JWSS - Journal of Water and Soil Science

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In order to analyze the genetics of tobacco cultivars’ resistance to blue mould (Peronospora tabacina A.) and to estimate the combining ability of the four resistant and susceptible tobacco cultivars (Bel 61-10, Bergerac C, Samsoun and Trumpf) received from CORESTA, these cultivars were crossed in 1997 in a half-diallel crossing system and in 1998 their parents and their progenies were sowed. Also to estimate gene effects controlling resistance and susceptibility of cultivars and to obtain F₂, BC₁ and BC₂ generations, F₁ generation was selfed and backcrossed with their parents. In 1999, the six generations (P₁, P₂, F₁, F₂, BC₁ and BC₂) from six families were evaluated in a complete block design with 3 replications and the resistance of genotypes against Peronospora tabacina were evaluated by the standard method of CORESTA and the parents with their F₁ and F₂ progenies were analyzed as a 4×4 half-diallel crossing system.

The ANOVA showed significant differences among genotypes concerning resistance to Peronospora tabacina and the generation means analysis indicated fit of families Bel 61-10 x Bergerac C and Bel 61-10 x Trumpf with additive-dominance model. The rest of the families showed non-allelic digenic interactions (epistasis) and were fit to 6-parameter model of Mather and Jinks. Because of moderate to high narrow sense heritability of resistance to Peronospora tabacina (from 34 to 85% for different families), the selection of resistant lines may be successful. The results showed significant GCA of parents for the resistance to Peronospora tabacina. So, the role of additive effects of genes concerning resistance to Peronospora tabacina was recognized and the estimated heritability (in the narrow sense) was from 72% to 75% for this trait.

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The results of diallel analysis of 7 parents and 21 F2 progenies of Burley tobacco cultivars, which were transplanted in 2002 as RCBD with 3 replications in the Tobacco Research Center, Rasht, Iran, showed significant genetic differences among genotypes and high GCA and SCA for most of the traits. Therefore the role of additive and non-additive (dominant) effects of genes on the formation of the corresponding characteristics was important. The graphical analysis of progenies of diallel crosses showed partial dominant effect for nicotine percentage in leaves. The distribution of parents around regression line showed most dominant genes in cultivars B.CDL 28, B.Banket, and B.21 while cultivars B.14 and B.TN 86 hade most recessive genes for this characteristic. Also the low and high percentages of nicotine were controlled by dominant and recessive genes, respectively. Estimated simple (phenotypic) relationships between characteristics showed significant correlation between dry leaf yield of tobacco cultivars and its components such as leaf area index (LAI) (r = 0.482**), time to flowering (r = 0.440*), appearance of leaves (r = 0.648**) and percent age of dry matter of leaves. The path coefficient analysis showed very high direct influence of dry matter percent age of leaves, appearance of leaves, and LAI in dry leaf yield. These characteristics would be a favorite selection index for increasing tobacco yield, since characteristics such as leaves per plant, plant height and time to flowering have no significant direct influence on dry leaf yield of tobacco cultivars. These yield components explain 82% of variance of tobacco yield. Results of factor analysis, using principal Component Analysis (PCA) with Varimax rotation showed that characteristics such as leaves per plant, LAI, and plant height with high positive and significant factor loading as a morphological factor explain 44% of data variance. The second factor including such traits as time to flowering, appearance, and percent of dry matter of leaves with high positive and significant factor loading, form a physiological factor. These two factors together explains 65% of variance of dry leaf yield of tobacco cultivars.