Abstract: In this study, ‘Xinluzhong 54’ cotton seeds coated with three different homemade seed coating agents (Mian 1, Mian 2 and Mian 3) were sown in plastic nutrition boxes and cultured at 25℃. On the 4th and 7th days when cotton seed radicles had grown into cotton seedlings, the coated cotton seeds were exposed to four low-temperature treatments at 5℃, 10℃, 15℃ and 20℃. The uncoated cotton seeds exposed to the same low-temperature treatments served as the control group (CK). The external morphologies and ultrastructures of the cotton radicles and seedlings were observed at 24 hours after low-temperature exposure to explore the effects of seed coating agents on improving the cold resistance of cotton by simulating the late spring cold climate in Xinjiang Province. The results showed that there was no significant difference between the coated and uncoated treatments at 20℃. Cotton seeds germinated normally at the germination rate of 92.24%-95.51%. The radicle growth lengths of the three different seed coating agents were 3.66-3.81cm, and the cotton seedlings grew well with dark green leaves and erect stems. The mitochondrial structure of radicle cells was complete, and the structures of mitochondria and chloroplasts in mesophyll cells were normal. Compared with the control, the seed germination rate of the coated seeds increased by 4.45-6.98 percentage points at 15℃. The radicle growth speed declined, but the growth amount increased by 25.27%-36.05%. The cotton seedlings grew well with dark green leaves and erect stems. Mitochondria cristae in radicle cells were clear. In addition, the structures of chloroplasts and mitochondria in mesophyll cells were clear, which were significantly better than after CK treatment. Cotton seed germination was inhibited at 10℃, but the germination rate of the coated seeds increased by 0.73-4.25 percentage points compared with that of the uncoated seeds. Radicle growth was limited, and the leaves were dark green. Approximately 19.15%-42.55% of the seedling leaves drooped and wilted. There were more mitochondria and endoplasmic reticula in the radicle cells of seedlings compared with that in seedlings from uncoated seeds. The outline of the chloroplast in mesophyll cells was clear with few inclusions. The Main 1 treatment was better than the other treatments. The cotton germination rate after different treatments was lower than 50.00% at 5℃. The seedling leaves in the uncoated group lost color and wilted with a wilting rate of 74.47%, whereas that of the coated treatment group was 60.42%-67.45%. The mitochondrial structure in the radicle cells was clear, and the mitochondrial structure in mesophyll cells was normal, while chloroplasts expanded into a spherical shape. The grana lamellar structure disintegrated, inclusions were turbid, and no significant difference was observed between coated and uncoated treatment groups. Our results show that seed coating agents can stimulate cotton seed germination, increase the seed germination rate and radicle growth speed under low-temperature treatments, protect the external morphology and cell ultrastructure stability of cotton radicles and seedlings, maintain normal growth, and enhance the ability of seedlings to resist damage from the low-temperature climate.

Key words: Seed coating agents, Cotton, Low temperature stress, External morphology, Ultrastructure