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Genetic Variability, Heterosis, and Combining Ability Estimates for Root Weights of Maryland Tobacco¹

Based on the hypothesis that a larger root system will increase lodging resistance in Maryland tobacco (Nicotiana tabacum L.), the present study was initiated with two objectives: (i) to obtain root and top data on several Maryland tobacco cultivars and hybrids, and (ii) to determine the importance of heterosis, general combining ability, and specific combining ability for root and top weights. Greenhouse studies conducted in 1977 and 1978 included seven parents and 21 hybrids. Fresh weights of roots and tops plus the top/root ratio were obtained at 20, 35, and 52 days after transplanting (DAT) for plants grown in 15 cm plastic pots. Significant genetic variability was found among the parents and hybrids within each year. Due to a large entry x year interaction in the combined analysis, no significant variation was found among the parents and hybrids over the 2 years. Estimates of heterosis ranged from 0.3% at 35 DAT for top/root ratio to 31.9% for top weight at 20 DAT. General combining ability (GCA) and specific combining ability (SCA) estimates were significant for most traits at the three sampling dates within each year. For the combined analysis over the 2 years, GCA x Y and SCA x Y interactions were significant in most cases. The results from this study suggest that selection for increased lodging resistance in tobacco is possible based on the amount of genetic variability and additive genetic variance present in the population. However, the presence of a large genotype x environment interaction may delay progress. Possible breeding techniques, problems, and procedures for reducing the G xE interaction are considered.

Key Words: Diallel crosses • Lodging resistance • Genotype x environment interaction • Nicotiana tabacum L.

² Professors of agronomy.

Received for publication December 15, 1983.