Under natural pollination （NP）, early-fertilized ovaries at the base of ear promote kernel abortion of late-fertilized ovades from the tip in maize （Zea mays L.）. Synchronous pollination （SP） improves maize kernel set, but the physiological masons behind this response are yet unclear. We e^amined maize kernel growth at the tip of ear subjected to NP or SP with different ~.ant density of 6 plants/rr~ or 9 plants/n~. Synchronous pollination of ears baged before silking was obtained by hand pollination on 3 days after silking （DAS） and samples were taken from natural and hand-pollinated ears on 8, 13, 18, and 23 DAS. At each sampling date, kemel fresh weight, volume and dry weight at the tip of ear were all higher for maize grown under SP than NP, and the contents of soluble sugar, sucrose, starch, nitro- gen and the ratios of soluble sugar to nitrogen （C/N） in kernel at the tip of ear were all higher too for maize grown under SP than NP.
There are significant differences in nitrogen absorption and utilization effi- ciency among different maize varieties. So it is very essential to determine nitrogen efficiencies of different maize varieties. The effect of nitrogen application on maize growth can be reflected at the grain filling stage. Many scholars have researched the differences in grain filling characteristics among different maize varieties under the same nitrogen application conditions, but there are reare reports on the differ- ences under different nitrogen application conditions. In this study, the grain filling dynamics of maize were observed so as to determine the differences in nitrogen efficiency and to further compare the differences in grain filling characteristics a- mong different maize varieties. The test was carried out with Tunyu 99, Luyu 19 and Xianyu 335 as the materials during May 1st to October 7th, 2013. Under the nitrogen application levels of NO （0 kg/hm2 of pure nitrogen）, N1 （140 kg/hm2 of pure nitrogen） and N2 （210 kg/hm2 of pure nitrogen）, the filling dynamics of upper- and middle- lower-part grains of spring maize were studied. The results showed that among the three nitrogen application level, the grain fresh weight and dry weight of Tunyu 99 ranked as N2〉NI〉N0; the grain fresh weight and dry weight of Luyu 19 ranked as N1〉N2〉N0; the grain fresh weight and dry weight of Xianyu 335 ranked as （N1, N2）〉N0, and there was no significant difference between N1 and N2. Un- der the nitrogen application level of N1, Luyu 19 showed the best growth; under the nitrogen application level of N2, Tunyu 99 showed the best growth, The growth ad- vantage of Xianyu 335 was unobvious under neither N1 nor N2 nitrogen application levels. The development-promoting effect of nitrogen application was more obvious in upper-part grains than that in middle-lower-part grains.
