The Mechanisms Of Wound-Induced Resistance In Apple Fruit After Harvest

Mechanical injury, caused during harvesting and postharvest handling, is an important locus for infection by many postharvest pathogens of apple fruit. Avoidance of injuries during harvesting and postharvest handling is usually considered an important disease-reduction measure. However, wounded fruit that escape detection at the sorting table are placed into cold storage along with healthy fruit and may serve as foci for secondary spread of decay, leading to a large decay loss of apple fruit. Such condition would be avoided if a healing process intervened, but little about wound healing was reported in apple fruit after harvest. Therefore, in this paper, we selected apple fruit as experimental material, and our purpose is to research the effect and mechanism of wound healing on disease resistance and heat treatment on wound-induced response in apple fruit after harvest.The contents and results are as follows:1 Enhance wound resistance to P. expansum and B. cinerea by healing in apple fruit after harvestApple fruit was artificially wounded and incubated at 20℃(RH 85%). Results showed that healed wounds were more resistant to P. expansum and B. cinerea when compared with fresh wounds. Decay incidence and lesion development decreased significantly as the delay of healing time. After wounding, Phenylalanine ammonia-lyase (PAL), peroxidease (POD) and polyohenoloxidase (PPO) activity significantly increased, leading to increased accumulation of phenolics and lignin in wounded tissue. Histochemical tests of cell walls near healed wounds were positive for phenolics and lignin after 96 h at 20℃. Thus, a healing period of 96 h at 20℃(RH 85%) was more effective on increasing wound resistance.2 Effect of apple cultivars, healing temperatures and fruit maturity on wound-induced defense resistanceApple cultivars affect the ability of wound-healing. After healing at 20℃(RH 85%) for 96 h, wounds of Gala, Golden Delicious and Red Fuji apple fruit showed different resistance to P. expansum and B. cinerea. The ability of increased PAL activity and lignin content by wound stress was cultivar depend. Gala apple fruit showed a higher increase in wound-induced PAL activity and lignin content, and healed wounds showed higher decrease rate of P. expansum and B. cinerea decay development when compared with Golden Delicious and Red Fuji apple fruit. Moreover, fruit firmness and sugar content play an important role in wound resistance.Wounded Gala apple fruit were incubated at 0℃,20℃and 38℃for 96 h. Results showed that wounds healing at 20℃were more resistant to P. expansum and B. cinerea than healing at 0℃and 38℃. Wounded fruit healed at 0℃showed a lower increase in ethylene production, associated with lower PAL activity, leading to lower resistance to P. expansum and B. cinerea infenction when compared with those healed at 20℃. Ethylene production in wounded fruit healed 38℃was significantly lower than that healed at 20℃, associated with lower wound-induced PAL activity, leading to lower accumulation of phenolis and lignin in wounded tissue. Those also suggested that ethylene may play an important role in wound-induced defense responses. Moreover, heat healing at 38℃for 96 h increased water loss of wounded fruit, which may result in decreased wound healing ability.Using'Gala' apples harvested at different stages of ripening, we demonstrated that wounding can activate initial H2O2 accumulation and wound healing ability to defend against P. expansum and B. cinerea penetration. Though more mature fruit emit higher ethylene production, the reduce wound resistance caused by fruit ripens is associated with a loss of more mature fruit to produce PAL and lignin in response to wounding. Thus, H2O2 accumulation in response to wounding is modulated by fruit maturity and required for efficient wound healing and resistance to P. expansum and B. cinerea.3 Effect of wounding on ethylene synthesis and ethylene on wound induced defense resistanceEthylene production increased immediately after wounding, and maintained a higher level than non wounded fruit after 48 h. We survey the gene expression of key enzymes involved in ethylene synthesis by real-time PCR analysis. Results showed that the gene expression of Md-ACS5A and Md-ACO1 increased significantly at the early 24 h after wounding. And Md-ACS1 gene expression increased after 24 h during healing time. Those may contribute to increased ethylene production in wounded fruit.Treatments by 0.5 and 1.0μL L-1 1-MCP and 500μL L-1 NBD before wounding inhibited ethylene production of wounded fruit, and decreased wound resistance to B. cinerea infection. Treatments with 0.5μg L-1 and 1.0μg L-1 ethephon before wounding enhanced ethylene production, the gray mold decay of treated fruit was lower than non treated fruit. However, treatment with 4.0μg L-1 ethephon increased B. cinerea. development. When ethylene production was suppressed, wound-induced PAL and POD activity decreased, resulted lower phenolics and lignin accumulation in wound tissue, thus decrease wound-induced defense resistance. Appropriate induction of ethylene production can enhance wound-induced response, increase wound resistance. Those suggested that ethylene play an important role in defense resistance.4 Effect of wounding on H2O2 content and the role of H2O2 in wound induced defense resistanceThe H2O2 content increased immediately after wounding in Gala apple fruit, this is because the activation of superoxide dismutase (SOD) and deactivation of catalase (CAT) at the early wound response. Thereafter, the increase of CAT, ascorbate peroxides (APX) and POD activity decreased the accumulation of H2O2 in wounded tissue, protected cell around wounds from oxidative damage.Gala apple fruit treated by 10 mM DMTU immediately after wounding showed significantly lower H2O2 level than non treatment, associated with decreased resistance to B. cinerea in healed wounds. DMTU treatment inhibited wound-induced PAL and POD activity, suppressed phenolics polymerization, decreased phenolics and lignin accumulation in wound tissue. Thus, the timely generation of H2O2 after wounding was necessary for wound-induced defense resistance.5 Relationship of ethylene and H2O2 in wound-induced defense responseEthylene synthesis may partly need the activation of timely produced H2O2 immediately after wounding, because elimination of H2O2 by DMTU suppressed ethylene emission. However, inhibition of ethylene decreased wound-induced H2O2 accumulation. Ethylene actived those enzymes involved in H2O2 metabolism, leading to lower level of H2O2 in healed wounds. Thus, H2O2 metabolism was regulated by ethylene, and both involved in wound-induced defense response.