Shoot growth potential drives N uptake in maize plants and correlates with root growth in the soil

Variation in nitrogen (N) acquisition ability is known to exist among maize genotypes. Field experiments were conducted and the N-efficient maize inbred line 478 and the N-inefficient line Wu312 were employed to illustrate whether the amount of N taken up in maize plants with different N acquisition ability was determined by the shoot growth potential or by the root size. To meet the accelerated growth of the shoot from the jointing stage to the grain-filling stage, the net N gain in whole plants of both genotypes increased dramatically and accounted for 77% and 74% of the total N increment in 478 and Wu312, respectively. Similarly, the 4th to 8th nodal root whorls were initiated predominantly between 35 and 76 d after sowing, which accounted for about 90% of the total root length on 93 d after sowing. The whole plant N content of the N-efficient 478 was significantly higher than that of the N-inefficient Wu312. 478 had also longer root length, including axial and lateral roots, of the embryonic roots and each whorl of shoot-borne roots, and greater root length density (RLD) than Wu312. In spite of the smaller root size, Wu312 had higher shoot N concentration than 478 during the whole growth period, implying that N was not limited for shoot growth in Wu312. It was concluded that maize root growth, especially initiation and development of the shoot-borne roots, as well as the amount of N taken up were coordinated with shoot growth and demand for nutrients. Although a large root system and high RLD in the soil profile were beneficial for efficient N acquisition, amount of N taken up by the two maize genotypes in the presence of sufficient N supply was determined by the shoot growth potential, and not by the root size.