عنوان مقاله [English]
Drought stress is one of the most important abiotic stresses all around the world. The aim of breeding studies and breeding for resistance to drought is that breeders seek to identify varieties and genetic resources to drought resistant and comparison of drought resistance among the varieties and the introduction of superior varieties to farmers. Drought or imbalance between supply and demand for water is one of the most important limiting factors affecting crop production which is very important in this context, effective and economic use of water resources especially for areas with arid and semi-arid climatic conditions which covers about two-thirds of the total area of Iran (Shahram & Daneshi, 2005). Breeders have been trying that by testing different varieties under normal and stress conditions to identify varieties and use them to plant breeding programs.
Cowpea (Vigna unguiculata L. Walp), a member of the family leguminous (Fabaceae) is a crop grown under the tropical and sub-tropical areas covering Africa, Asia, South America, and parts of Southern Europe and United States (Singh et al., 1997). Dry seeds of cowpea contain 20-25% protein, 1.8% fat, and 60.3% carbohydrate and are rich sources of iron and calcium (Majnoon Hoseini, 2008).
In this study, various drought tolerance indices were used to identify drought resistant in varieties. Indices included drought tolerance, Tolerance Index (TOL), Mean Productivity (MP), Geometric Mean Productivity (GMP), Stress Susceptibility Index (SSI), Yield Stability Index (YSI), Yield Index (YI), Stress Tolerance Index(STI), and Harmonic Mean (HM) (Ahmadi et al., 2000; Fernandez, 1992; Safari et al., 2007; Bouslama & Schapaugh,1984; Gavuzzi et al.,1997).
Materials and Methods
In order to study and determine the most effective traits, drought tolerance indices and identify tolerant genotypes in vegetative drought stress on the cowpea genotypes, All 32 cowpea genotypes were cultivated in a randomized complete block design with three replications which each replication consisted of 32 experimental units, each unit or plot, three lines with a length of two meters with line spacing of 70 cm were planted. The distance between rows of plants, 10 cm and 50 cm was considered the distance between each plot, in two separate experiments including normal irrigation and water stress conditions. The study was conducted at Experimental Research Farm, University of Tehran, Karaj Agricultural Research Institute at College of Agriculture and Natural Resources in Karaj, Tehran, Iran during 2014. Drought stress was imposed by doubling the irrigation time about 50 days after planting against normal irrigation on thirty-two cowpea genotypes. Evaluation of drought resistant in different genotypes was conducted using eight indices including Tolerance Index (TOL), Mean Productivity (MP), Geometric Mean Productivity (GMP), Stress Susceptibility Index (SSI), Yield Stability Index (YSI), Yield Index (YI), Stress Tolerance Index (STI), and Harmonic Mean (HM).
Results and Discussion
Analysis of variance showed that there is a significant difference between genotypes for all the indices of drought tolerance and grain yield in both normal and stress conditions (P0.01). This result suggested that the genetic variation among genotypes is capable of selection for drought tolerance.
A simple calculation of statistical parameters (mean and standard deviation) for drought tolerance indices indicated that there is a great diversity among the study genotypes which it can be used as rich genetic resources to help breeders to improve and identify resistant varieties.
The average yield of all genotypes under drought stress and normal irrigation condition was Ys = 83.57, and Yp =101.82, respectively. Significant differences between two different conditions indicated that cowpea plant has a high potential for tolerance under drought stress condition. TOL index revealed the lowest average value among various indices (TOL =18.24).
The low level of stress tolerance index shows a high relative tolerance genotype. In fact, stress tolerance index showed the changes of stress condition in genotypes. It means that genotypes with low TOL index indicate less changes and genotypes with high TOL index show more changes.
Correlation coefficient was calculated to determine the relationship between grain yield and drought tolerance indices. The STI, MP, HM and GMP indices which have the most positive and significant correlation with grain yield under stress and non-stress conditions were introduced as the best indices for screening tolerant genotypes to drought and high-yielding in both environmental conditions. Using Biplot scatter graph in 32 cowpea genotypes and according to genotypes situation in Biplot display, genotypes 998, 313, 291 and 7 were identified as tolerant genotypes with high-yield. Cluster analysis based on investigated indices and yield under drought stress and non-stress conditions showed that genotypes were grouped in four clusters and most of the drought tolerant genotypes with high yield were grouped in the second cluster، while most of drought sensitive genotypes were grouped in the fourth cluster.
In this study, genotypes showed high genetic diversity in terms of drought tolerance using drought tolerance indices. Based on the results obtained in this study genotypes 291, 7, 313, and the Mashhad cultivar (998) can be proposed as drought tolerant genotypes.