evaluation of drought tolerance in rapeseed (brassica napus l.) cultivars using drought tolerance indices

drought is the most significant constraint for crop production in arid and semi-arid regions. in order to assess drought tolerance in fifteen rapeseed (brassica napus) cultivars using yield-based drought tolerance indices, two experiments were conducted at the research station of college of agriculture, shiraz university, shiraz, iran during 2009-2010 growing season. the cultivars were arranged in a randomized complete block design with four replications in each experiment. the experiments differed in respect to irrigation regimes. the well-watered and water-limited experiments were irrigated after 40% and 70% depletion of available soil moisture, respectively. analysis of variance showed highly significant differences among the rapeseed cultivars for yield in normal and stress conditions as well as all the drought tolerance indices. karun cultivar had the maximum seed yield in both conditions. yield in non-stress (yp) and stress (ys) conditions showed positive and significant correlations with stress tolerance index (sti), mean productivity (mp), geometric mean productivity (gmp) and modified stress tolerance indices  (k1sti and k2sti). biplot analysis also indicated that sti, mp, gmp, k1sti and k2sti were more reliable indices to identify drought tolerant rapeseed cultivars. the results of biplot and cluster analysis revealed that karun, nk aviator and nk octans were the drought tolerant rapeseed cultivars. therefore, they may be recommended to cultivate in drought prone regions and also can be used in rapeseed breeding programs aimed at improving drought tolerance