Changes in the panicle-related traits of different rice varieties under high temperature condition

The size and structure of panicles are important factors which contribute to both yield and quality of rice. Changes in the panicle-related traits by elevated temperature were studied using 40 rice varieties including 27 japonica and 13 Tongil-types (derived from indica-japonica crosses). Tongil-type rices showed larger sink size than japonica either in ambient and high temperature conditions, mainly due to the higher number of secondary rachis-branches (SRBs) and spikelets on SRBs per panicle, which showed highly positive correlations (>0.95) with the numbers of spikelets and rachis-branches per panicle. On the other hand, percentages of ripened grains and head rice (%) were significantly higher in japonica than in Tongil-type under both conditions. Under high temperature, Tongil-type showed significant increase in the numbers of spikelets, rachis-branches, SRBs and spikelets on SRBs per panicle, while japonica did not show any significant change in sink size. However, the percentages of ripened grains and head rice were significantly reduced by high temperature in both ecotypes. While the decrease in ripened grains percentage was higher in Tongil-type, head rice percentage reduced more in japonica than in Tongil-type. For both ecotypes of rice, the percentage of ripened grains was higher in primary rachis-branches (PRBs) than in SRBs in ambient and high temperature conditions. Moreover, the decrease in the ripened grains percentage by temperature rise was higher in SRBs than PRBs. Therefore, it would be desirable to breed varieties with increased number of PRBs rather than SRBs to minimize the negative impact of warmer climate on the ripening of rice grains.