RADIATION INTERCEPTION AND BIOMASS ACCUMULATION IN A SUGARCANE CROP GROWN UNDER IRRIGATED TROPICAL CONDITIONS

Little quantitative information relating yield accumulation in sugarcane to climatic factors is available to allow the maximum yield in different seasons and locations to be determined. By comparison of actual yield with the climatically determined maximum yield for a given crop, the extent of yield limitation due to management and soil and pest factors can be assessed. This paper analyses the relationship between radiation interception and biomass accumulation for an autumn-planted sugarcane crop grown under irrigated conditions at Ayr, Qld (lat. 195 degrees S.). Crop samplings were conducted from 167 to 445 days after planting (DAP). Less than 60 degrees of the seasonal incident solar radiation was intercepted by the crop. A radiation extinction coefficient of 0.38 was estimated from the relationship between green leaf area index and the fraction of the radiation intercepted (fi). A maximum crop radiation (SW, 0.35-2 5 mu m) use efficiency (RUE) of 1.75 g MJ(-l) was determined. The maximum crop growth rate over a 140 day period was 41.1g m(-2) d(-1). However, this value is dependent on fi and the incident radiation (S), and accordingly would be expected to vary across locations. In contrast, the RUE value of 1.75 g MJ(-1) is independent of fi and S, and can be used as a baseline value to assess the extent of yield limitation and the scope for yield improvement at different locations. The maximum biomass production was 72 t ha(-l) and the maximum fresh cane yield was 201 t ha(-l). However, these maximum yields were attained up to 4 months before the final sampling. Future research should examine the wider applicability of this early yield plateau, and focus on the factors responsible for the early cessation in yield accumulation.