Abstract: In this study, Jiashi melons and 86-1 melons were artificially wounded and inoculated with Alternaria alternata to investigate the difference in disease resistance in peel and flesh between the two varieties. The inoculated fruits were stored at (7 ± 1) ℃ and relative humidity (RH) of 85%–90% for up to 30 days. During the storage period, lesion size was measured, lesion microstructure was observed, the expression levels of CmGLU, CmCHT1, CmCHT2 genes were determined by real-time polymerase chain reaction (real-time PCR), and the activities of chitinase (CHT) and β-1,3-glucanase (GLU) were measured. The results revealed that the lesion size in Jiashi melons was smaller than that in 86-1 melons during the whole storage period, and the lesion size in peel was smaller than that in pulp. Moreover, the lesion size in the peel and pulp of 86-1 melons was 1.28 and 1.29 times greater than that in Jiashi melons on day 30. It was also found that the cell structure in the peel of both cultivars was thickened to resist infection by this pathogenic fungus; however, disease resistance of the pulp was weaker than that of the peel. At the late stage of infection, the cell wall in the peel and pulp became stratified and shrunk, and numerous hyphae and spores existed inside the cells. Moreover, some pulp cells were disintegrated so that the cellular structure could not be discerned. The relative expression levels of CHT and GLU genes were higher in Jiashi melons versus 86-1 melons, and in the peel versus the pulp over the entire storage period. The relative expression level of CmCHT1 was higher during the early to middle period, and the relative expression level of CmCHT2 was higher during the middle period, suggesting that infection with A. alternata stimulated the expression of CHT gene. Moreover, the relative expression level of CmGLU was higher during the middle to late period, suggesting that infection with A. alternata stimulated the expression of GLU gene. After infection with A. alternata, the enzyme activities of CHT and GLU in peel and pulp tissues were significantly increased and then decreased with storage time, showing higher values in Jiashi melons relative to 86-1 melons, and in peel relative to pulp. Collectively, these results revealed that Jiashi melons showed stronger disease resistance than 86-1 melons and so did the peel compared with the pulp, which may be closely related with the expression levels and enzyme activities of CHT and GLU.

Key words: muskmelon; Alternaria alternata; chitinase; β-1,3-glucanase; gene expression; enzyme activities