Designing a rice variety for high yielding environments using source-sink balance analysis

To identify possible reasons for varietal differences in grain yield in rice under favourable conditions, it is common to determine whether yield is limited by grain sink size, that is the product of grain number meter -2 and 1,000 grain weight, or conversely the assimilate source availability to fill all grains. This review examined the results of such source-sink balance studies conducted under favourable conditions with the yield of at least one variety exceeding 11.5 t ha -1 . The ratio of source availability and sink size of 28 varieties determined in 9 papers was found to be related to seed set (filled grain percentage), and the yield of a variety with the seed set exceeding 90% was considered sink limited as further increase in source availability is unlikely to increase grain yield. Among 23 papers where seed set was determined for two or more varieties, there were more cases of sink rather than source limitation in the lower yielding variety. In these cases, larger sink size and yield were often achieved through increased spikelets panicle -1 , but there were also cases that had a larger number of panicles m -2 . Varieties with large sink size need to have matching capacity to produce assimilate supply, particularly during the grain filling period. Physiological and morphological traits to increase sink size and source availability were described. Varietal improvement programs in high yielding environments could apply the source-sink balance analysis to identify limitations of the current varieties, which could be used to drive further yield improvement.