Effect Of Oxalic Acid Treatment On Alleviating Chilling Injury In Chilling-Sensitive Fruits And Its Involved Mechanism

Mango (Mangifera indica L.) and tomato (Solanum lycopersicum L.) fruit belong to chilling sensitive fruits, and harvested mango and tomato fruit are susceptible to chilling injury during storage at low temperature, which result in decrease in storage period and fruit quality extremely. Thus, it is very important to alleviate chilling injury in these fruit for their preservation and quality maintaining during storage at low temperature. Oxalic acid is an organic acid ubiquitously occurring in various organisms. Recently, pre-storage application of oxalic acid for postharvest fruit has received more attention, and it has been found that oxalic acid plays important roles such as delaying the ripening process, inducing disease resistance, and alleviating browning and chilling injury in postharvest fruit. In order to further understanding the effect of oxalic acid on alleviating the chilling injury in chilling-sensitive fruit, influences of oxalic acid in chilling injury, fruit quality, antioxidant capacity, proline acumulation, energy metabolism and expression of differential genes in mango and tomato fruit under chilling stress were investigated in this paper. The main results were as follows:(1) Chilling injury index in mango and tomato fruit gradually increased with increase in storage time under chilling stress. Compared to the control, the chilling injury index in both fruit decreased by25.32%and19.42%at the end of storage respectively, which showed oxalic acid treatment could effectively alleviate chilling injury in mango and tomato fruit during storage under chilling stress. Oxialic acid significantly inhibited respiration rate and ethylene production, retarded the decrease in firmnes and titratable acid (TA) and the increase in soluble solid content (SSC) in mango fruit, whereas it had no significant efficency in SSC and promoted the decrease in firmness in tomato fruit as transferring to room temperature from chilling stress. These effects of oxalic acid might collectively contribute to delaying the ripening and senescent process of postharvest fruit and thus to maintaining fruit quality for mango and tomato in storage under chilling stress.(2) Oxalic acid treatment obviously increased ascorbic acid (AsA) content and activities of superoxide disumutase (SOD), peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR), which resulted in improvement on scavenging capacity for reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) and the production rate of superoxide anion (O2’") in mango and tomato fruit, and thus contributed to maintaining membrane integrity associated with alleviating the chilling injury in these fruit under chilling stress.(3) Proline content in mango fruit decreased during storage under chilling stress, but oxalic acid inhibited the decrease in proline content accompanied with increase in△1-pyrroline-5-carboxylate synthetase (P5CS) activity and decrease in proline dehydrogenase (PrDH) activity. Proline content in tomato fruit also decreased under chilling stress and increaseed thereafter decreased as transfering to room temperature. However, oxalic acid treatment gradually increased proline content accompanied with increase in activities of P5CS and ornithine-5-aminotransferase (OAT) and decrease in PrDH activity. Also, the gene expression of P5CS, OAT and PrDH in tomato fruit during storage was further studied. The data showed that the expression of P5CS, OAT and PrDH in tomato fruit were greatly enhanced during storage, and oxalic acid significantly improved the expression of P5CS and OAT except for PrDH, so that it seemed that proline accumulation in tomato fruit was associated with glutamic acid pathway and ornithine pathway, and oxalic acid might regulate proline accumulation for alleviating chilling injury in chilling sensitive fruit under chilling stress.(4) Oxalic acid treatment enhanced the content of ATP and ADP in mango fruit, and inhibited the increase in AMP and the decrease in energy charge during cold storage. In tomato fruit, neither the contents of ATP and ADP during cold storage nor AMP content and energy charge during the whole storage were affected by oxalic acid treatment, but the contents of ATP and ADP were enhanced as fruit transferred to room temperature.. Furthermore, oxalic acid treatment increased activities of succinodehydrogenase (SDH), H+-ATPase and Ca2+-ATPase both in mango and tomato fruit, it was suggested that oxalic acid treatment could regulate energy metabolism for alleviating chilling injury in mango and tomato fruit under chilling stress.(5) Lycopene, as an important indicator for ripening and a primary antioxidant in tomato fruit, is closely related with stress resistance for fruit.. Oxalic acid treatment did not affect lycopene in tomato fruit during cold storage, but resulted in higher lycopene content at the latter days (20+9d and20+12d) as fruit were removed to room temperature. Also, oxalic acid treatment enhanced the gene expressions of phytoene synthasel (PSY1),ζ-carotene desaturase (ZDS) and phytoene desaturase (PDS). Thus, it was suggested that effect of oxalic acid on the accumulation of lycopene might also contribute to improving antioxidant ability in tomato fruit, and in turn for alleviating chilling injury in tomato fruit under chilling stress.(6) For obtaining an overall view on transcript modification and finding differentially expressed genes in tomato fruit with oxalic acid treatment during storage under chilling stress, a microarrary analysis was carried out by using Affymetrix tomato genechip arrays. As compared to control fruit,1623genes expressed more than two folds in oxalic acid treated fruit in storage under chilling stress for20days, in which540and1083genes were up-regulated and down-regulated, respectively. As fruit were removed to room temperature for9days (20+9d),1313genes still expressed more than two folds in treated fruit, in which361and952genes were up-regulated and down-regulated, respectively.The differentially expressed genes were analyzed by GO annotation. The differentially expressed genes in oxalic acid treated tomato fruit mainly involved in stress response, defense response, metabolic process, oxidation-reduction process, phosphorylation and cellular process, which were associated to binding, catalytic activity, lyase activity and peroxidase activity. Also, GO enrichment analysis showed that the differentially expressed genes in20d and20+9d tomato sample had55and47annotation cluster, respectively. Annotation cluster with more enrichment genes mainly participated in stress response, primary metabolism, cell metabolism process and secondary metabolism.12genes, including pathogenesis related protein (PR protein), heat shock protein, PR-P2pathogenesis-related proteins, isocitrate lyase and sucrose synthetase, participated in annotation cluster both in20d and20+9d tomato sample, while PSY1, PSY2, ZDS, chitinase, Beta-1,3-glucanase, succinodehydrogenase and phosphoenolpyruvate carboxylase participated in annotation cluster only in20d or20+9d sample. Based on the fold change of those differentially expressed genes, it was suggested that oxalic acid regulated the protein expressions including resistance protein and key enzymes involed in energy metabolism and lycopene metabolism, which might contribute to alleviating chilling injury in tomato fruit.