Rice breeding for weed competitiveness requires improved screening tools. An ongoing breeding program uses O. glaberrima as genetic donor for growth vigor and weed competitiveness. This study investigates morphophysiological characteristics relevant to seedling vigor, namely, the kinetics of reserve mobilization, dry matter partitioning among organs, and specific leaf area (SLA). Five diverse cultivars (O. glaberrima upland rice, improved and traditional tropical O. sativa japonica upland rices, an improved O. sativa indica cultivar and an interspecific progeny) were grown in the screenhouse for 18 days on wet soil. Dry matter fractions of plant organs were measured daily, leaf area (LA) and SLA were measured 9, 14 and 18 days after seed soaking (DAS). SLA measurements were repeated using crowded populations simulating farmers' seedbed nurseries. Seedlings achieved photo-autotrophic growth between 7.6 and 9.3 DAS, and had compensated for respiration losses during heterotrophic growth at 10.3-12.4 DAS, with O. glaberrima and improved indica cultivars requiring shorter periods than tropical japonica cultivars, For all cultivars. dry matter partitioning coefficients (PC) for roots were initially large but dropped temporarily to near zero during the transition from heterotrophic to autotrophic growth, The O. glaberrima cultivar had the largest PC for laminae and the largest SLA among the cultivars. It is concluded that the superior early growth vigor of the O. glaberrima cultivar, as documented in previous studies, is partly due to (1) early onset of autotrophic growth, (2) high PC for leaves, and (3) large SLA. Based on the previous observation that SLA is correlated with weed competitiveness, it appears that SLA measured at about 14 days after germination might be employed as a rapid screen for weed competitiveness. (C) 1999 Elsevier Science B.V. All rights reserved.
