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Genetic studies of earliness and growth stages of Lycopersicon esculentum Mill. Li, Shin-Chai

Abstract

It is desirable to develop tomato (Lycopersicon esculentum Mill.) cultivars which have the characteristics of earliness to fit the relatively short and cool growing season in Canada. Earliness was studied by parti-tioning the life cycle of the tomato plant into 7 component growth stages and using these as a basis for attempts to recombine quantitative genes which control the earliness of different stages from different parents to obtain progeny earlier than both parents. The mode of inheritance of the earliness in the 7 growth component stages was studied with 3 approaches. First, a complete diallel cross experiments used 3 parental cultivars: Bonny Best, Immur Prior Beta and Cold Set. The progenies were grown under 2 temperature regimes (17.0-21.0°C and 10.0-13.0°C). The data for days required for each stage were analyzed first by the Hayman and Jinks method which estimated the following 4 genetic parameters; variation due to differences in additive and dominant gene action; asymmetry of positive and negative effects of genes; relative frequencies of dominant and recessive alleles; and 5 genetic estimators: average degree of dominance; proportion of dominant and recessive alleles; ratio of the total numbers of dominant to recessive genes in the parents; number of effective factors which exhibit some degree of dominance and the heritability. The calculated genetic parameters and estimators differed in the 2 temperature regimes indicating there could be differences in gene action such as overdominant instead of partial dominant gene action depending on the temperature conditions. There were differences in heritabilities for the component stages, and some of the longer stages had potentially useful high heritabilities. The data were also analyzed by the Griffing method which estimated the general combining ability and specific combining ability. The analyses showed that both the additive and dominant gene action had significant effects in most of the component stages, and in most cases, the additive variance was larger than the dominant variance. The second approach employed reciprocal cross experiments with 2 parental cultivars, Bonny Best and Immur Prior Beta, and their reciprocal hybrids under the 2 temperature regimes in greenhouses and growth chambers. The nuclear and/or cytoplasmic effect on the 7 growth component stages, net photosynthesis rate and leaf area were studied. There was some evidence that cytoplasmic effects were relatively important for some of these characteristics, and these effects were more noticeable in the cool regime. In the third approach, field selection experiments on the earliness of 2 major stages were commenced in the F₃ of Bonny Best and Immur Prior Beta reciprocal cross populations. The mean values for both stages in the F₅ reciprocal populations were earlier than the 2 original parents indicating recombination of genes for earliness from parental cultivars. These results indicate that the methods which were used in these studies are a feasible way to increase the quantitative characteristic of earliness in the tomato.

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