Rainfed lowland rice breeding strategies for Northeast Thailand. I. Genotypic variation and genotype x environment interactions for grain yield

The magnitude and nature of genotype-by-environment (G x E) interactions fo r grain yield, days-to-flower and plant height of rainfed lowland rice in N ortheast Thailand were examined using random F-7 lines from seven crosses d eveloped by the Thai breeding program. A total of 1116 lines and checks wer e evaluated in a multi-environment trial conducted across three years (1995 -1997) and eight sites. The G x E interaction was partitioned into componen ts attributed to genotype-by-site (G x S), genotype-by-year (G x Y) and gen otype-by-site-by-year (G x S x Y) interactions. The G x S x Y interaction w as the largest G x E interaction component of variance for all three traits . There was little G x S interaction for grain yield and days-to-flower. Th e G x S interaction was significant for plant height, but was the smallest component of variance for that trait. The G x Y interaction component of va riance was significant for all three traits, but was small relative to the genotypic component for days-to-flower and plant height. For grain yield th e G x Y interaction component was comparable in size to the genotypic compo nent. Partitioning the genotypic and G x E interaction components of varian ce into among-cross and within-cross components indicated that there was si gnificant variation both among and within the crosses for each trait. The r elationships between the three traits differed among the crosses and the en vironments. A major factor contributing to the large G x S x Y interactions for grain yield was the genotypic variation for days-to-flower in combinat ion with environmental variation for the timing and intensity of drought. S ome of the interactions associated with timing of drought were repeatable a cross the environments sampled in the multi-environment trial, and to some extent the environments were characterised on the basis of whether there wa s pre-flowering, intermittent, or terminal drought. There was genotypic var iation for grain yield after taking into consideration the influences of ti ming of drought in relation to plant development, Three of the seven crosse s involved the Thai cultivars KDML105 and RD6 as parents. These crosses pro duced an array of progeny with lower yield than the Thai cultivars, suggest ing it would be difficult to improve on the yield of these cultivars. In co ntrast three of the remaining four crosses, which did not have a Thai culti var as a parent, produced progeny that had higher yield than KDML105 and RD 6 indicating that yield of rainfed lowland rice could be improved above tha t of the these popular cultivars. (C) 1999 Elsevier Science B.V. All rights reserved.