Microscopic studies were conducted on weather fleck, a non-parasitic plant disorder of flue-cured tobacco in Ontario. Within a few hours of the initial appearance of the flecks, damage was restricted to the palisade parenchyma cells; after about one day some of the spongy parenchyma cells also were affected. There was no apparent difference between flecked and healthy tissue with respect to thickness or structure of the cell wall. In affected tissue, the nuclei of the palisade cells were shrunken to spherical, ellipsoid, or irregularly ellipsoid shapes with average measurements of 6.0 × 5.1 μ; in adjacent undamaged cells, they averaged 9.1 × 7.3 μ. The nuclei of damaged cells stained more heavily than nuclei of normal cells and their structural details were lost. Chloroplasts of flecked and healthy palisade cells were almost of an equivalent size, the average lengths being 4.3 μ and 4.4 μ, respectively. However, chloroplasts in flecked cells had often been disrupted. Evidence is presented to support the hypothesis that the unknown agent or agents causing weather fleck first affect the cytoplasm and nucleus, then the chloroplasts.
The eight tobacco cultivars Delcrest, Hicks Broadleaf, Yellow Gold and Jamaica Wrapper of the flue-cured type and Burley 1, Burley 21, Virginia B29 and Kentucky 12 of the burley type and their crosses and reciprocals were grown in an 8 × 8 diallel-cross experiment. Diallel analyses were completed according to procedures described by Hayman, Griffing and Robinson.The mean squares for general combining ability and for Hayman's a made up a considerably higher proportion of the total variability than the other subdivisions. The variances for special combining ability were in all instances smaller than those for general combining ability, although there were differences in size and in levels of significance between Griffing's and Robinson's methods. The statistic due to additive effects of genes (Hayman's D) was significant for days to flower, height, and number of leaves, and the statistic due to dominance effects significant for days to flower and height. The greatest effect in decreasing the number of days to flower was exhibited by the cultivar Delcrest, and the greatest effect in increasing the number of days to flower was by Yellow Gold and Kentucky 12. Kentucky 12 showed the greatest effect in increasing height and number of leaves.
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