Selection of tolerant genotypes to broomrape Orobanche cernua stress in oriental tobacco Nicotiana tabacum genotypes using stress tolerance indices

Tobacco is one of the most important industrial and economic plants in many countries of the world which is cultivated mainly for harvesting and collecting its leaves. In order to study the response of 92 oriental tobacco genotypes to broomrape stress, an experiment was conducted in a randomized complete block design with three replications under normal (without broomrape) and broomrape stress conditions during 2007 and 2008 cropping seasons. Nine stress tolerance indices including tolerance index (TOL), relative stress index (RSI), mean productivity (MP), harmonic mean (HM), yield stability index (YSI), geometric mean productivity (GMP), stress susceptibility index (SSI), stress tolerance index (STI) and yield index (YI) were calculated based on leaf dry weight (yield) under normal (Yp) and broomrape stress conditions (Ys). Genotypes 24, 39 and 40 had the highest mean yield in normal and broomrape stress conditions. Yield value in stress (Ys) and non-stress (Yp) conditions were significantly and positively correlated with MP, HM, GMP, STI and YI indices. Using the ranking method, genotypes 40, 10 and 45 were introduced as the most tolerant genotypes to broomrape stress. Based on the results of biplot, genotypes 40, 39, 24, 19 and 4 were identified as high yielding and tolerant genotypes and genotypes 70, 69, 72, 67, 78, 71 and 8 as the most sensitive genotypes to broomrape stress. Grouping of genotypes by Ward method also classified high yielding and stress tolerant genotypes including genotypes 40, 39, 24, 19 and 4 in one group. Susceptible and low yield genotypes under stress and normal conditions including genotypes 70 and 69 were classified in the other group. Due to high diversity among the studied genotypes in view of response to broomrape stress, it is possible to select appropriate parents for breeding programs, produce diverse populations for identifying genetic loci controlling broomrape stress tolerance and study changes in the expression of tolerance genes.