Phenological responses of Oryza sativa, O-glaberrima and inter-specific rice cultivars on a toposquence in West Africa

Phenological properties of rice cultivars, particularly crop duration, dete rmine their yield potential, local agronomic suitability and ability to esc ape from drought. Crop duration of a given cultivar depends mainly on photo period (PP) and temperature, but is also affected by the crop establishment practice and environmental stresses. A sample of 84 ecologically and genet ically diverse rice cultivars was sown on five dates between May and Septem ber 1997 on the flooded-lowland (transplant), hydromorphic and upland level s of a toposequence at 7 degrees 52' N in Cote d'Ivoire, in order to charac terize the cultivars' phenological responses. In the upland, life-saving sp rinkler irrigation was applied when drought symptoms were visible. A non-re plicated design augmented with four replicated checks (four replications pe r ecosystem) was used. Phenology was characterized by date of emergence, fi rst heading, 50% flowering and maturity. The period from emergence to flowe ring was subdivided into three phases following a simple model used at IRRI to characterize germplasm for photoperiodism. For each ecosystem and culti var, the basic vegetative period (BVP) was estimated by subtracting 30 d fr om the duration to flowering at the sowing date associated with the shortes t duration, and expressed in degree-days (dd), assuming a base temperature of 10 degrees C. The PP-sensitive phase (PSP) was estimated by subtracting BVP+30 d from the time to flowering. PP-sensitivity (PS) was calculated fro m the apparent change in PSP between 12.0 and 12.5 h mean astronomic daylen gth during the PSP, by regression across dates. Cultivars differed strongly in BVP (300 to 1200 dd) and PS (0 to 1000 dd). The BVP was generally longe r in the lowland than in the hydromorph, and mostly longer in the upland th an in the hydromorph, possibly due to transplanting shock (lowland) and dro ught (upland). Many cultivars, particularly upland-adapted japonicas, had a greater PS in the lowland than in the upland. Principal-component and clus ter analyses based on BVP and PS in each of the three ecosystems establishe d three large and three small groups of cultivars having common phenologica l responses. The linkage groups were associated with ecotypes (lowland vs u pland, traditional vs improved) and genetic groups (O. sativa japonica and indica, O. glaberrima, inter-specific progenies). The groups were seen to r epresent past selection strategies by farmers and recent breeding strategie s, with respect to achieving yield stability in the various ecosystems. For example, indigenous selection strategies for O. sativa upland rices seem t o have favored a long BVP, whereas from O. glaberrima, which generally has a superior initial vigor, cultivars with a short BVP have been selected. Th e authors conclude that the modern upland rice breeding strategy for the re gion on the basis O. sativa, which aims at drought escape using a short BVP , is paralleled by existing indigenous O. glaberrima materials. Efforts to utilize these materials for breeding are ongoing.