Objectives We investigated the changes in shoot and root traits of maize under different plant densities for better understanding of the mechanism underlying maize adaptation to high-density tolerance.

Methods A field experiment was carried out with 18 maize hybrid cultivars commonly used in China, under high (75000 plants/hm²) and low (60000 plants/hm²) plant densities. A total of 14 shoot traits and 8 root architecture traits were measured at silking and maturity stages. The shoot and root cooperative responses to varying densities were assessed by regression and correlation of the traits.

Results Compared to low plant density, high plant density decreased shoot and root biomass and yield per plant, but significantly increased population shoot biomass and grain yield. Based on the population yield under the two plant densities, 6 cultivars were rated as high yield type under the two densities (DH), 3 as high yield type under high density only (HH), 7 cultivars as low yield type under the two densities (DL), and 2 cultivars as high yield under low density (HL). Under high plant density, DH type cultivars had higher dry matter accumulation than the DL type cultivars at silking and maturity stages. DH cultivars had higher root area and total root length with decreasing root dry weight, number of nodes and root width. Under the two plant densities, stem dry matter at the silking stage, grain dry matter and harvest index at the maturity stage positively contributed to yield, while the number of root nodes negatively contributed to yield.

Conclusions DH cultivars coordinate the relationship between shoot and root well and allocated the limited carbon resources more reasonably irrespective of the plant density treatments. By reducing the number of node root, and increasing the root length and root absorption surface area, DH cultivars efficiently improved nutrient absorption of root system and the shoot growth of maize. Moreover, DH cultivars coordinated the source-sink relationship by increasing dry matter accumulation before silking and dry matter distribution to grains after silking, thereby increasing yield.