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Combining Ability Estimates for Weather Fleck in Nicotiana tabacum L.¹

Seven cultivars of Nicotiana tabacum L. (four Maryland and three cigar types) and 21 F₁ hybrids were grown at three locations for 2 years. During August, data were collected on leaf damage caused by air pollutants, primarily ozone. This condition was referred to as weather fleck. Ozone causes small necrotic lesions usually 0.2 to 0.5 cm in diameter. Plants were scored on a scale of 1 to 5 (1 = no damage, 5 = severe damage). Comparison of parent (2.34) and F₁ hybrid (2.37) performance indicated no significant amounts of heterotic resistance to weather fleck. Diallel analyses of variance of the F₁ hybrid generation suggested an appreciable amount of variance due to general combining ability (GCA) and the absence of variance due to specific combining ability (SCA). Genetic effects were stable with respect to locations and years. The genotype x environment interactions for the six environments were small. The predominance of additive genetic variance in this material suggests that progress could be made through selection.

Key Words: Air pollution • Diallel cross • Heritability

² Associate Professor of Agronomy, University of Maryland.

Received for publication April 13, 1972.