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This paper reports the results obtained from an investigation of the potential of two crosses that were taken at random from the new plant type (NPT) programme initiated recently at the International Rice Research Institute, whose objective is to increase the yield potential of the direct-seeded, irrigated crop in the lowland tropics to 13–15 t/ha. The pedigrees in the NPT programme have been initiated by crossing temperate japonica varieties from China, Japan and Korea with tropical javanica land races from southeast Asia and are, thus, of a novel kind. The parents, F1 and F3 families of each of the two crosses were raised in completely randomised blocks and their individuals scored for twelve quantitative characters, nine of which were those specified in the NPT ideotype. The results showed that every character was heritable in both crosses, except for harvest index in the first, their heritabilities being mostly moderate to high. An assessment of the potential of these crosses indicated that it should be relatively easy to obtain transgressive segregants from both for the six characters where the NPT targets take the form of an interval, such as days to heading, culm length, number of panicles per plant and number of spikelets per panicle, but more difficult, if not impossible to accomplish this for percentage of filled spikelets and grain yield, whose targets lie well above the parental range. A striking feature of the results obtained from the second cross was a marked loss of fertility, in terms of percentage of filled spikelets, in the F2 and backcross generations which, it is argued, was caused by the disruption of the independently co-adapted gene complexes of the parents by recombination and segregation at F1 meiosis. The pattern of genetic correlations between characters differed markedly between the crosses, suggesting that the chief cause of these correlations was the linkage disequilibrium of genes linked in their inheritance. |
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