ALLOCATION AND RETRANSLOCATION OF N-15 BY MAIZE (ZEA-MAYS L) HYBRIDS UNDER FIELD CONDITIONS OF LOW AND HIGH-N FERTILITY

A field experiment was conducted with high and low N fertility regimes to evaluate root-absorbed N-15, and its subsequent remobilisation in four maize hybrids. In previous experiments, potential yield and patterns of senescence had varied among these hybrids. Individual plants were supplied with N-15 at the 12-leaf stage and the uptake of the labelled N and its subsequent partitioning during successive harvests was followed. Grain yield and N accumulation at physiological maturity (PM) were reduced 20 and 40%, respectively, under low N compared with high N fertility. Neither hybrid type nor N fertility level initially affected the amount of N-15 absorbed. Maximum N-15 content was observed at mid grain-fill and a significant 10% loss of N-15 from plants was found at PM. Compared with those under low N, plants cultured under high N maintained higher percentages of N-15 in the lower stalk and roots over hybrids and across harvests. Kernels were a sink for remobilised N-15 from all vegetative and 'other' reproductive tissues. Patterns of N remobilisation varied among different vegetative tissues. N was remobilised from stalks earlier than from leaves, while N from roots was remobilised throughout the season. Stalk and leaves were the major sources of N-15 for kernels (about 45% each) while roots contributed much less. Physiological differences between the hybrid with the highest yield at both fertility levels, and with other hybrids, are discussed.