GROWTH OF SUGARCANE UNDER HIGH INPUT CONDITIONS IN TROPICAL AUSTRALIA.3. ACCUMULATION, PARTITIONING AND USE OF NITROGEN

Leaf nitrogen (g N m(-2) leaf) is an important determinant of crop rad iation-use efficiency. It is not known to what extent sugarcane can ma intain high leaf N over its long growth duration. This study analyses the accumulation of N in two contrasting cultivars of sugarcane (Q117, Q138) under plant and ratoon crop conditions, and the partitioning of N to the various plant components, including the leaf. The crops were grown for 15 months under irrigated conditions in the same season and received 34.4 g N m(-2) as fertiliser over the first 100-120 days of the season. Higher early N accumulation by the ratoon crop was associa ted with higher early biomass production. However, maximum N accumulat ion was unrelated to maximum biomass accumulation, with the plant crop (25.9 g N m(-2)) accumulating more than the ratoon crop (21.3 g N m(- 2)), which resulted in widely varying values for biomass/N ratio. N ac cumulation ceased later in the plant (200 days) than in the ratoon (15 0 days) crop, and this occurred 100-140 days before maximum biomass. M ore work is needed to determine if this is due to exhaustion of soil N supply, reduced root activity, or a lowered crop N requirement. Leaf N was maintained above 1.2 g N m(-2) for 300 days of the 450 day seaso n. Decline in leaf N below 1.2 g N m(-2) at the end of the season was associated with loss in leaf and total crop N accumulation, and appare ntly unrelated to the timing of cessation in biomass accumulation. Thr oughout the season, leaf N was higher in the plant than the ratoon cro p (averaged over cultivars), and Q117 versus Q138 (averaged over crop classes), however these differences could not explain crop-class diffe rences in RUE. Cultivar differences in leaf N were due to higher N acc umulation because specific leaf area and partitioning of biomass and N to leaf were unaffected by crop class or cultivar. The biomass/N rati o varied with cultivar and crop class, and increased with crop age.