Longer mesocotyl contributes to quick seedling establishment, improved root anchorage, and early vigor of deep-sown rice

Abstract

Dry direct seeding of rice (DDSR) is practiced as a technique to address drought in tropical Asia, as it requires less water than transplanting. However, rice has a narrow range of optimal sowing depth, and deep sowing often causes poor seedling emergence. The objective of this study was to elucidate differences among rice cultivars in their tolerance to deep sowing (i.e., the ability to show vigorous seedling emergence from deep soil) and the traits associated with this vigor. Two-year on-farm research in rainfed lowlands showed mean seeding depths of 16.1–35.0 mm (a coefficient of variation of 27%–49%). The deep-sowing tolerance differed significantly among 16 cultivars in research station field experiments. The emergence of seedlings from sowing at a depth of 85 mm was >80% in tolerant cultivars versus <1% in cultivars popular with farmers. Deep-sowing tolerance was attributable primarily to the elongation ability of the mesocotyl and, to a lesser extent, of the 1st internode. In a pot experiment that simulated drought around the time of sowing, seedlings that germinated from deeper soil had greater emergence and greater shoot and root growth than those that emerged from shallow soil, as the deeper seeds could acquire residual soil moisture. Our results suggest that improving the tolerance of rice to deep sowing should stabilize seedling establishment in DDSR. Cultivars that tolerate deep sowing will allow further development of the deep sowing technique and reduce the risk of sowing failure caused by drought during seedling emergence in water-scarce environments.