[Effects of different nitrogen application rates on low temperature resistance of late rice at heading and flowering stage.]

Four levels of nitrogen application (N ₁ , low nitrogen, 90 kg N·hm ^-2 ; N ₂ , normal nitrogen, 180 kg N·hm ^-2 ; N ₃ , high nitrogen, 250 kg N·hm ^-2 ; N ₄ , ultra high nitrogen, 330 kg N·hm ^-2 ) were set to examine the effects of nitrogen application on the growth and development, yield formation and related physiological characteristics of late rice under low temperature condition during heading and flowering stage, with super hybrid late rice variety Wufengyou T025 as test material. The results showed that yield of all treatments decreased in different degrees under the low temperature conditions, with the yield of N ₂ being the highest, and N ₁ the lowest. The effective panicles per plant and total grains per panicle in N ₃ and N ₄ were higher than those of the other treatments, but their seed setting rate and harvest index were significantly decreased. With the increases of nitrogen application rates, pocketed panicle rate, the total dry mass and chlorophyll contents (SPAD value) increased to different extent, while the panicle dry mass of N ₂ was the highest. After low temperature, chlorophyll content, net photosynthetic rate and transpiration rate of the total treatments decreased to varying degrees, and the declined range of the net photosynthetic rate and trans-piration rate in N ₃ and N ₄ were higher than other treatments. The contents of free proline, malondialdehyde and peroxidase activity of all treatments increased to varying extent, while superoxide dismutase, catalyes activities and the soluble protein contents decreased, with the magnitude of variation in N ₃ and N ₄ being the greatest. It demonstrated that high amounts of nitrogen application were unfavorable to late rice to resist low temperature at heading and flowering stage and thus misuse of nitrogen fertilizer application should be avoided in production. The suitable amount of nitrogen application in double cropping late rice was not more than 180 kg·hm ^-2 .