VARIABILITY AND GENETICS OF TOLERANCE FOR ALUMINUM TOXICITY IN RICE (ORYZA-SATIVA L)

A study was undertaken to investigate the variability among lowland ri ce cultivars and the mode of gene action of aluminum (A1) toxicity tol erance in rice. Pregerminated seeds were grown in a nutrient solution containing 30 ppm A1 and in normal nutrient solution, and relative roo t 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 us ed to investigate the genetics of tolerance by diallel analysis. Of th e 62 cultivars tested, only 3 were found to be sensitive to A1 toxicit y. Among the tolerant cultivars identified, 11 ('Siyam Kuning', 'Gudab ang Putih', 'Siyam', 'Lemo', 'Khao Daeng', 'Siyamhalus', 'Bjm-12', 'Ke tan', 'Seribu Gantang', 'Bayer Raden Rati', and 'Padi Kanji') were fou nd to possess higher levels of tolerance than the improved tolerant up land cultivar 'IRAT104'. Diallel analysis revealed that high RRL is go verned 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 environmental effec ts were low. Findings suggest that when breeding for A1 toxicity toler ance, 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 combi ning ability (SCA) in the genetics of al toxicity tolerance in rice. G CA was more prevalent than SCA. Tolerant parens 'Azucena', 'IRAT104', and 'Moroberekan' were the best general combiners. The presence of rec iprocal effects among crosses suggested the proper choice of parents i n hybridization programs. Results indicated that 'Azucena', 'IRAT 104' , and 'Moroberekan' should be used as the female in crosses for A1 tox icity tolerance.