| Kiwifruit (Actinidia deliciosa. cv. Bruno), as one of the fruits most cultivated in Zhejiang province, is highly perishable and prone to storage diseases. Penicillium expansion, the causal agent of blue mould rot, is prevalent in the orchard environment and easy to infect the mechanical wounds, of kiwifruits. Apart from destroying postharvest quality of fruit, P. expansum is also regarded as the main producer of patulin, a mycotoxin and secondary metabolites toxin to humans, thus maybe causing the problems in food safety.In this study, the delay of ripening and control of postharvest disease in kiwifruits (Actinidia deliciosa. cv. Bruno) by pre-harvest spraying with oxalic acid (OA) or salicylic acid (SA) were investigated. The effects of pre-harvest treatments on several indexes associated with quality of kiwifruit were measured. By being inoculated with P. expansum, additionally, the effects of pre-OA and pre-SA treatment on quiescent infection, disease incidence and toxin control, as well as the mechanism of postharvest disease resistance involved in defensive enzymes activities, resistance related substances, reactive oxygen species (ROS) metabolism and histology of kiwifruit were evaluated. The main results were as follows:1) Effect of pre-harvest OA or SA treatment on postharvest quality in kiwifruitsTrees of kiwifruit were sprayed with5mmol·L-1OA or1.5mmol·L-1SA at130,137and144d after full blossom. The fruits were harvested and stored at room temperature (20±1℃) after mature. Results showed that per-OA or pre-SA treatment efficiently inhibited the decrease of firmness and soluble solid content (SSC) in fruits during storage, and maintained a lower rate of sugar and acid. The levels of vitamin C (Vc) and chlorophyll were increased in the treated fruits compared to the control. Thus, the process of fruit ripening was delayed, and the storage quality of kiwifruit was enhanced by pre-SA and pre-OA treatment as well. Compared to SA, pre-OA treatment conducted more efficiently.2) Effect of pre-harvest OA or SA treatment on disease control and toxic content in kiwifruits during storageFour main quiescent fungi were separated, purificated and identified from the peel of kiwifruits, e.g. Pestalotiopsis, Alternaria, Penicillium and Fusarium. Spraying with5mmol·L-1OA or1.5mmol·L-1SA during fruit growth had an effective decline on disease index and inoculated lesion diameter by P. expansum in kiwifruits. In addition, the in vitro experiment showed that OA and SA could inhibit P. expansum growth directly. Moreover, the total content of patulin was declined by52.17%,80.54%in treated fruits compared to the control at6d after inoculation, respectively. Pre-harvest spraying with OA and SA significantly limited patulin content in the decay tissue of kiwifruits inoculated with P. expansum. It was showed that pre-harvest OA and SA treatment could efficiently control the disease and toxic level in kiwifruits.3) Effect of pre-harvest OA or SA treatment on developing disease resistance in kiwifruits during storageTrees of kiwifruit were sprayed with5mmol·L-1OA and1.5mmol·L-1SA in the field before inoculated with P. expansum. Results showed that pre-OA and pre-SA treatment made kiwifruit maintain a strong resistance system during storage, respectively. Activities of defense enzymes, including chitinase (CHT), β-1,3-glucanase (GLU), phenylalanine ammonia-lyase (PAL),4-coumarate CoA ligase (4CL), peroxidase (POD) and polyphenoloxidase (PPO), were enhanced apparently. Contents of total phenol, flavonoids and hydroxyproline-rich glycoprotein (HRGP) in treated fruits significantly increased during storage, and elevated content of lignin by OA or SA treatment was also manifested at beginning time of storage. Moreover, pre-harvest OA and SA treatment stimulated the activities of catalase (CAT), superoxide dismutase (SOD), ascorbic peroxidase (APX), as well as controlled the increase of H2O2content effectively. Furthermore, pre-harvest OA or SA application decreased the relative electrolytic leakage and enhanced the soluble protein content at beginning time of storage. Compared to the control, the thicknesses of the cuticle and cell wall were increased by pre-harvest treatment with OA or SA, and the array of cell became tighter. |
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