EFFECT OF NITROGEN ON YIELD DETERMINATION IN IRRIGATED MAIZE IN TROPICAL AND SUBTROPICAL ENVIRONMENTS

An understanding of the physiological basis of yield accumulation in r esponse to nitrogen (N) supply can assist the development and testing of mechanistic crop growth models. Models are important tools to asses s the scope for improving N fertiliser management by taking account of variation in climatic and soil N conditions across sites and seasons. Accordingly, the interaction of soil N supply, N fertilisation and cl imatic conditions on yield determination in maize was assessed by cond ucting seven held experiments with the same hybrid in tropical and sub tropical environments. The physiological analysis examined crop N upta ke and allocation to vegetative tissues, leaf growth, radiation interc eption, biomass and grain yield accumulation, and mobilisation of vege tative N to grain N during grain-filling. Where N limited yield, grain yield was positively correlated with crop N uptake, whereas under hig h N supply, yields were higher in the subtropical environment due to t he lower-temperature regime extending the crop duration. Thr: response of grain yield to N supply and climatic conditions in the different e xperiments was associated with much larger effects on biomass producti on than on harvest index. Where N supply limited yield, the decrease i n biomass production was associated with a much larger decrease in rad iation-use efficiency (RUE) than in radiation interception (S-i), wher eas under high N supply, differences in biomass production between exp eriments were associated with differences in Si. Under both low and hi gh N supply, the grain demand for N could not be met solely by soil N uptake during grain-filling, and there was significant mobilisation of vegetative N to grain N. Consequently, leaf N and RUE declined during grain-filling in all situations.