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Coleoptile and Mesocotyl Lengths in Semidwarf Rice Seedlings¹

Drill-seeded semidwarf rice (Oryza sativa L.) emerges slower and with less consistency than taller plant types. To determine if short mesocotyls or coleoptiles cause emergence problems, three semidwarf cultivars (‘Bellemont’, ‘M-101’ and ‘IR-24’), two taller cultivars (‘Labelle’ and ‘Starbonnet’), and strawhulled red rice were planted at 1, 2.5, 4, 6, and 10 cm below the soil surface. Mesocotyl and coleoptile lengths measured on 14-day-old seedlings incubated at 30 C showed that semidwarf plant types had shorter mesocotyls at all planting depths and coleoptiles equal to or longer than the taller cultivars when planted 1, 2.5, 4, and 6 cm deep. For the 10 cm planting depth mesocotyl lengths were 7.9, 13.5, and 35.6 mm for the semidwarfs Bellemont, M-101, and IR-24, while the tall types, Labelle, Starbonnet and red rice, exhibited mesocotyl lengths of 83.5, 53.9 and 24.0 mm, respectively. Coleoptile lengths were 38.3, 47.1, and 63.2 mm for the same semidwarfs and 21.0, 53.9 and 62.7 mm for the tall plant types, respectively. These data illustrate the difference in contribution of mesocotyl and coleoptile toward emergence of a specific cultivar.

Planting depth also influenced mesocotyl and coleoptile length and their relative contributions to emergence. The sum of coleoptile and mesocotyl length of an emerged seedling equaled the planting depth of that seedling. Generally, within a cultivar at a given planting depth, the coefficient of variation for total length was low. However, coefficients of variation for mesocotyl or coleoptile lengths were high. For example, at the 6 cm planting depth, coefficient of variation for total length was only up to 14%, while it was as high as 46 % for mesocotyl or coleoptile length.

Short mesocotyls can contribute to emergence problems of Bellemont and M-101 semidwarf cultlvars when planted 10 cm deep. Labelle emerged from 10-cm depth primarily because of mesocotyl elongation while the other tall plant types and IR-24 emerged because of both mesocotyl and coleoptile elongation.

Key Words: Rice emergence • Stand establishment • Red rice • Drill or direct-seeded rice • Planting depth

² Associate professor and technician, Texas Agric. Exp. Stn. and research agronomist, USDA-SEA/AR, respectively.

Received for publication February 9, 1981.