Abstract: 【Objective】 Stem lodging is an important adversity affecting summer maize in the Huang-Huai-Hai China Plain under current husbandry practices, and may limit progress towards the achievement of higher yields via increased plant population density. For this problem, the influence of nitrogen application time on the stem development and lodging property of summer maize was studied under field conditions for two successive years. The aim of the experiment is to provide a scientific basis for the nitrogen management of summer maize under high-yielding conditions to improve the stem lodging-resistance capability in the Huang-Huai-Hai Plain.【Method】The experiment was conducted with three different morphological characteristics hybrids and six nitrogen application times from 2011 to 2012. The hybrids were Xianyu335 with higher plant height and lower ear height (hereafter refers to as a XY), Xundan20 with medium plant height and higher ear height (hereafter refers to as a XD) and Jingdan28 with medium plant height and lower ear height (hereafter refers to as a JD). The nitrogen application times included seed manure (N1), seedling fertilizer (N2), jointing fertilizer (N3), big-bell stage fertilizer (N4), tasseling stage fertilizer (N5), and no nitrogen treatment (N0) as the control.【Result】The results showed that nitrogen application time had significant impacts on the morphological, anatomical and mechanical characteristics of summer maize stem (p＜0.05). N1, N2 and N3 treatments promoted the stem growth which the ear height coefficient and center gravity of plant, diameter, stem wall thickness, cortex thickness, big vascular bundles number and small vascular bundles number, stalk lateral breaking strength and stalk rind penetration strength of the third internodes and bending strength of plant were significantly higher than N0. The ratio of the length to diameter of the third internodes and the field lodging rate significantly reduced in N1 and N2, while slightly increased in N3 as compared to N0. There were no significant differences in the ear height coefficient and internode length under N4 treatment, but the diameter, anatomical and mechanical characteristics of the third internode were significantly higher than N0, and the ratio of internode length to diameter was slightly lower. And the field lodging rate was also significantly lower than N0. Nitrogen application at the tasseling stage had no differences on stem growth of summer maize, and the field lodging rate significant reduction was induced by the reduction of ear height coefficient and center gravity of plant. Furthermore, it was found that the grain weight, kernel numbers per ear and yield of summer maize under all the nitrogen application treatments were significantly higher than N0. The yield increasing rate of N3 and N4 which had higher grain weight and kernel number per ear were the first, followed by N1 and N2 which had more kernel number per ear and lower grain weight, and then was N5 which had higher grain weight and significant lower kernel number per ear. 【Conclusion】 The reasonable nitrogen application time could promote the stem growth of summer maize, significantly reduced the ratio of the length to diameter of the basal internodes and improved the stem lodging-resistance capability. The effects of N1 and N2 were the best, but the yield increasing rate was a little lower due to lower grain weight. The stem lodging-resistance capability was lower in N3 which resulted from the higher ratio of the internode length to diameter, and should be avoided in the summer maize culture. The stem lodging-resistance capability, yield and yield components in N4 were the best. Therefore, combining with previous researches, split nitrogen application with small amount in N1 or N2 while with high ratio in N4 is beneficial to promote stem and ear of summer maize growth robustly, and then promote grain yield and stem lodging-resistance of plant.

Key words: summer maize, nitrogen application times, stalks traits, lodging