experiments carried out in 2016 and 2017 seasons, in which the best results in term of yield and fruit quality attributes were obtained.
The results of this PhD Thesis have shown that MeJa preharvest treatments at 0.1 mM accelerated berry ripening, mainly colour evolution due to increased anthocyanin biosynthesis, leading to earlier harvest and increasing vine yield in both table grape cultivars. In addition, berry quality parameters, such as size, weight, firmness, total soluble solids and bioactive compound content were also enhanced by this treatment, leading to berries with increased health beneficial effects. Respect to pomegranate crop, MeJa at 5 mM could be selected for practical application purposes in order to get earlier harvest and increase pomegranate crop yield, fruit quality, mainly colour development, and its content in phenolics, anthocyanins and ascorbic acid at harvest and during storage at 10 °C. Combined 5 mM MeJa treatments (Pre- + Postharvest) were also performed and results showed that Pre- + Postharvest MeJa treatments reduced external and internal CI symptoms in pomegranate husk, likely by better maintaining the cell membrane structure through enhancing the unsaturated fatty acids (UFA)/saturated fatty acids (SFA) ratio. However, preharvest MeJa treatment is recommended over Pre- + Postharvest application to increase pomegranate fruit storability during cold storage since no significant differences between both treatments were observed.
On the other hand, it is worth noting the effects of preharvest treatments with salicylates at 10 mM, and especially SA, on increasing total and individual anthocyanin concentration in pomegranate arils which reached a deeper red colour and, in turn, would be more appreciated in the international market. In addition, SA at 10 mM improved pomegranate fruit quality and their content on antioxidant compounds at harvest and during storage at 10 °C. Contradictorily on table grape, considering the overall results, it could be concluded that 0.1 mM MeSa treatment could be a useful tool to increase crop yield and accelerate on-vine ripening process on both cultivars studied, which would lead to improve the economic profit of table grape crop. Furthermore, this treatment was the most effective on enhancing anthocyanin biosynthesis and berry colour in these poorly colored cultivars as well as inducing resistance to Botrytis cinerea spoilage, probably due to enhanced levels of phenolic compounds and activity of antioxidant enzymes.
Finally, 10 mM OA treatment led to an early harvest of ‘Mollar de Elche’ cultivar since tree yield and number of fruits were higher at the first harvest date. This concentration of OA resulted in the greatest effects improving fruit quality parameters, beneficial health effects to consumers and sensory quality properties. Nevertheless, preharvest applications with OA at 5 mM improved the skin colour of table grapes at harvest by the up-regulation of Vitis vinifera 9-cis-epoxycarotenoid dioxygenase 1 gene (VvNCED1) and abscisic acid (ABA) homeostasis during berry development and on-vine ripening. Additionally, OA delayed table grape postharvest ripening and senescence processes during storage, which was mainly mediated by the stimulation of enzymatic and non-enzymatic antioxidant systems, as well as by the reduction in the ABA metabolism.
Therefore, it has been demonstrated that preharvest treatments with MeJa, salicylates and OA, applied at the best tested concentrations, could be considered a safe strategy, based on naturally occurring plant compounds, in preharvest to improve fruit quality attributes, mainly colour, at harvest and during storage of table grape and pomegranate fruit. Overall, a solution is provided to a regional problem. The results would help to the horticultural companies since they would provide fruits with higher quality standards at harvest and after their postharvest handling, storage and marketing.
