Stability and yield performance of genotypes: A proposal for regrouping world rice area into mega-environments

Rice ecosystems are characterized by elevation, rainfall pattern, depth of flooding and drainage, and by the adaptation of rice to these agroecologica l factors. Many divisions were made in the attempts at defining and identif ying specific ecosystems and their breeding objectives. Genotypes are evalu ated every year in multilocational trials. The aim was to study the perform ance of breeding lines developed for various ecosystems and to identify sta ble genotypes with wide adaptability. The data from 1341 all-India coordina ted experiments (1970-94), and 1305 international experiments (1975-94) on the performance of rice cultivars were used. Mean yield, coefficient of var iation and regression analyses identified several cultivars, developed at d ifferent centres, as stable for yield. These genotypes, with varying geneti c makeup that crossed geographic boundaries and spread over ecosystems base d on their sensitivity or insensitivity to photoperiod and maturity duratio n, are identified as universal genotypes for a specific mega-environment. T hey differ in the genetic expression of maturity period and photosensitivit y. The diversity in these genotypes has successfully prevented vulnerabilit y and yield instability in mega-environments. In this study, no genotype wa s found to significantly surpass the grain yield of 10 t/ha, established by the photoinsensitive universal genotypes, IR 8 and Jaya with medium maturi ty period, developed in 1966 and 1968, respectively. The singular reason fo r this dismal performance is the plethora of breeding objectives assumed es sential for the targeted ecosystems. We, therefore, suggest a regrouping of rice habitats into four mega-environments: (1) rainfed unfavourable upland s requiring varieties with photoinsensitivity and very early maturity (< 90 days); (2) rainfed favourable uplands and irrigated areas requiring variet ies with photoinsensitivity and early maturity (90-110 days); (3) irrigated areas requiring varieties with photoinsensitivity and medium maturity (120 -135 days); and (4) rainfed lowlands requiring varieties with photosensitiv ity or insensitivity and late maturity (> 140 days). This delineation can m otivate rice breeders to push yields of universal genotypes beyond the 10 t /ha barrier.