Introduction: Tobacco is one of the important agricultural, industrial, and commercial plants that plays an important role in the economies of producer and consumer countries. Broomrape is an absolute parasitic plant that is attached to the roots of many important crops such as tomato, sunflower, cucumber, and tobacco. due to its lack of chlorophyll, it absorbs water and nutrients from the host plant and then reduces the growth and yield of the host plants. In the process of crop breeding, knowledge of relationships between traits is important for indirect selection of traits that are not easily measurable or that have low heritability. So far, few studies have been conducted to investigate the relationship between yield and yield components of tobacco under broomrape stress conditions. The aim of this study was to investigate the relationship between morphological traits and leaf yield (dry weight of leaf) and to determine the important traits effective in increasing the leaf yield of tobacco under normal and broomrape stress conditions.
Materials and Methods: In order to determine the most important traits affecting tobacco yield, 92 oriental tobacco genotypes were studied in a completely randomized block design with three replications in the presence and absence of broomrape during two years in the Urmia tobacco research center. The leaves of tobacco genotypes were harvested during industrial ripening and sun -cured. Characteristics such as plant height (cm), day to flowering (day), number of leaves, leaf area (square centimeters), fresh weight of leaf (g), dry weight of leaf (g), fresh weight of root (g), dry weight of root (g), and fresh and dry weight of shoot (g) were measured under normal and stressful conditions. Genotypic correlation coefficients were calculated among traits using Restricted (or residual, or reduced) maximum likelihood in SAS software. The stepwise multiple regressions were performed to identify traits affecting the leaf yield under normal and broomrape stress conditions. By path analysis based on genotypic correlation coefficients, the direct and indirect effects of traits affecting leaf yield were calculated. By path analysis based on genotypic correlation coefficients, the direct and indirect effects of traits affecting leaf yield were calculated.
Results and Discussion: Based on step-wise regression analysis, under the absence of broomrape conditions, fresh weight of leaf and leaf area were explained, and in the presence of broomrape conditions, fresh weight of leaf explained 80 and 73% of leaf yield variation, respectively. Based on path analysis, in both conditions, the fresh weight of the leaf showed the highest direct effect on leaf yield. In regression analysis for fresh weight of leaf in the absence of broomrape, aerial part fresh weight and in the presence of broomrape, aerial part fresh weight, leaf area, day to flowering, and plant height explained 61 and 72 % of fresh weight of leaf variation, respectively. In path analysis for fresh weight of leaf, in both the presence and absence of broomrape conditions, aerial part fresh weight showed the highest direct effect on fresh weight of leaf.
Conclusion: By comparing the results of step-wise regression and path analysis between two normal and broomrape stress conditions, it was observed that fresh weight of leaf is one of the traits that was effective on tobacco leaf yield in both conditions. In both conditions, the fresh weight of the leaf had a high and positive direct effect on leaf yield and explained a high percentage of changes in leaf yield, which shows the importance of this trait in the selection of high-yielding oriental tobacco genotypes. Of course, under normal conditions, the direct effect of the fresh weight of the leaf on tobacco leaf yield was greater than in the broomrape stress conditions. Therefore, the fresh weight of the leaf is introduced as the most important factor in both normal and broomrape stress conditions for increasing oriental tobacco leaf yield in breeding programs.