| The study was conducted on the Experimental Farm of Jiangsu Provincial Key Lab of Crop Genetics and Physiology, Yangzhou University, Jiangsu Province, from 2008 to 2009, to investigate the effects of low temperature at seedling stage and jointing stage on plant growth and physiological parameters in wheat Yangmai 16. A pot experiment was conducted in a temperature and moisture controlled artificial greenhouse. The main results were as follows.1. Under the stresses of low temperatures, the tips of wheat seedling leaves became yellow, curly, and wilted, and the growth of wheat seedlings were temporarily inhibited. The longer and the lower the temperature was, the more serious the cold damage was produced on wheat plants. At seeding stage, there were no plants whose damage were identified at greater than the 3rd level under -4℃and -6℃lasting for 24 h. Most culms and tillers died when wheat plants were treated for 24 h and 48 h at -8℃. The proportion of injured plants at the 5th level under -8℃stress increased significantly than at -4℃and -6℃. At jointing stage, most culms and tillers died under the temperature of -2℃for only 1 h. The temperature of -2℃was identified as the fatal temperature of wheat Yangmai 16 at elongation stage. 2. At seedling stage and jointing stage, the contents of chlorophyll, chlorophyll-a, and chlorophyll-b in seedling leaves decreased significantly under low temperatures. The ratio of chlorophyll-a to chlorophyll-b decreased at the preliminary stage, and then increased. The decrease in net photosynthetic rate of wheat plants at elongation stage was caused partially by stomatal effects and non-stomatal effects besides the decrease in chlorophyll content. Under the stress of -2℃lasting for 18 h and the stress of -4℃lasting for 6 h, the decrease in photosynthetic rate was mainly caused by stomatal factors. Under the stress of -2℃lasting for 24 h and the stress of -4℃lasting for 12 h, the decrease in photosynthetic rate was primarily caused by non-stomatal factors.3. When wheat plants were subjected to low temperature stress at jointing stage, 14C-photosynthate produced in main stems was mainly transported from culms to tillers. In the controlled treatment, however, over 90% of 14C-photosynthate was distributed in culms, only less than 10% was transported from culms to tillers.4. At seedling stage, the activities of SOD, CAT and POD in the penultimate leaf increased at the beginning under low temperature, then SOD activity began to decline under the temperature of -4℃and -6℃. Under the temperature of -8℃, the activities of SOD and POD declined. At jointing stage, the activities of SOD, CAT and POD increased at the beginning under low temperature (-2℃and -4℃), then declined. MDA content increased with the lowering of temperature at seedling stage and jointing stage.5. At seedling stage, soluble sugar and soluble protein contents increased significantly as the lowering of temperature and the lasting of treating time under the temperature of -4℃and -6℃, which was helpful for wheat plants to counter cold damage, but the content of proline decreased. The contents of soluble sugar under the temperature of -8℃increased at the beginning, then declined, while the contents of soluble protein increased. At jointing stage, the contents of proline increased at the beginning under low temperature (-2℃and -4℃), then declined, while the contents of soluble protein increased. The soluble sugar contents which increased under the temperature of -2℃, increased at the beginning under the temperature of -4℃, then declined.6. Low temperature stress at elongation stage decreased significantly the number of spike and the number of grain per ear. But the effects of low temperature on 1000-grain weight were less significant. The longer and lower the temperature was, the more the yield declined. The grain yield didn't decrease significantly when the wheat plants were subjected to the stress of -2℃lasting for 6 h and the stress of -4℃lasting for 3 h. Compare to CK, the grain yield decreased by more than 30% under the temperature of -4℃for 24 hour, and the grain yield decreased by more than 50% under the temperature of -4℃for 48 hour. Under the stresses of -8℃at seedling stage and -2℃at jointing stage, tillers at high tillering nodes could grow into spikes and make a good compensation for the yield loss caused by cold damage.
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