Higher crop performance of rice hybrids than of elite inbreds in the tropics : 1. Hybrids accumulate more biomass during each phenological phase

The better performance of hybrid rice compared with elite inbreds has been associated in previous studies with higher harvest index and higher shoot biomass at maturity, with the occurrence of one excluding the other and depending on weather conditions. These outputs were examined here by comparing high-yielding genotypes of similar crop duration and through an intense sampling schedule to identify key crop stages and processes involved in hybrid rice performance. Four contrasting hybrids and four contrasting inbreds were grown on the IRRI farm during a wet and a dry season under the same cultural practices. The grain yield of hybrids was significantly higher than that of inbreds by 14% in the wet season and 18% in the dry season. Crop growth rate (CGR) during each phenological phase, blade growth rate during the vegetative phase, stem growth rate during the reproductive phase and panicle growth rate during the ripening phase were consistently higher for hybrids than for inbreds. As a consequence, the shoot biomass of hybrids increased quicker with time than that of inbreds. And the larger biomass accumulation observed for hybrids at maturity, 2139 and 1906 g m_2 vs. 1952 and 1736 g m_2 for the dry and wet seasons, respectively, was systematically associated with a higher harvest index. In contrast to most of the previous studies aiming at characterizing the higher performance of hybrid rice, differences in leaf area production and in specific leaf area among plant types during the vegetative phase were inconsistent and rarely significant in this study and could not support the higher performance of hybrids. The tillering dynamics was similar between both plant types. This work highlighted the key role of each phenological phase in contributing to the higher biomass production of hybrid rice regardless of the cropping season, dry or wet, while the processes underlying this higher performance are still unclear. (Résumé d'auteur)