New high-yielding, weed competitive rice plant types drawing from O. sativa and O. glaberrima genepools

Abstract

Weed competition is the most important yield-reducing factor in upland and hydromorphic rice environments in Africa. Many of the commonly grown tropical japonica varieties have a high yield potential, but they compete poorly with weeds. By contrast, the indigenous cultivated Oryza glaberrima landraces are highly competitive due to high tillering ability, vigour and leaf area during vegetative growth. But their yield potential is low because of O. glaberrima’s specific panicle type and tendency to lodge. Rice breeders at WARDA recently developed stable and fertile progenies from O. sativa × O. glaberrima crosses that combine the O. saliva panicle type with vegetative growth characteristics of O. glaberrima. We conducted detailed growth analyses for the two parents (CG14 O. glaberrima and WAB56-I04 O. sativa) and four F7 progenies under moist upland conditions and four levels of N fertilization in Ivory Coast during the 1995 wet season. The objective was to develop crop growth models that help define interspecific plant types combining the weed competitiveness of O. glaberrima with the yield potential of O. saliva. The superior vegetative vigour and leaf area index (LAI) of the CG14 parent was due to at least five factors, a high specific leaf area (SLA; around 30m²kg⁻¹) which remained constant from seedling stage to maturity, and that was responsible for the pale appearance of the crop; high (>0.6) initial assimilate partitioning to leaves; high leaf N concentration on a dry weight basis; droopy leaves; and extremely high tillering rate. The glaberrima parent CG14 had two to three times the LAI, and 1.5 to 2 times the tiller number of WAB56-104, the O. saliva parent. The progenies had intermediate SLA, LAI, leaf N content and assimilate partitioning patterns. They combined the superior vigour of CG14 with the panicle structure and sturdy stems of WAB56-104. Preliminary modelling studies using ORYZAI indicated that early groundcover and high yield potential can be achieved with the same plant type if it has high SLA (glaberrima type) during vegetative growth, followed by low SLA (saliva type) during the reproductive phase. This was partially achieved in some progenies, resulting in yields up to 5.6tonha⁻¹ on the basis of 25×25 cm hill spacing and 80kgNha⁻¹ applied. We will further refine the model and our plant type concepts during 1996 on the basis of weed competition and drought studies, accompanied by a detailed analysis of canopy architecture and leaf gas exchange.