A study was undertaken to investigate the variability among lowland rice cultivars and the mode of gene action of aluminum (Al) toxicity tolerance in rice. Pregerminated seeds were grown in a nutrient solution containing 30 ppm Al and in normal nutrient solution, and relative root length (RRL) was determined at the 14-day-old stage to characterize genotypes for tolerance. Sixty-two traditional rice cultivars grown on lowland acid sulfate soil areas of Asia and West Africa were tested. Tolerant varieties 'Azucena', 'IRAT104', and 'Moroberekan', moderately sensitive 'IR29' and 'IR43', and sensitive 'IR45' and 'IR1552' were used to investigate the genetics of tolerance by diallel analysis. Of the 62 cultivars tested, only 3 were found to be sensitive to A l toxicity. Among the tolerant cultivars identified, 11 ('Siyam Kuning', 'Gudabang Putih', 'Siyam', 'Lemo', 'Khao Daeng', 'Siyamhalus', 'Bjm-12', 'Ketan', 'Seribu Gantang', 'Bayer Raden Rati', and 'Padi Kanji') were found to possess higher levels of tolerance than the improved tolerant upland cultivar 'IRAT104'. Diallel analysis revealed that high RRL is governed by both additive and dominance effects with a preponderance of additive effects. The trait exhibited partial dominance, and one group of genes was detected. Heritability was high, and environmenal effects were low. Findings suggest that when breeding for A1 toxicity tolerance, selection can be made in early generations. The pedigree method of breeding would be suitable. Combining ability analysis revealed the importance of both general combining ability (GCA) and specific combining ability (SCA) in the genetics of A1 toxicity tolerance in rice. GCA was more prevalent than SCA. Tolerant parens 'Azucena', 'IRAT104', and 'Moroberekan' were the best general combiners. The presence of reciprocal effects among crosses suggested the proper choice of parents in hybridization programs. Results indicated that 'Azucena', 'IRAT 104', and 'Moroberekan' should be used as the female in crosses for A1 toxicity tolerance.
A representative group of 100 elite lines, 67 of which are restorers and 33 maintainers, from 68 crosses made at IRRI and 18 improved varieties from five countries were studied, using Mahalonobis, D(2)-statistic and canonical analysis, to understand the nature and magnitude of divergence and to assess the importance of a set of quantitative characters related to yield in genetic differentiation. The 100 genotypes were grouped into 13 clusters. There were three single variety clusters and the number of lines in the remaining clusters ranged from 2-36. Canonical vector values indicated the importance of yield followed by 1,000-grain weight, days to maturity and plant height in primary as well as in secondary differentiation. Results showed that yield, number of tillers per plant, days to maturity and 1,000-grain weight contributed largely to the divergence. There were no indications of a relationship between geographical diversity and genetic diversity in the present study. Disposition of IRRI developed maintainers and restorers into various clusters indicated the presence of large amounts of diversity within the IRRI elite lines which suggested that these materials could be used in crossing programs to produce heterotic F1 hybrids. Crossing of maintainers and restorers among the highly diverse groups was suggested as it may produce F1S that will give higher magnitudes of heterosis.
An experiment to locate QTL conferring resistance against Peronosclerospora philippinensis causing Philippine downy mildew was observed in a BC1F2 population developed from a cross between the susceptible inbred line Pi 23 and the resistant inbred line P 345. Phenotypic data were collected in Los Baños, UPLB for disease incidence. A total of four regions were associated with disease incidence. Some of QTL associated with disease incidence have been reported previously, and the remaining QTL are described here for the first time. The results on QTL analysis using molecular markers also agree with the previous study using classical generation mean analysis of the cross combination Pi 23 x P 345.
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