Abstract: 【Objective】The specific gravity of maize grains is positively and significantly correlated with test weight. Low test weight has been one of the major problems of low grain commodity quality in maize. The aim of this experiment was to investigate the dynamic of maize specific gravity in grain filling process and the correlation between specific gravity and grain filling traits, in order to improve the specific gravity and test weight, and provide a theoretical basis for improving the grain commodity quality. 【Method】 Four maize varieties including the normal type variety Nongda 108(ND108), durum variety Feiyu4(FY4), high starch varieties Feiyu3(FY3) and Zhengdan18(ZD18) were planted in this experiment. The specific gravity of maize grains and grain filling indexes including 100-kernel dry weight, individual fresh kernel volume and percent water content were determined after pollination. Regression analysis was applied to discuss the relationship between specific gravity and grain filling characteristics.【Result】Along with the development of the grain, the fresh specific gravity showed a rising trend after pollination and tended to be stable in the mature period. However, the dry specific gravity showed a downward trend in early grain filling. A period of 20-35 days was a rapid growth period of dry specific gravity after pollination, and dry specific gravity tended to be stable at the late grain filling stage. The dry specific gravity and fresh specific gravity of FY4, FY3, and ZD18 were higher than those of ND108 at maturity. The 100-kernel dry weight and individual fresh kernel volume increased rapidly at early filling stage, and then their growth rate became slow, and finally became stable at maturity. Their dynamics could be simulated well with Logistic curve after pollination, while the correlation coefficient for each regression equation was in the range of 0.986-0.999 and reached a significant level, respectively. The 100-kernel dry weight and individual fresh kernel volume of FY4, FY3, and ZD18 were higher than those of ND108 at maturity. With dry matter accumulation after pollination, percent water content began to decline rapidly by 1.302% on the average everyday. Fresh specific gravity was positively and significantly correlated with 100- kernel dry weight (R²=0.851，P＜0.01) and individual fresh kernel volume (R²=0.594，P＜0.05). On the contrary, there was a negative and significant correlation between fresh specific gravity and percent water content (R²=0.803，P＜0.01). Taking the 100-kernel dry weight, individual fresh kernel volume and percent water content at filling stage as the independent variable x, and the dry specific gravity as dependent variable y, a quadratic curve equation y=a+bx+cx² was used to fit the regression relationship between them. The variance analysis showed that the regression equation coefficients were in the 0.623-0.748, and F test was significant (P＜0.01). It is worth mentioning that the dry specific gravity was most closely related to percent water content (r=0.731，P＜0.01). Whenhundred dry kernel weight, individual fresh kernel volume and percent water content were 18.75g, 0.589cm3 and 61.5%, the dry specific gravity was at a minimum. The corresponding days after pollination of each variety were 24-28.1 d, 16.3-20.7 d and 21.1-23.6 d, which were all in the fast increasing period. So 16-28 days after pollination is a key period for the formation of the dry specific gravity. 【Conclusion】The dry specific gravity first decreased at the early stage of grain filling, then increased rapidly, and finally stabilized at the late grain filling stage. The fast increasing period of grain filling was a critical period of the formation of the dry specific gravity, at the same time, the effects of this grain filling period will significantly affect the dry specific gravity. The correlation between dry specific gravity and percent water content was the most close, and the regression coefficient is 0.731.

Key words: maize (Zea mays L.), specific gravity, grain filling, regression analysis