GROWTH OF SUGARCANE UNDER HIGH INPUT CONDITIONS IN TROPICAL AUSTRALIA.1. RADIATION USE, BIOMASS ACCUMULATION AND PARTITIONING

There is little detailed information on yield accumulation in sugarcan e under high-input conditions, which can be used to quantify the key p hysiological parameters contributing to yield variation. Sugarcane is grown under plant and ratoon crop conditions. This study analysed cano py development, radiation interception and biomass accumulation of two contrasting cultivars of sugarcane under irrigation during the same s eason under plant and ratoon crop conditions. Over the 15 month season , 11 crop samplings were conducted. Biomass partitioning to stalk was also measured to determine to what extent differences in partitioning between cultivars under ratoon and plant crop conditions contribute to differential productivity. The key findings were: (1) The ratoon crop accumulated biomass more quickly than the plant crop during the first 100 days of growth due to higher stalk number, faster canopy developm ent and greater radiation interception. For similar reasons, cultivar Q138 had higher early biomass production than cultivar Q117 in the pla nt crop, (2) Early differences in biomass accumulation due to crop cla ss became negligible at about 220 days because maximum RUE of the plan t crop (1.72 +/- 0.01 g MJ(-1)) was 8% higher than in the first ratoon crop (1.59 +/- 0.08 g MJ(-1)). The higher maximum RUE in the plant cr op was consistent with a higher crop growth rate (35.1 +/- 2.3 versus 31.0 +/- 3.4 g m(-2) d(-1)) during the Linear phase of biomass accumul ation. (3) Biomass accumulation, which ceased about 300 days after pla nting/ratooning and 140 days before final harvest, attained similar le vels of 53-58 t ha(-1) in all four crops. (4) The plateau in biomass w as associated with loss of live millable stalks, and not a cessation i n the growth rate of individual stalks. The crops continued to interce pt radiation while on the biomass plateau, so that average RUEs at fin al harvest were much lower than the maximum values. (5) There was no e ffect of crop class or cultivar on the fraction of biomass in the leaf and millable stalk components. This study emphasizes that maximising early radiation interception and biomass accumulation will not necessa rily lead to higher yield in an environment where biomass production r eaches a plateau well before final harvest. Loss of live millable stal ks late in the crop cycle results in poor utilisation of intercepted r adiation.