Studies on competition in plants VIII. Chromosome number, hybridity and competitive ability inOryza Sativa L

Abstract

In 1953, four varieties of theJaponica rice group were compared for their competitive ability with their respective tetraploid plants. It was found that tetraploids were always inferior to their diploid prototypes in this respect. In 1954, these diploid and tetraploid races ofJaponica varieties, two diploid varieties ofIndica group and diploid and tetraploidF 1 hybrids between two of the fourJaponica varieties or betweenJaponica andIndica varieties were compared for their competitive ability by the use of a standard tester. It was found that theF 1 hybrids often show very notable superiority to both parents. Doubling of the chromosome number in hybrids always decreases their competitive ability just as doubling of chromosomes in pure-line varieties does. It may be possible to find a tetraploid hybrid with higher competitive ability than its parental varieties if the original hybrid possesses a very high competitive ability. This possibility may probably explain the competitive superiority of amphidiploids to their parental species. The very high competitive ability of hybrids is assumed to be due to over-dominance of competitive ability genes in heterozygous condition.