Abstract: 【Objective】Maize is the first food crop in China, which plays an important role in national food security. Using density-tolerance hybrids and increasing plant density is one of the primary measures to achieve high yields of maize in modern times. However, the high planting density increased the pressure of maize growth space, resulting in the growth of single plant inhibited and the yield per plant decreased, at the same time, as the major organ to absorb moisture and nutrients from soil, the roots' growth can be inhibited by high plant density. To ascertain the relationship between the root characteristics of density-tolerance hybrids and grain yield, and nitrogen utilization under high plant density is the base of studying the genotype differences of root characteristics of different summer maize hybrids to plant density, and which is significant for root improvement of density-intolerance hybrids and management of nutrients and moisture under high planting density.【Method】This experiment was conducted during 2014-2015 at the Huanghuaihai Regional Corn Research Center of Shandong Agricultural University. With Zhengdan 958 (ZD958, density-tolerance hybrid) and Ludan 981 (LD981, density-intolerance hybrid) as the experimental materials, using the soil column culture in combination with the ¹⁵N-labeling technique, the responses of root characteristics, as well as nitrogen uptake and utilization, of different density-tolerance varieties to increased density were investigated at two planting densities (D1,  52 500 plants/hm² and D2, 82 500 plants/hm²). 【Result】Grain yield of maize significantly increased with the increase of plant density, while the grain yield per plant of both hybrids significantly reduced. Over the growing process of both hybrids, the root biomass, length, surface area and active absorbing area of both hybrids were decreased with the increase of plant density. In D1 treatment, all root indicators of LD981 were higher than those of ZD958 at early growth stage but then turned to be lower or significantly lower than ZD958 after milk stage. In D2 treatment, no significant differences in various root indicators were observed between the two hybrids at early growth stage; however, the root indicators of LD981 were significantly lower than those of ZD958 at late growth stage. The leaf area per plant and net photosynthetic rate of ear leaf changed in a trend consistent with that of roots. The difference in root-shoot ratio in biomass under the impact of density increase was not significant between the two hybrids; but their root-shoot ratio in active area was significantly reduced. The N accumulation amount (NAA) per plant and N use efficiency (NUE) of both hybrids were significantly reduced, but the N fertilizer recovery rate (NRR) and the nitrogen partial factor productivity (NPFP) significantly improved with plant density increased. In addition, the proportion of N from fertilizer in NAA was not affected by the changes of density. In D2 treatment, the N content per plant, ratio of fertilizer N, NRR and NPFP of ZD958 were significantly higher than LD981. 【Conclusion】The roots of ZD958 proved to be less affected by plant density. At high density, it could maintain relatively high root weight, length, absorbing area and activity, and longer high value duration, which were beneficial to N uptake, the photosynthetic production and obtaining higher grain yield. This suggests that the well-developed roots can guarantee the plant nitrogen uptake at high density, contributing to the photosynthetic production of the aboveground part and thus achieving higher grain yield. The bigger seed set and stronger seed set adjustment ability of ZD958 promoting nitrogen were the primary reason of its higher NUE and NPFP than LD981 at high plant density.

Key words: summer maize, density-tolerance hybrid, plant density, soil column, root characteristics, ¹⁵N-label