Effect of 1- MCP treatment on chlorophyll degradation in postharvest ‘Granny Smith’ apple fruit

Abstract：【Objective】The effect of 1- methylcyclopropene (1-MCP) on the post- harvest chlorophyll degradation and metabolism was studied in‘Granny Smith’apple fruit in order to provide new understanding for improved post-harvest storage and green preservation techniques.【Methods】The‘Granny Smith’apple used in the experiment were picked from the Haisheng Apple Co., Ltd., (Qianyang, Baoji, China) during commercial harvesting period. 400 health fruit with even size and maturity and no mechanical damage were selected. Half of the fruit were fumigated with 1 μL·L^-1 1-MCP at room temperature for 24 hours, and the remaining half were air-sealed as the control group (CK). After treatment, the fruit were packed in PE bags with 40 fruit per bag and stored at room temperature (20±2 ℃), with a relative humidity of 80%-90%. Physiological parameters such as fruit color, ethylene production, and respiratory intensity were regularly measured, and qualitative and quantitative analyses of chlorophylls and their degradation metabolites in the peel were performed using high-performance liquid chromatography (HPLC). Real-time fluorescence quantitative PCR was used to detect expression of key genes involved in chlorophyll degradation.【Results】The results showed that with the prolonged storage time, the color of‘Granny Smith’apple peel became yellow, and the color values L* and h˚ changed significantly. The magnitude of change in the color value was significantly reduced. 1-MCP treatment delayedthe occurrence of ethylene peak in‘Granny Smith’fruit and inhibited ethylene production and respiration intensity. The CK group showed an ethylene peak at 60 days and the ethylene production rate was 25.9 μL· kg^-1 · h^-1 , while the 1-MCP treatment group showed an ethylene peak at 70 days, and the ethylene production rate was 22.1 μL·kg^-1 ·h^-1 . Fruit respiration intensity gradually increased in the late storage period. The peak of respiration occurred at 70 days in both groups. The maximum respiration intensity in the control and the treatment was 15.1 mg·kg^-1 ·h^-1 and 10.6 mg·kg^-1 ·h^-1 , respectively. The respiratory intensity of the CK group was significantly higher than that of the 1-MCP group. HPLC results showed that after 1-MCP treatment, chlorophyll a and chlorophyll b decreased slowly, and their contents were significantly higher than in the CK group. The accumulation levels of pheophytin a and pheophorbide a after 1-MCP treatment were lower than those in the CK group, indicating that 1-MCP was effective to delay the chlorophyll degradation by inhibiting the further generation of pheophytin a and pheophorbide a. qRT-PCR analysis showed that 1-MCP treatment inhibited the expression of related genes in chlorophyll degradation pathway, especially MdPAO and MdRCCR, which regulate the degradation of pheophytin a and pheophorbide a, respectively, in‘Granny Smith’. It shows that 1-MCP regulates chlorophyll degradation and metabolism during peel yellowing by delaying gene expression and accumulation of metabolites downstream of chlorophyll degradation pathway. In the fruit of CK group, MdNYC1 was significantly positively correlated; the L* value was significantly negatively correlated; and the h˚ value was extremely significantly positively correlated with the content of chlorophyll a and pheophorbide a. Expression of MdSGR was significantly positively correlated with L* value and significantly negatively correlated with chlorophyll b content. In the fruit of 1-MCP treatment group, the expression of MdPAO was significantly negatively correlated with the L* value, and chlorophyll a, chlorophyll b, and pheophorbide a were extremely significantly positively correlated. The expression levels of MdNYC1, MdPAO, and MdRCCR were negatively correlated with L* values and positively correlated with h value. MdPPH, MdSGR, and MdHCAR expression levels were positively correlated with L* value and negatively correlated with h˚ value.【Conclusion】Under normal temperature storage, 1- MCP treatment can delay the occurrence of ethylene and respiration peaks in‘Granny Smith’apple fruit, reduce ethylene release and respiration intensity, and plays a role in delaying fruit senescence. When the L* value is higher than 75 or h value lower than 112°, the peel will enter a significant yellowing period, and 1-MCP treatment significantly inhibits the coloration process in the peel. 1-MCP treatment inhibited the expression of genes MdPAO and MdRCCR, which regulate the degradation of pheophorbide a and chlorophyll a, delay the expression of genes downstream of the chlorophyll degradation pathway and the accumulation of its metabolites, and reduced the production of pheophytin a and pheophorbide a. Therefore, 1-MCP is effective in regulating chlorophyll degradation and metabolism during the yellowing of the peel. The genes MdNYC1, MdPPH, MdPAO, and MdSGR are significantly correlated with the peel color of‘Granny Smith’apple. MdPAO in the 1-MCP treatment group showed a strong positive correlation with chlorophyll content and is likely a key gene regulating chlorophyll degradation.