Abstract:  Summer squash is an annual trailing herbaceous crop of cucurbit. Its fruit is thin in skin, thick in meat, rich in juice, rich in nutrients, and can be eaten as vegetables, which is deeply loved by the public. In production, summer squash can get higher economic benefits when cultivated out of season in winter-spring season, and temperature is the main limiting factor in winter-spring cultivation, so improving the cold resistance of summer squash seedlings is the key to ensure the production benefits of summer squash in winter-spring season. Seed initiation is a simple and quick means to improve seed germination vigor and enhance seedling quality under stress conditions. As a natural organic polymer mixture, humic acid can affect various physiological activities in crops and stimulate crop growth, which has high application value in agriculture and is an excellent seed initiator. In order to explore the effect of humic acid initiation on seed germination and seedling growth of summer squash at low temperature, 0.05% fulvic acid (FA), humic acid (HA) and biochemical humic acid (BFA) were selected separately as initiation solutions, with deionized water caused by contrast, through the analysis of the germination activity, indices of root and shoot, activity of antioxidant enzyme and content of osmotic regulators and peroxides of summer squash under low temperature stress, application of humic acid cause raised summer squash seed resistance to low temperature stress. The results showed that the humic acid cause could improve under low temperature stress summer squash seed germination rate, germination potential, germination index and vigor index, ease to summer squash seed germination inhibition in low temperature, increase seedling bud high, hypocotyl diameter, radicle length, plant dry weight and root cap ratio, at the same time promote seedling lateral root meristem, making branch root diameter, forks and tips, total root length, root surface area and volume also increased, to some extent promote the summer squash seedlings growth, improve the quality of the summer squash seedlings under the condition of low temperature. Humic acid cause summer squash seedlings could improve the low temperature in the content of soluble sugar, soluble proteins and proline, enhance seedling low temperature adaptability, and could improve the seedling superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activity and lower malondiadehyde (MDA) and superoxide anion free radical content, improving oxidation resistance of the summer squash seedlings. Among them, biochemical humic acid induced the most obvious mitigation effect on low temperature injury of summer squash.

Key words:  Cucurbita pepo L., Low temperature stress, Humic acid, Seed germination