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

Little quantitative information relating yield accumulation in sugarca ne to climatic factors is available to allow the maximum yield in diff erent 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 fa ctors can be assessed. This paper analyses the relationship between ra diation interception and biomass accumulation for an autumn-planted su garcane crop grown under irrigated conditions at Ayr, Qld (lat. 195 de grees S.). Crop samplings were conducted from 167 to 445 days after pl anting (DAP). Less than 60 degrees of the seasonal incident solar radi ation was intercepted by the crop. A radiation extinction coefficient of 0.38 was estimated from the relationship between green leaf area in dex 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) w as 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 inci dent radiation (S), and accordingly would be expected to vary across l ocations. In contrast, the RUE value of 1.75 g MJ(-1) is independent o f fi and S, and can be used as a baseline value to assess the extent o f yield limitation and the scope for yield improvement at different lo cations. The maximum biomass production was 72 t ha(-l) and the maximu m fresh cane yield was 201 t ha(-l). However, these maximum yields wer e attained up to 4 months before the final sampling. Future research s hould examine the wider applicability of this early yield plateau, and focus on the factors responsible for the early cessation in yield acc umulation.