Study on Physiological Basis of Seed Aging in Non-Heading Chinese Cabbage

The non-heading Chinese cabbage (Brassica campestris L. ssp. chinensis Makino.), with its multiple cropping area and production as the larger species of vegetables, is a widely consumed vegetable. Seed is the most essential production means of cultivated crops. As a large amount of seeds are used in Brassica cultivation, the seed quality is directly related to its performance and profit in production. However, aging is a common phenomenon in the seed storage process. Seed aging affects not only seed germination and seedling growth, but also plant later growth, yield and quality. Therefore, understanding the physiological and biochemical changes of non-heading Chinese cabbage seed aging is of important significance, for reasonable storage in order to maintain or enhance the vitality of the seed, to ensure the sowing quality and meet the needs of vegetable production.In this experiment, using non-heading Chinese cabbage’Hanxiao’seeds as materials, seed artificial aging was carried out at 42℃and RH 100% to explore the physiological, biochemical and cytological mechanism of seed aging, and the physiological and biochemical changes such as seed vigor, antioxidant characteristics and respiration during artificial aging process, the microstructure, ultrastructure of aged seeds were studied. In addition, we analyzed and verified the effects of seed aging on non-heading Chinese cabbage growth, yield and quality. The major results were as follows:1. In this study, using non-heading Chinese cabbage variety’Hanxiao’seed as material, seed vigor and antioxidant-related indexes were studied during artificial aging at 42℃and RH 100%. The results showed that with the extending of the aging time, the seed germination rate, germination energy, germination index and vigor index decreased, abnormal seedling rate after germination rose; the O2 production speed first went up and then went down; the H2O2 content kept increasing, the relative electrolyte leakage was increased. Activity of LOX and the contents of MDA enhanced; Activities of SOD, CAT and GR declined gradually with the aging process. Both of the APX and GPX activity increased during the initial phase of artificial aging, APX activity decreased afterward, while the GPX activity showed no obvious change; the contents of AsA and GSH dropped significantly after Id-aging, then no remarkable change afterward. It suggested that the disorder of antioxidant system led to reactive oxygen metabolism imbalance, this might be the major cause leading to seed aging in non-heading Chinese cabbage.2. Non-heading Chinese cabbage variety’Hanxiao’seed was used as material to study changes in seed germination and respiratory characteristics of artificial aged seeds at 42℃and RH 100%. The results showed that:during the aging process, the seed moisture content increased with the extending of aging time, respiratory enzymes PGI and MDH activity increased in the 2 day’s aging followed by rapid decline, the respiration rate of seed first increased and then decreased with the increase of aging time, the contents of seed soluble sugar, starch and sucrose remarkbally reduced during the aging progresses, soluble protein content of seeds increased significantly after two day’s aging, and then showed no obvious change; seeds were placed in the normal (25℃wet) conditions after aging at high temperature and humidity, respiration of aged seed slowed down during the 6-days breathing process. The changes in moisture content during the initial stage of germination of aged and control seeds were similar, both showed increasing trend, but the respiration rate of aged seeds was significantly lower than the control seeds under the same germination state. It suggested that the consumption of storage material and the disorder of respiratory metabolism during accelerated aging process was one of the major causes leading to seed aging in non-heading Chinese cabbage.3. Non-heading Chinese cabbage variety’Hanxiao’seeds were used to get mild and severe aged seed at 42℃,100% relative humidity. Scanning electron microscope and transmission electron microscope were used to observe the differences of cell structure between control and aged seeds in microstructure and cell ultrastructure. The results showed that the differences between mild aged and control seed in cotyledon and radicle tissue microstructure were not obvious, but the outline of some mild aged cells were not easily distinguished; the rupture and fuzzy edge of radicle and cotyledon cell were observed in severe aged seed, and the cells showed part degradation and mortality. There were obvious changes in radicle cell ultrastructure including protein body, lipid, mitochondrion and nucleolus of aged seed. In radicle cells of severe aged seed, the protein body was seriously degraded, showing a grid and reduced, the fat body disintegrated and formed electron dense material, mitochondrial damage occurs within the ridge, even showed vacuolization and disintegration; the nucleus structure was unclear, with condensation in chromatin, disappearance in nucleolus, the shrinkage in nuclear edge, ect. This showed that the malformation, degradation and death in the radicle and cotyledons cells of aged seed, with the loss of structural integrity of organelles, which also contributed to cytology and histology reasons of slow seed germination and abnormal seedling.4. In this study, using non-heading Chinese cabbage variety’Hanxiao’seed as material, the effects of seed artificial aging on seed germination, field emergence and plant growth were studied. The results showed that:the seed germination rate, germination energy, germination index, vigor index, field emergence rate, seedling rate and speed of emergence significantly decreased along with the extending of aging time, and seeds with the same aging time performed worse in field emergence than in the laboratory germination. With the aging time, non-heading Chinese cabbage seedling growed weaker and root activity decreased significantly. The contents of Vc, soluble sugar and soluble sugar in non-heading Chinese cabbage leaves reduced while the nitrate content increased, seed aging significantly reduced the nutritional quality of non-heading Chinese cabbage. The plant height, diameter, number of leaves, fresh weight and dry weight of shoot, and filed yield of non-heading Chinese cabbage showed the downtrend with the seed aging time. It indicated that seed aging directly affect plant growth throughout the growing season, the final yield and quality in the field, moreover, this effect was more evident with the longer aging time.