Abstract
To clarify the high yielding characteristics of new rice varieties recently bred in the cold region of Japan, we examined their yielding ability, yield component and harvest index in 1996 (24 varieties) and 1997 (21 varieties) in comparison with those of four popular varieties refistered after 1970 in the Tohoku region. The grain yield of these varieties was 716∼845 g/m2 in 1996 and 696∼799 g/m2 in 1997, and the difference among the rice varieties was 100∼130 g/m2 in both years. Twenty varieties of the new rice varieties recorded more than 750 n/m2 grain yield in 1996 and 15 varieties in 1997. Compared with popular varieties, the new rice varieties were classified into four groups:1) the varieties having a larger sink due to larger number of spikelets per m2, including Ouu 316 and Ouu 339, 2) the varieties having a larger sink due to a heavier grain weight and similar number of spikelets per m2, including Fukuhibiki, Akita 59 and Manamusume, 3) the varieties having a similar sink size due to a heavier grain weight but smaller number of spikelets per m2, including Okiniiri and Iwanan 6 and 4) the varieties having a similar sink showing a similar number of spikelets per m2 and similar grain weight, including Hananomai and Jyoudeki. These results suggest that in order to breed high yielding rice varieties in the cold region of Japan, we must enlarge the sink capacity by either increasing the number of spikelets per m2 or incresing the grain weight maintaining a similar number of spikelets per m2.
