Rainfed lowland rice breeding strategies for Northeast Thailand.: I. Genotypic variation and genotype × environment interactions for grain yield
Introduction
Investigations of genotypic variation and genotype-by-environment (G × E) interactions for grain yield of rainfed lowland rice have been conducted in and across a number of Asian countries (Henderson et al., 1996; Cooper and Somrith, 1997; Wade et al., 1997, Wade et al., 1999). Generally these studies used sets of lines that had been selected to represent recognised genotypic diversity for adaptation to a range of the rainfed lowland rice ecosystems. A major objective of these exploratory studies was to investigate environmental and genetic constraints to the improvement of broad and specific adaptation of rainfed lowland rice for a range of target environments (Cooper, 1999). An understanding of these constraints was sought as a basis for defining breeding strategies that would contribute to higher and more stable grain yields for the heterogeneous rainfed lowland ecosystems. These studies consistently identified large crossover type G × E interactions for yield. The presence of these interactions has important implications for breeding strategies that aim to improve either broad or specific adaptation or some combination of both components of adaptation. Crossover G × E interactions can be a significant impediment to selection strategies that aim to improve broad adaptation. Alternatively, where some aspects of the G × E interactions are repeatable, it may be possible to select for components of specific adaptation to the relevant target environments. It has been argued that a breeding program aimed at recombining components of specific adaptation to drought environments may be a strategy for improving drought resistance and broad adaptation of rainfed lowland rice (Fukai and Cooper, 1995; Nyguen et al., 1997; Cooper, 1999).
The use of selected sets of diverse lines in the previous adaptation investigations makes it difficult to evaluate the implications of their results for the design and optimisation of crossing and selection strategies within a breeding program. To enable this step it is necessary to obtain estimates of the magnitude and form of genetic, G × E interaction and error components of variation that are relevant to both the target population of environments and the germplasm used in the breeding program. The focus of the present study is to investigate the implications of G × E interactions for genetic improvement of the grain yield of rainfed lowland rice in drought-prone Northeast Thailand. The objectives were to: (1) document the magnitude of sources of genotypic, G × E interaction, and error variation for grain yield, based on a sample of breeding lines relevant to the Thai breeding program for Northeast Thailand; (2) examine the influence of timing of drought and genotypic variation for flowering time and plant height on G × E interactions for grain yield; and (3) evaluate potential sources of repeatable G × E interactions for grain yield. A companion paper (Cooper et al., 1999) evaluates suggested modifications to the current breeding strategy used in Northeast Thailand.
Section snippets
Reference population: development of experimental lines
Seven biparental crosses were sampled from the crossing program of the Thai rainfed lowland rice breeding program in 1992 (Table 1). A series of random inbred lines were derived from each cross. These inbred lines, their parents, the five check cultivars KDML105, RD6, RD23, NSG19, Chiangsaen, and one check line IR57514-PMI-5-B-1-2, were used as the experimental lines in the multi-environment trial (MET). Seed was not available for one of the parents of Cross 4, NR15013-40-10-7, so this line
Experimental conditions and environmental characterisation
The seeding and transplanting dates of the experiments differed among the site–year combinations (Table 3). In each of the three years, two experiments were seeded late. With early seeding and no drought or stress conditions (e.g. 1995 PSL), the time from seeding to flowering was longer for the strongly photoperiod sensitive check lines. As expected with later seeding, the differences in flowering time between the strongly photoperiod sensitive and less sensitive check lines decreased and in
Discussion
The analysis of the sources of variation based on the results of the METs identified significant components of genotypic, G × E interaction and error variation for the traits grain yield, days-to-flower and plant height. Partitioning the G × E interaction component indicated that the three-factor G × S ×Y interaction was consistently the largest source of G × E interaction for all three traits. The G × S × Y interaction was 4.3 times the size of the genotypic component of variance for grain yield, but for
Acknowledgements
The contributions and commitment of the many Thai scientists involved in conducting the experiments is gratefully acknowledged. The Australian Centre for International Agricultural Research provided the financial support for conduct of the experiments.
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