Water Use Efficiency and Water Deficit Tolerance Indices in Terminal Growth Stages in Promising Bread Wheat genotypes

During growth stages of wheat, anthesis and grain filling periods are the most susceptible to drought. Wheat cultivars that are more tolerant to terminal drought are more suitable to Mediterranean conditions. To increase water use efficiency, the target environment should be taken into account, because one trait might be effective in an environment but ineffective in another environment. In general, some traits like early vigour and root absorbtion capacity are so important in water deficient conditions. In recent years, increasing grain yield was due to increasing grain numbers. Although both the source and sink is considered as the limitation factors in increasing grain yield in old cultivars, even in the new cultivars sink seems to be more important. In fact, phenological adjustment adapted with seasonal precipitation pattern can improve water use efficiency in drought conditions. Suitable flowering time is the most important trait that is correlated with increasing water use efficiency in drought conditions. In order to evaluate the level of drought tolerance in promising bread wheat lines, a split plot arrangements using randomized complete block design with three replications was carried out in 2008-09 and 2009-10 growing seasons at Torogh Agricultural Research Field Station, Mashhad. in. water limited conditions at three levels Optimum moisture conditions (L1), removal irrigation and using rain shelter from milky grain stage to maturity (L2), removal irrigation and using rainshelter from anthesis to maturity (L3) were assigned to main plots. Ten bread wheat lines include suitable for cold and dry regions (V1: (Toos), V2: (C-81-10), V3: (pishgam), V4: (C-84-4), V5: (C-84-8), V6: (C-D-85-15), V7: (C-D-85-9), V8: (C-D-84-5502), V9: (C-D-85-5502) and V10: (C-85-6) were randomized in sub-plots. Stress susceptibility index (SSI), stress tolerance index (STI) and tolerance (TOL) were calculated using following equations: D = 1- (Yd/Yp), SSI = (1-(Ydi/Ypi)) / D, STI = (Ypi×Ysi)/ (Yp) 2, TOL= Ypi – Ysi In which D is environment stress intensity; Yp, average of grain yield for all genotypes in optimum; Ys, in water limited conditions; Ypi, grain yield of one genotype in optimum; and Ysi, grain yield of one genotype in water limited conditions. Anthesis and physiological maturity were determined by observing of anthers in %50 spikes and changing color of %50 pedancles to yellow, respectively. The results revealed that water stress (L2 and L3 treatments) reduced grain yield (18.6% and 45.3%, respectively). Genotypes V5, V4 and V10 showed maximum water use efficiency (WUE) (1.885, 1.756 and 1.833 kg.m-3 respectively). A highly significant relationship was found between grain yield under moisture limited conditions and STI (r = 0.93**) and TOL (r = 0.85**). Grain yield under optimum irrigation condition was significantly (r = 0.50**) correlated with STI. Therefore, stress tolerance index (STI) was more efficient index for estimation the grain yield under either conditions as well as grouping the genotypes with higher grain yield and tolerant to water limited condition. So, stress tolerance index (STI) was suitable for classifying the higher yielding genotypes adapted to drought prone environment. Since stress tolerance index (STI) was highly and significantly associated with grain yield in both optimum (r = 0.50**) and limited moisture (r = 0.93**) conditions, it can be used an efficient index for evaluation in the field. Our results indicated that genotypes V5, V4 and V10 with high stress tolerance index (STI) values and the greatest WUE, had the best performance among the other genotypes, respectively. These cultivars had higher grain yield in both optimum and stress conditions than other genotypes So, these cultivars could be recommended to cultivate for similar conditions.