N-UPTAKE, N-PARTITIONING, AND PHOTOSYNTHETIC N-USE EFFICIENCY OF AN OLD AND A NEW MAIZE HYBRID

An old maize (Zea mays L.) hybrid (Pride 5) has been shown to be less tolerant to N stress than a new maize hybrid (Pioneer 3902) during ear ly phases of development. The objective of this study was to quantify the response of the two hybids to N supply in terms of N uptake, N par titioning, and photosynthetic N-use efficiency. Plants were grown unde r controlled-environment conditions until the 12-leaf stage at three l evels of N supply (i.e., 15 mM N, 2.5 mM N, and 0.5 mM N) and were sam pled at the 4-, 8-, and 12-leaf stages. Rates of N uptake per unit gro und area were higher for Pioneer 3902 than for Pride 5 under maximum N stress during the 8- to 12-leaf phase, but rates were higher for Prid e 5 at high N. Rates of N uptake per unit root weight were higher for Pioneer 3902 than for Pride 5 under both medium and low N supply. The old hybrid (Pride 5) partitioned more dry matter and N to leaves than the new hybrid under low N supply, but leaf N per unit leaf area was h igher for the new hybrid. The new hybrid (Pioneer 3902) maintained gre ater rates of leaf photosynthesis per unit leaf N regardless of N supp ly. Consequently, results indicate that the higher N-use efficiency of Pioneer 3902 under low N supply is associated with higher N uptake an d a higher leaf N per unit leaf area.