GROWTH OF SUGARCANE UNDER HIGH INPUT CONDITIONS IN TROPICAL AUSTRALIA.2. SUCROSE ACCUMULATION AND COMMERCIAL YIELD

Information on the processes controlling the accumulation of sucrose o ver time can be used to assess the physiological basis of yield variat ion and consequently the scope for yield improvement in sugarcane, As commercial yield is commonly expressed on a fresh weight basis, and dr y weight measures of sucrose accumulation aid biological interpretatio n, there is a need to study yield accumulation on both a fresh and dry weight basis. This study analysed the pattern of sucrose accumulation in the stalk in relation to crop biomass, and the concentration of su crose in the stalk on a fresh and dry weight basis of two contrasting cultivars (Q117 and Q138) of sugarcane during the same season in tropi cal Australia, with irrigation and under plant and ratoon crop conditi ons. Over the 15 month season, 11 crop samplings were conducted, The k ey findings were that (1) greater than 95% of the aboveground sucrose accumulation is in the stalk; (2) a logistic relationship described th e time trend in stalk sucrose accumulation, with maximum sucrose yield occurring 100 days before final harvest; (3) stalk biomass rather tha n stalk sucrose concentration was the major determinant of stalk sucro se accumulation; (4) maximum stalk sucrose concentration was stable ac ross cultivars and crop classes at a value of 0.48 g g(-1) stalk dry w eight; (5) commercial yield expressed as the fresh millable stalk yiel d plateaued up to 230 days before final harvest and well before the at tainment of maximum stalk biomass and sucrose yield due to increases i n dry matter content during growth; (6) maximum stalk sucrose concentr ation on a fresh weight basis was obtained at a later crop age and was more variable across crops than maximum stalk sucrose concentration o n a dry weight basis. The study highlighted the major influence that s talk dry matter content has on the relationship between sucrose yield and commercial yield, and that biological interpretation of crop respo nse to climatic and management factors is difficult based on commonly available fresh weight measures of productivity. Optimising economic r eturn from commercial sugarcane production requires further understand ing on the factors controlling the dynamics of stalk dry matter conten t.