Changes of Physiological and Biochemical Indexes of the Quality of Fistular Onion Leaves at Different Drying Grades After Harvest

The changes of quality indexes of fistular onion leaves at different drying grades of shallot after harvest were studied, thus to provide theoretical basis for the storage and transportation of shallot. In order to investigate the agronomic characteristics of large-leaved variety AF37 and small-leaved variety AF45 in the mature period, they were stored at room temperature (25±1)℃ and relative humidity (70%±5%) after harvest. Then, the contents of moisture content, chlorophyll, total protein, soluble sugar, vitamin C and nitrate nitrogen of the two varieties of Allium fistulosum L.var.caespitosum Makino at different grades were determined and compared according to the drying degree of fistular onion leaves (which was graded to grades 0, 1, 5, 9 and 10). The results showed that there were differences in agronomic traits between the two varieties. The plant type of the large-leaved variety AF37 was larger, with an average plant height of 57.51 cm and leaf transverse diameter of 1.02 cm. The plant type of small-leaved variety AF45 was small, with an average plant height of 42.52 cm and leaf transverse diameter of 0.53 cm, and the number of leaves per plant was up to 4.With the increase of drying grades of fistular onion leaves, the moisture content of fistular onion leaves of the large-leaved variety AF37 and small-leaved variety AF45 decreased from 87.97% and 90.29% at grade 0 to 7.28% and 11.15% at grade 10, and the bunching onion has lost its commodity value when the drying grade was 5. The contents of chlorophyll a, chlorophyll b and total chlorophyll decreased rapidly with the increase of the drying grade, and the decrease was extremely significant before the drying grade was 5. The content of total protein decreased slowly with the deepening of the drying degree, from 0.44 and 0.60 mg·g^-1 at grade 0 to 0.05 and 0.07 mg·g^-1 at grade 10, respectively. The content of soluble sugar in fistular onion leaves decreased from 30.83 and 40.02 mg·g^-1 at grade 0 to 2.86 and 3.81 mg·g^-1 at grade 10, respectively. The content of vitamin C decreased significantly, from 1.73 and 1.14 mg·g^-1 at grade 0 to 0.03 and 0.04 mg·g^-1 at grade 10, respectively. Before grade 1, the content of vitamin C decreased significantly and then decreased gradually. The content of nitrate nitrogen also showed a decreasing trend with the drying of shallot. There was no significant difference in the reduction of nitrate nitrogen content in the large-leaved variety AF37 (P>0.01), while there was significant difference in the reduction of nitrate nitrogen content in the small-leaved variety AF45 (P<0.01). The correlation analysis results showed that the drying degree of green onion increased after harvest, accompanied by the process of water loss, chlorosis and degradation of the fistular onion leaves. There was a significant or extremely significant correlation between the dry grade and the main quality indexes of shallot, such as moisture content, chlorophyll, protein, soluble sugar, vitamin C and nitrate nitrogen content. So, its division has practical significance. During the storage process after harvest, the physical and chemical indexes of various qualities of shallot tended to decline or senescence with the process of water loss, chlorosis and degradation of the fistular onion leaves. In addition, there may be a critical point (at the drying grade 5) for quality degradation during the storage of shallot, and when the dry grade exceeded the critical level, the shallot began to accelerate senescence. Therefore, while considering the reduction of storage and transportation time, the emphasis should be placed on water retention, green preservation and the prevention of the degradation of storage reserves such as proteins and soluble sugars in the early stage of storage and transportation.