Adaptation and performance of rice genotypes in tropical and subtropical environments

Standardized field experiments were carried out to study the performance of five rice genotypes derived from different germplasm in terms of yield, harvest index (HI) and grain quality at eight agro-ecological sites of the tropics and subtropics across Asia during 2001 and 2002. Considering that indica and javanica genotypes adapt to warm climatic conditions, and japonica genotypes to cool agro-climatic conditions, it is hypothesized that indica × japonica hybrids may combine high yields and good quality traits under a wide range of agro-climatic conditions. Grain yield, HI, protein content and amylose content varied considerably among genotypes and environments. Mean rice yields of genotypes ranged from 1.5 to 11 t ha-1 across the eight sites; on average yields were 7.2 t ha-1 under subtropical and 2.7 t ha-1 under tropical conditions. The much lower yields in tropical environments resulted from a low biomass as well as a low HI. Among the genotypes, the indica × japonica hybrid showed the highest yield under subtropical conditions, and a higher yield than the japonica genotypes and the indica × javanica hybrid but lower than the indica genotype under tropical conditions. Phenology of genotypes varied strongly across environments. Low yields at tropical locations were associated with a low light capture due to short growth duration. Post-anthesis light-use efficiencies and the photothermal quotient explained much of the variation in yield. Protein content varied among genotypes depending on location and year. Variation in amylose content of rice grains was mainly associated with genotypic differences and much less with environmental conditions, but contents decreased with higher post-anthesis ambient temperatures. The indica × japonica hybrid combined high yields with a favourable amylose content and showed a better ability to adapt to cool and to warm agro-climatic conditions than the indica or japonica genotypes. Our study showed the magnitude of yield penalties associated with growing rice genotypes in environments to which they are not adapted. The consequences of these findings for improved adaptation of rice are discussed