Spikelet numbers, sink size and potential yield in rice

Abstract

A new plant type (NPT) has been developed at the International Rice Research Institute (IRRI) using tropical japonica germplasm in an attempt to increase yields. In direct contrast to high yielding indicas, which have large numbers of tillers and relatively small panicles, the NPT has large panicles and low tiller numbers. To determine whether the NPT had a higher potential for spikelet production, and possibly yield, than an elite indica, the total number of juvenile spikelets (glumous flower primordia) was determined for plants grown in the field as a crop or widely spaced. The maximum number of juvenile spikelets produced by the temporal pattern of tillers in a hill was recorded at the time of late differentiation of each tiller’s panicle. The number was greater for spaced plants than crop plants and it decreased linearly with tiller age. For crop plants approximately one juvenile spikelet was produced per square centimeter of leaf area. In 1997, a large number of the juvenile spikelets were lost before harvest, that number increased significantly in the wetter year of 1999. To quantify potential yield and its reduction by the loss of spikelets between the juvenile and mature stages, the number of spikelets at harvest and the filled fraction (grains) was determined for each tiller and a simple empirical model of yield was constructed. The model predicted that if all of the juvenile spikelets were to be transformed into grains a potential yield more than double the actual yield would be produced in crops of high yielding indicas or the NPT, suggesting that source strength rather than sink size limited yield in this study. Differences in spikelet number at harvest could be offset by differences in individual grain weight and percentage filling of spikelets. Even in rice crops grown with large applications of fertilizer, at current atmospheric concentrations of CO 2, yields are limited by the supply of resources necessary for the development and synthesis of the grain rather than sink capacity. The potential sink size of the crop is almost double that actually realized, even though the productive tillers of crop plants do not reach their full potential for spikelet production compared with spaced plants.