2021 Volume 90 Issue 4 Pages 382-392
Almost all manuals and textbooks for direct seeding of rice (Oryza sativa L.) in well-drained paddy fields in Japan recommend a sowing depth of 2 to 3 cm. This practical sowing depth is empirical and the eco-physiological basis is unknown. We carried out field experiments using sowing machines similar to those used by farmers and the leading variety in Japan, Koshihikari. The correlations between sowing depth and the lengths of coleoptiles, 1st leaves, and 2nd leaf sheaths were significant, irrespective of cumulative temperatures from sowing to sampling. Mesocotyl length was added to the length of these organs and plotted against sowing depth. These plots indicated that the net length of coleoptiles and 1st leaves increased according to the sowing depth up to 4 cm. The net length of these organs fell short of sowing depth when sown deeper than 4 cm, and the number of greening seedlings declined sharply. In suitable temperature and water conditions in artificial climate chambers, the net length of coleoptiles, 1st leaves, and 2nd leaf sheaths fell short of sowing depth if seeds were sown deeper than 5 cm, and seedling establishment declined significantly. These results revealed that the practical sowing depth was determined by the ability of the rice plant to elongate the net length of the coleoptiles and the 1st leaves, that penetrated the covering soil.
