Rapeseed is a main oilseed crop in the world. Water deficiency is the main cause of reduced crop yield under drought stress. The highest rapeseed yield reduce was resulted when water deficit occurred at flowering and pod developmental stages (Kahrizi and Allahvarand, 2012). Knowing about the traits that affect grain yield, so that by improving these traits, grain yield can be increased, plays an important role for being successful breeding programs. Factor analysis of multivariate statistical methods is for a deeper understanding of the relationships between traits and is an effective statistical method in reducing data volume (Cooper, 1983). The aim of this study was to study the effect of late-season drought stress on some traits affecting the yield of studied rapeseed genotypes and to identify genotypes compatible with normal and drought stress conditions using some stress tolerance indices.
Material and Methods
This experiment was carried out at the research field of Seed and Plant Improvement Institute, Karaj, Iran, on 2016-2018 growing seasons under full irrigation conditions and late-season drought stress (from beginning pod setting stage) on new winter rapeseed genotypes in a randomized complete block design with three replications. Some of morphological, physiological, phenological traits, yield and yield components were measured. Factor analysis was performed to determine the most influential variables. To identify drought tolerance genotypes, drought tolerance indices such as tolerance index (TOL), stress susceptibility index (SSI), mean productivity (MP), geometric mean productivity (GMP), harmonic mean (HMP), stress tolerance index (STI), Yield index (YI), Sensitivity drought index (SDI) and yield stability index (YSI) were used.
Results and Discussion
The combined analysis of variance showed that the effect of drought stress on all evaluation traits was significant except for height of the first branch, days to end of flowering, flowering period and oil percentage. The results showed that drought stress decreased Grain yeald, Pods per plant, Seeds per pod, 1000 Seed weight, plant height, Chlorophyll a and b, Cartenoid, Relative water content and Days to maturity; while it increased Chlorophyll Index (SPAD).
The results of factor analysis exhibited three factors in normal and drought stress conditions, which explained 79.7% and 82.51% of the variation, respectively. This factors in normal conditions were named "Yield", "Plant pigment", "Phenological traits" respectively, and in stress conditions were named " productivity", "Plant pigment" and " sink" , respectively. It seems that its seams possible to use these traits as selection criteria in breeding programs for improve grain yield of winter rapeseed genotypes results showed that GMP, MP, HMP and STI indices were positively correlated with grain yield under both stress and non-stress conditions. Therefore, they can be exploited not only to screen drought tolerance but also to identify superior genotypes for both stress and non-stress field conditions (Fernandez, 1992). Principal component analysis using grain yield under both conditions and drought tolerance indices showed that the genotypes Nafis, Vesuvio, Medea, HL3721 and HL2012 were the most tolerant genotypes to drought conditions, while the genotypes Extorm, Excalibur, Danube and Expower were the most sensitive genotypes. With cluster analysis using WARD procedure based on drought tolerance, the studied genotypes were grouped in 3 separate clusters. The first group consisted of genotypes that have high yield and drought tolerant and had appropriate and reliable values of indices such as STI, MP, GMP, HMP. Therefore, this group (genotypes Nafis, Vesuvio, Medea, HL3721 and HL2012) are drought tolerant group. The second group had genotypes with low values of STI, MP, GMP, HMP as well as Ys and Yp and higher values of SSI. Therefore, genotypes Danube, Excalibur, Expower, Harnus and Extorm with low yield and drought sensitivity were placed in this cluster. The next group (Okapi, Ahmadi, Nima, Natalie, SLM046, Artist) with average yield in both non-stress and stress conditions, and less sensitivity to drought stress condition, were stable in yield, and therefore considered as semi-resistant cultivars.