Studies On Salt Tolerance And QTL Mapping For Some Related Traits In Barley

Soil salinity is one of major abiotic factors affecting crop yields in arid and semi-arid irrigated areas, and has become a serious global agricultural issue. Approximately, one third of the world's irrigated soils and a large proportion of soils in dry areas has been salinizated. Salinization reduces crop production and is threatening sustainability in agricultural development. To cope with the problem, it is quite imperative to develop salt-tolerant crops or genotypes to improve utilization efficiency of saline land. Nevertheless, Complexity of salt-tolerant genetics, deficiency in reasonable salt-tolerant germplasm and efficient methodology for evaluating and identifying genetic materials with high salt tolerance pose a bottleneck for successful development of salt-tolerant breeding. In China, there are a huge number of saline and latent saline soils with total area of about 9.913×10⁷ ha. In the past two decades, phosphorous fertilizers containing high Cd content were extensively applied to these saline soils for alleviating salt stress. As a result, Cd content in these soils has been dramatically increased. Thus, the plants growing in the saline soil may suffer from both salinity and Cd stresses.Barley (Hordeum vulgare L.) is a promising crop for saline areas as it is one of the most salt tolerant field crops. However, its growth and development are still greatly affected by salt stress. Many physiological changes occur when barley plants are exposed to salt stress. It is well documented that enhancement of crop salt tolerance will require pyramiding several traits. Therefore, elucidation of biochemical and physiological mechanisms in the response of plants to salt stress is necessary to enhance salt tolerance of crop plants through agronomic approaches, genetic improvement or gene manipulation. The objectives of the present work were to illustrate the mechanisms in salt tolerance of barley and to map some salt tolerance related QTLs.1. Genotypic differences in physiological traits in response to salt stress The effect of salinity on some physiological parameters in 16 barley genotypes with different salt tolerance was investigated. The results showed that 50 mM NaCl treatment increased Na⁺/K⁺ ratio, malondialdehyde (MDA) and proline contents, and decreased cell membrane stability index (CMSI) and fresh shoot biomass (FSB) of all tested genotypes. Salt stress also resulted in a decreased chlorophyll (Chl) content and net photosynthesis (Pn) for most genotypes. Under higher salt stress (300 mM NaCl), the marked increase for Na⁺/K⁺, MDA and proline content, and decrease for other parameters were found for all genotypes. The affected extent of these parameters by salt stress varied with genotypes. Proline accumulation in barley was positively associated with injured extent under salt stress, indicating it is not a defensive reaction to the stress. K⁺ uptake was less affected while Na⁺ accumulation in plants was enhanced under high salt stress. The correlation analysis showed that MDA and proline content, Na⁺ concentration and Na⁺/K⁺ were negatively correlated with FSB, while other parameters examined in the study were positively correlated with FSB.2. The Difference in isozymes and activities of peroxidase and superoxide dismutase in two barley genotypes under salt stressDifference in isozymes and activities of peroxidase (POD) and superoxide dismutase (SOD) in two barley genotypes differing in salt tolerance (Gebeina -tolerant; Quzhou - sensitive) was investigated using a hydroponic experiment with 0, 50, 100 and 200 mM NaCl treatments. The activities of both enzymes were significantly increased when the plants of the two barley genotypes were exposed to salt stress, with salt tolerant genotype being higher than the sensitive one in any salt treatment. The variation of POD and SOD isozymes was dependent on barley genotype, salt level and exposure time. When the plants were exposed to salt stress for 1 d and 10 d, two new POD isozymes, R_m0.19 (R_m-relative mobility of enzyme to dye) and R_m0.23 were found in Gebeina, but not in Quzhou. Meanwhile, higher POD activity was detected in Gebeina than in Quzhou, indicating that increased POD activity in barley plants exposed to salt stress might result from synthesis of new isozymes. A new SOD isozyme of R_m0.80 was found in Gebeina when exposed to 50 and 100 mM NaCl for 10 d, but not in Quzhou, suggesting that the synthesis of the new SOD isozyme might contribute to increased SOD activity under salt stress. In 200 mM NaCl treatment, different new SOD isozymes could be found in the two genotypes, with R_m0.72 for Gebeina and R_m0.64, R_m0.72 and R_m0.80 for Quzhou. It is suggested that the up-regulation of the isozyme R_m0.72 is closely related to higher SOD activity in the salt tolerant genotype, while the isozymes Rm₀.64 and Rm₀.80 might have less contribution to enzyme activity.3. Interaction of salinity and Cadmium stresses on mineral nutrients, sodium, and cadmium accumulation in barleyInteraction of salinity (Na) and cadmium (Cd) on growth, mineral nutrients, Na and Cd accumulation in four barley genotypes differing in salt tolerance was studied in a hydroponic experiment. Cd, Na and their combined stresses reduced Ca and Mg concentrations in roots and shoots, K concentration in shoots, increased K and Cu concentrations in roots relative to control, but had non-significant effect on Cu, Fe and Mn concentrations in shoot. The three stress treatments all reduced accumulation of most nutrients in both roots and shoots, except salinity (Na and Na+Cd) stresses for root K and shoot Cu accumulation in salt tolerant genotypes. The salt tolerant genotypes did not have higher nutrient concentration and accumulation than the sensitive ones when exposed to Cd and Na stresses. In conclusion, the mechanisms in affecting nutrient uptake and accumulation differ between Cd and salinity stresses, and the effect caused by Cd+Na stress is not simply pool of those caused by Cd and Na stress. It is assumed that there is a complex interaction between the two stresses in their effects on nutrient uptake and accumulation, probably due to the different in valence and competitive site of Na⁺ and Cd²⁺. NaCl addition in the Cd-containing medium caused remarkable reductions in both Cd concentration and accumulation, and the extent of the reduction was also dependant on genotypes. The salt-tolerant genotypes had lower Na concentration than sensitive ones. 4. SSR linkage mapping and QTLs analysis for salt tolerance in barleyUsing a DH population developed from the cross between salt tolerant genotype CM72 and salt sensitive genotype Gairdner, genetic linkage map was made through SSR markers. Some salt tolerant related QTLs were located on chromosome. Under 0.05 significant level, one QTL for plant height (PH) was mapped in the interval of HVM62-GBM1285 on Chromosome(CH) 3; two QTLs for tillers per plant (TPP) were located on CH2(qtpp2.2)and CH4 (qtpp4.12); two QTLs for root biomass(RB), qrb2.1å’Œqrb7.6,were located on CH2 and CH7 respectively; no QTL for root length (RL) or shoot biomass (SB) was detected..Under salt stress and no stress environments, one additive QTL for MDA was detected on CH7 and two additive by additive QTLs for MDA, i.e. qmdac1.6 and qmdac2.2,, were detected on CH1 and CH2, respectively; one additive QTL and one additive by additive QTL for proline concentration were located in the interval of EBMAC0775-GBM1422 on CH4 and in the interval of GMS002-GBM1462 on CH2. Under salt stress environment, one additive by additive QTL for K⁺/Na⁺, i.e. qkna1.2, was mapped in the interval of BMAC0032-EBMAC0656 on CH1.Pleiotropism or gene linkage was found in salt tolerance related loci of barley. F values of QTLs as qph3.10, qtpp4.12, qmdacl.6 and qknal.2 were over 10, suggesting that these QTLs were reliable and valuable for refine mapping.