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Heterosis and Combining Ability Estimates for Air Pollution Damage, Primarily Ozone, in Maryland Tobacco¹

Estimates of heterosis and general and specific combining ability for air pollution damage, primarily ozone, on Maryland tobacco (Nicotiana tabacum L.) were obtained under greenhouse and field conditions. A diallel set of crosses (excluding reciprocals) among seven Maryland tobacco cultivars was used for the greenhouse fumigation-chamber study. Tobacco plants, aged 10 to 12 weeks, were exposed to ozone at 50 pphm for 4 hours in a fumigation chamber. Significant differences in ozone susceptibility were observed among the seven parents and their 21 F₁ hybrids. The heterotic response of the hybrids was toward greater ozone susceptibility. Diallel analysis of variance of the F₁ hybrid generation indicated a large amount of variance due to general combining ability and a small amount due to specific combining ability.

An additional cultivar was added to the diallel set of crosses for the field study and was grown at three locations. Air pollution data (commonly referred to as weather fleck under field conditions) for the eight parents and 28 F₁ hybrids were similar to the data obtained in the greenhouse. However, heterosis for weather fleck was small and not significant in the combined analysis over locations. Variability for general combining ability was significant for the three locations and in the combined analysis over locations, and specific combining ability was significant at one location only. These estimates suggest a predominance of additive genetic variance in the population for weather fleck.

Key Words: Diallel cross • Fumigation chamber • Greenhouse • Wheather fleck

² Research aide, Dep. of Genetics, Univ. of Wisconsin, Madison, WI 53706 (former graduate assistant, Dep. of Agronomy, Univ. of Maryland), associate professor, and assistant professor, Dep. of Agronomy, Univ. of Maryland, respectively.

Received for publication April 17, 1975.