Root size and nitrogen-uptake activity in two maize (Zea mays) inbred lines differing in nitrogen-use efficiency

Considerable differences in response to nitrogen (N) availability among plant species and cultivars have been well documented. Focusing on the uptake of N, it is not clear which factor or factors determine efficient N acquisition. Two maize (Zea mays L.) inbred lines (478, N-efficient; W312, N-inefficient) were used to compare the relative contribution of root uptake activity and root size to IN acquisition. Nitrogen-efficient inbred 478 had higher yields and accumulated more N underfield conditions than W312 under both high- (135 kg IN ha(-1)) and low-N (no N supplied) conditions. The root system of 478, as indicated by total root length, root biomass, and root-to-shoot ratio, was larger and more responsive to low IN stress. Especially, 478 developed more and longer axial roots at low N stress. On the contrary, the average N-accumulation rate in 478 was lower than that of Wu312. In solution culture, (NO3-)-N-13 influx in 478 was lower than in W312 after 8 h of nitrate provision. The expression of nitrate-transporter genes ZmNRT1.1, ZmNRT2.1, ZmNRT2.2, and ZmNAR2.1 was stronger and lasted for a longer time after NO3- induction in W312. It is concluded that the efficient N acquisition in 478 is due to (1) a larger root system and (2) a stronger response of root growth to low IN induction.