Genetic analysis of drought tolerance in wheat using morpho-physiological traits and molecular markers.

In order to study drought tolerance in bread wheat, 142 recombinant inbred lines in F8 generation, derived from a cross between Azar2 (winter type) and 87Zhong291 (facultative type) was used. Experiment was conducted using randomized block design with three replications under rainfed (drought stress) and supplementary irrigation during 2006-07 seasons at Maragheh experimental station of the Dryland Agricultural Research Institute (DARI). Studied traits included morpho-physiological traits such as plant height, days to heading, days to anthesis, days to maturity, 1000 kernel weight, grain filling relative, degree day growth, yield components, and fluorescence parameters including Fo (minimal fluorescence level from dark-adapted leaves), Fm (maximal fluorescence level from darkadapted leaves), Fv (variable fluorescence level (Fv = Fm - Fo)), Fv/Fm (maximum quantum efficiency of PSII photochemistry), NPQ (non-photochemical quenching), QUE (Photochemical Quenching), LWP (Leaf Water Potential in relation to Drought resistance), YPEC (Yield of Photochemical Energy Conversion) and PI (Performance Index). Analysis of variance revealed significant differences among genotypes in two experimental conditions with respect to all the evaluated traits. Average grain yield under drought and irrigated conditions were 928 and 2547 Kg/ha, respectively. Under drought and irrigated conditions, grain yields ranged from 272 to 1935 Kg/ha and 1086 to 3805.4 Kg/ha, respectively. Accumulation of photosynthesis assimilate in grain were 10.64, 24.89, 52.61 and 11.86 percentage in first, second, third and fourth week after anthesis, respectively, under drought condition. More than 88 percentage of photosynthesis assimilate, accumulated in the 3rd week after anthesis. Correlation coefficient grain weight with grain filling rate in first and second week after anthesis were r = 0.76** and r = 0.048 ** respectively. In the very early lines total of photosynthesis assimilate, accumulation finished in the 3rd week after anthesis. The results indicated that traits such as days to heading, days to anthesis, 1000 kernel weight and grain filling relative were important selection criteria for drought tolerance in wheat breeding programs. Regression analysis revealed positive and significant relationship between all the chlorophyll fluorescence parameters except QUE and grain yield under drought condition, whereas the association for QUE was negative and significant. Under irrigated condition no significant relationships were identified between these parameters and grain yield. Significant positive or negative correlations were observed between fluorescence parameters under drought condition. Parental polymorphism was studied using 31amplifed fragment length polymorphism (AFLP) primer pairs and 17 simple sequence repeat (SSR) markers and polymorphic were screened in the population. The markers were assigned into four linkage groups with total length of 859.2 cM and 15.74 cM distance between two neighboring markers. Composite interval mapping (CIM) was used to analyze association between genotypic and phenotypic data. Based on CIM 13, 5, 7, 12, 4, 7 and 3 QTLs were identified for plant height, 1000 kernel weight, grain filling, QUE, Fv, Fv/Fm and YPEC, respectively under drought condition. The favorable alleles for the QTLs were contributed by both of the parents. The results indicated that chlorophyll fluorescence along with morphphysiological traits could be used as selection criteria for drought tolerance in wheat breeding programs. Keywords: Wheat, drought, Fluorescence parameters, earliness, SSR, AFLP, Markers and QTL.