Differences Of Inheritance Of Wheat Seedlings To Cadmium Tolerance And Its Physiological Bases

Cadmium (Cd) pollution is one of the most important environmental contaminants, and the most toxic metal to plants. Cd can be easily uptaken by plants and then enters into the food chain, resulting in serious health issues of human being. Wheat (Triticum aestivum L.) is an important crop extensively planted for diet of people worldwide. The production and safety of wheat affect the food and diet safety for people. On this condition, Cd tolerance and translocation in wheat varieties of different genotypes, of different type genomes and ploidy, different decades and a recombinant inbred line (RIL) population were investigated to determine the effect of Cd stress on the growth, physiology, and differences of Cd tolerance and translocation of wheat at seedling stage under a controlled solution condition. The mechanisms of physiological and Cd accumulation and quantitative trait loci (QTL) were studied in order to clarify the differences of inheritance in wheat seedlings responding to Cd stress and its physiological bases. The main results were summarized as follows:1 Clarified the relationship between wheat seedling growth, photosynthesis and Cd accumulation responding to Cd stressThe growth parameters, gas exchange parameters, and chlorophyll a fluorescence parameters of different wheat genotypes were generally depressed by Cd stress, especially under heavy Cd concentrations in the culture solution. Cd concentration and accumulation in both shoot and root increased with the increasing of Cd concentrations in the culture solution. Of the growth parameters such as shoot and root weight, shoot height, tiller and secondary root number, photosynthesis parameters such as net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr), chlorophyll a fluorescence parameters such as Fv/Fm, Fv/Fo,ΦPSⅡand qp were sensitive to Cd stress, and could be used as parameters for Cd identification. The optimum Cd concentration for identification was 50μmol·L-1. With a regression model of Y=K/(1+exp(a+bx)), the parameters of Pni/max, Fv/Fmi/max, OPSIIi/max, qpi/max, SDWi/max, RDWi/max, SHi/max, TNj/max, SRi/max can be well predicted by SCCi/max or RCCi/max.2 Studies on genetic variance in Cd tolerance and accumulation in wheat materials differing in ploidy and genome at seedling stage Cd tolerance and accumulation in wheat materials was investigated at seedling stage under 50μmol·L-1 Cd concentration in the culture solution. Growth parameters of shoot height (SH), secondary roots (SR), tiller numbers (TN), shoot dry weight (SDW) and root dry weight (RDW) as well as photosynthesis parameters of Pn, Gs and Tr were measured. Then each of the eight parameters was calculated by dividing a given parameter under Cd treatment by the one under the control, and was referred to the Cd-tolerant index of this parameter. The Cd tolerance and translocation of the 24 wheat materials were analyzed according to their ploidies and genomes, respectively. The results showed that Cd tolerance and distribution among these wheat materials varied with different ploidy and genome. Finally, the cluster analysis (CA) showed that the 24 wheat materials could be compartmentalized 5 groups according to the Cd-tolerant indexes or 5 groups according to Cd translocation. The B and D genome played a positive role in Cd tolerance while R and CCUU genome played a negative role in Cd tolerance. The Cd tolerance decreased from diploid to octoploid while Cd translocation of these wheat materials differing in ploidy was decreased from octoploid to diploid, except for the material containing CCUU genome. Of all the 24 materials, T. boeoticum Boiss was the most Cd-tolerant and Cd lowest-translocation wheat material while T. aestivum cv. Huixianhong and Jinghui 1 were the most Cd-sensitive and Cd highest-translocation wheat materials. The relatives of wheat Aegilops, Secale et al. showed diversity in Cd tolerance and Cd translocation.3 Studies on comparisons of Cd tolerance and accumulation of wheat seedlings among major varieties released in different decades from 1950s to 2000s in ChinaThe Cd tolerance and the accumulation of 46 wheat varieties differing in decades cultivated from 1950s to 2000s in Huanhuai Region and Yangzi River Region of China were compared. The Cd translocations were diversity in these 46 varieties replacing in China from 1950s to 2000s. The wheat varieties of 1950s and 1980s were more tolerant while of 2000s were more sensitive in the eight parameters to Cd stress. The CA showed that the 46 wheat materials could be compartmentalized 4 groups according to the Cd-tolerant indexes and 5 groups according to Cd translocation. The most Cd-tolerant and Cd lowest-translocation wheat varieties were Shannongfu 63, Yangmai 1 and Yangmai 158, as well as the most Cd-sensitive and Cd highest-translocation wheat varieties were Yumai 18 and Huaimai 20.4 Studies on the identification of Cd tolerance and genetic analysis of relative traits in a RIL population The Cd tolerance and Cd translocation were different in the RILs under Cd stress. Seven QTLs for Cd tolerance or Cd concentration and accumulation were detected with the RILs. Membership function analysis was used to select Cd-tolerant and Cd-sensitive lines from the RILs according to the parameters of SH, SR, SDW, RDW, TN and Fv/Fm. A large variation occurred in the RILs tolerance to Cd and generally fitted the normal distribution. As a consequence, the RILs were compartmentalized five groups according to the tolerance under Cd stress.Four wheat lines differing in Cd tolerance previously identified from a recombinant inbred line population were studied in root morphological traits, lipid peroxidation and nutrition metabolism. Cd-tolerant lines with lower in Cd accumulation, higher in antioxidative enzyme and photosynthesis than Cd-sensitive lines.The QTL for Cd tolerance and Cd accumulation traits in wheat were identified using the RIL population by the parameters of SH, SR, SDW, RDW, TN and Fv/Fm. Seven QTLs for Cd tolerance and Cd concentration and accumulation were detected with the RILs, of which five were designated for Cd tolerance and the other two for the Cd accumulation in root. The five QTLs for Cd tolerance separately explained 9.43% to 27.52% of the phenotypic variance, while the other two for Cd accumulation in root accounted for 12.81% to 26.16% of the phenotypic variance, respectively. For Cd tolerance, five QTLs for four growth or physiological traits were detected on Chromosomes 1A,2A,4A,5D and 7B. Two QTLs for RCA (root Cd accumulation) were detected on Chromosomes 4A and 5D. The additive effects of four QTLs were positive with the additive coming from Chuan 35050 while the remaining three QTLs were negative with the additive effects contributed by Shannong 483. Based on all above Cd tolerance and Cd accumulation should be dependent traits.5 Studies on the physiological mechanisms of Cd toleranceThe leaf reactive centre of PSII was damaged, and then the light energy conversion process was depressed (decreased Fv/Fo, Fv/Fm andΦPSⅡ, increased Fo) with more light energy harvested dispersing via non-photochemical approach (reduced qp), which could further contribute to the depressed Pn, Tr and Gs under enhanced Cd toxicity among the different types of wheat. MDA content increased and SOD activity decrease. CAT activity increased in Cd-tolerant lines while decreased in Cd-sensitive lines. Total soluble sugar (TSS) concentration decreased while free amino acids (FAA) concentration increased in both shoot and root, and resulted in the decreas of TSS/FAA. Cd concentration and accumulation in wheat differing in ploidy, genome and RILs were increased with the increase of Cd concentration and Cd accumulation was much higher in roots than in shoots. Cd tolerance lines could retain more Cd in roots.Thus,Cd accumulation in shoots and roots and translocation was lower than Cd sensitive lines.