Physiological characteristics of high-yielding rice inferred from cross-location experiments

Understanding the processes controlling yield in well-managed, high-input crops may offer ways of increasing currently recognized yield potentials. In this study, 13 crops of Koshihikari rice were managed with different nitrogen strategies to achieve maximum yield in three environments, Kyoto (6 t ha(-1)) and Ina (10 t ha(-1)) in Japan, and Yanco (13 t ha(-1)) in Australia. A common set of data on radiation interception, growth, yield, yield components, N uptake and non-structural carbohydrates was collected for each crop. There was a similar efficiency of dry matter production per unit of incident global radiation at the three sites, 0.78 g MJ(-1). At Yanco, incident and intercepted radiation were high but were offset by a low conversion of intercepted radiation to dry matter (radiation-conversion efficiency, RCE) of 1.0 g MJ(-1) compared to RCE = 1.4 at Kyoto and Ina. The RCE at Yanco was particularly low during the grain filling stage, apparently due to poor root function associated with low soil aeration and to low tiller survival and low minimum temperatures. If these constraints could be overcome and the RCE increased to the values in Kyoto and Ina, the estimated yield potential at Yanco would be about 18 t ha(-1). Given the relatively high values of RCE at Kyoto and Ina, the most promising way to increase yield potential would be to increase the efficiency of converting growth to grain yield. At all three locations the number of grains was less than the number of spikelets, The quantity of assimilate available for grain production was estimated from the amount of non-structural carbohydrate at heading plus the dry matter growth after heading. The potential sink for assimilate was estimated from the number of florets and the weight of a filled kernel. By comparing these amounts it appeared that some of the crops at Ina were source limited and some were sink limited, while at Kyoto crops were neither clearly source Limited nor sink limited. This paradoxical result is explained by a lack of assimilates soon after heading, leading to poor grain set, followed by a surplus of assimilates during late grain filling. On the basis of other evidence, there appears to be genetic variability for the ability to set grains in conditions of poor assimilate supply, suggesting that in the low-radiation environments in which much rice is grown, there may be scope for increasing yield by increasing the number of fertile florets.