Environmental, biochemical and genetic abnormalities can induce physiological disorder in tobacco. Energy conversion results in production of many air pollutants including ozone which causes weather fleck. High incidence of weather fleck results in earlier flowering, lower yields and lower total alkaloids. More mature leaves are more tolerant to ozone damage than younger leaves. Tolerance to ozone is determined by genetic makeup of the shoot and abaxial stomata. plant damage from ozone or sulfur dioxide is enhanced by the presence of the other pollutant. Frenching is the formation of progressively narrower apical leaves. The cause of frenching is not known but the substance(s) appears to be leached from soils, similar to thallium induced chlorosis and narrow leaves, most active in soil above 35°C, and altering amino acid metabolism in the plant. Genetic tumours form on certain Nicotiana hybrids. These are not of economic importance to N. tabacum production but may be significant as interspecific hybridization is used to improve commercial tobaccos. Tumour formation appears to be controlled by genes on the chromosomes and show conventional segregation, linkage and mutation.