Effect Of Cadmium Stress On Growth, Yield And Quality In Upland Cotton And Its Genetic Analysis For Cadmium Tolerance

Cadmium (Cd) is one of the most toxic heavy metals being contaminating our soils. It can be easily absorbed and accumulated by plants. The high accumulation of Cd not only inhibits the plant growth and negatively affects crop yield and quality, but also causes harm to human health through the food chain. It is therefore necessary to reduce soil Cd level to ensure the safety of agricultural products and ensure the sustainable and efficient use of the Cd contaminated soil. The most effective way of reducing soil Cd level is the breeding of Cd tolerant cultivars having no or very low Cd in their edible parts. Cotton (Gossypium hirsutum L.) is one of the important fibre crops in the world, which is widely used for fiber production. Thus it can also be used to be remediate Cd contaminated soil.In this study, we planted cotton cultivars namely ZD-90, SGK3and TM-1on Cd polluted soils in2009and2010. ZD-90is a transgenic glyphosate tolerant cotton germplasm with EPESP-G7developed by our laboratory. SGK3is an insect-resistant cultivar obtained from the Biotechnology Center of the Chinese Academy of Agricultural Sciences. TM-1is the upland cotton genetic standard line obtained from USDA, ARS, College Station, Texas, USA. A pot experiment was conducted to study the growth conditions, physiological traits and photosynthetic parameters, analyze the yield, fiber quality traits, seed quality traits, Cd accumulation and distribution, and explore the phytoremediation capability of these cultivars of Cd. Furthermore, we explored the genetic features and molecular mechanisms of the Cd resistance in cotton seedlings through the genetic models of ADM, by using sixteen transgenic insect-resistant cotton cultivars as parental with a diallel mating design. The main results were as follows:1. Effect of cadmium on the physiology and metabolism of cottonA pot experiment showed that cotton plant growth under Cd stress was dose-dependent. It is beneficial to cotton plants growth at low Cd concentrations, however, significant growth inhibition at high Cd concentrations. There are significant differences among cotton cultivars with respect to Cd stress. The physiolosical and biochemical analysis of cotton plant indicated that the activity of antioxidant enzymes such as POD, SOD, CAT and APX, etc. in plant cells greatly increased to avoid harmful effects at low Cd concentration. With the increase in Cd concentration and length of growth period, MDA content increased, chlorophyll decreased gradually, net photosynthetic rate increased at low Cd concentration and then declined. Transpiration rate and stomatal conductance decreased, stomatal limitation value increased gradually.2. Effect of cadmium on cotton yield and its componentsCotton yield significantly declined in the three cultivars under Cd stress. The mean value of lint was declined in TM-1and SGK3with an increase in Cd level, while it was increaced in ZD-90. A significant decreasing trend of bolls per plant was found in the three cultivars. As compared to control plants, boll weight increased to the highest at200μM Cd level and then declined significantly at600μM Cd level for SGK3, while a continuously decreasing trend was found for ZD-90and TM-1.3. Effect of cadmium on cotton fiber quality traitsFiber quality traits of the three cotton cultivars decreased to a little under Cd stress. The main difference exsised between fiber quality and cultivars. It was beneficial to fiber length at low Cd concentrations, which was inhibited at high Cd concentrations in TM-1and SGK3, but increased in ZD-90. Fiber strength was found increasing to the highest at200μM Cd level and there was no significant difference in the exposed cultivars. Micronaire was acceptable at low Cd concentrations, and showed a decline at high Cd concentrations. Higher micronaire was observed in SGK.3than that in ZD-90and TM-1. No significant difference was found for fiber elongation at all Cd levels in the three cultivars. It was lower at all Cd treatments than control.4. Effect of cadmium on cotton seed qualityThe physical quality traits of cottonseeds showed a little decrease in the three cotton cultivars under Cd stress, but cultivar specific response was different. TM-1was sensitive to Cd as a marked decrease was found in seed index, kernel index and kernel/hull. ZD-90and SGK3was not sensitive to Cd because there were no significant differences in seed physical traits. Seed protein content at higher Cd level was lower in comparison with lower Cd level, but was still higher than control. The change of oil content is opposite to that of protein content. Relative content of amino acids was less influenced by Cd stress, but there were some differences among cultivars. Cys content was increased in TM-1and decreased in SGK3. It was inconsistent with different fatty acids content under Cd stress. Palmitic acid conent in the seed decreased significantly with increasing Cd supply. There were not significant changes in the content of linoleic acid in seed under the various treatments compared to controls. A decreasing trend was observed for gossypol content in the three cultivars. Gossypol content in ZD-90and SGK3was lower than that in TM-1.5. Effect of cadmium on microstructure and ultra-microstructure of cotton seedsParaffin showed that the pigment gland density on surface of seeds was decreased with the increase in Cd concentration, which was consistant with the trend of gossypol content in cotton seeds. TEM results showed that the damage to the seed cells became more and more prominent with increasing concentration of cadmium as compared to control. The main alterations were such as plasmolysis, disintegration of nucleus, shrinkage of cytoplasm, thickening and constriction of cell wall, abnormal structures of organelle, cell collapse and disintegration. TEM observation and ED AX analysis confirmed that Cd was existed in the form of rings and crystals as well as electron dense granules, distributed in intercellular space, cytoplasm and cell wall.6. Accumulation and transportation of Cd and other metal elements in cotton plant under Cd stressThe results of Cd accumulation in cotton plant under Cd stress showed that petiole was the highest part in Cd accumulation, and fiber was the lowest. The ability of uptake and accumulation in different parts of the three varieties with the rise of Cd level was in order:fiber <seed<seed shell<root<leaf<shoot<boll shell<petiole. The translocation ability, bioconcentration factor and absorbency showed the same order in different organs and increaced with Cd concentration. It indicated that cotton has higher potential for absorbing and removing Cd from moderately contaminated soils and can be used for the phytoremediation of polluted soils. There was significant difference on Cd accumulation and transportation among cultivars, it was higher in ZD-90and SGK.3than that in TM-1. Correlation analysis showed that the uptake and accumulation of Mg, Mn, Cu, Zn in fiber and seed shell, and Mg, Mn, Zn in root, Mn, Zn in leaf, petiole and boll shell, and Mn, Cu in seed and shoot were greatly influenced by Cd stress.7. Genetic features analysis of the resistance to Cd of cotton seedlingsGenetic analysis on physiological traits showed that there were not genetic effects on plant dry weight, POD activity and MDA content in leaf and root, but the dominant effects and maternal effects. For the dominant effects and maternal effects, it was lower of seedling dry weight under Cd stress than that in control and increased with Cd levels of leaf POD activity. The dominant effects of root POD activity and MDA content were increasing to the highest at200p.M Cd level. For these physiological charecters, it could be selected according to maternal effects and using heterosis at longer treatment time. The heritability of shoot dry weight was higher than control in Cd treatments, while was lower in root dry weight. It showed that less influence was found on shoot dry weight and much influence on root dry weight under Cd stress.8. Effect of cadmium on cotton genomic DNA methylationMASP analysis showed that Cd may induce the changes of genomic methylation levels because of the rise in DNA methylation level and the reduction of unmethylation under Cd stress. The genetic frequency of CCGG cytosine methylation loci decreased and variation frequency increased under Cd stress. The frequency inherited from the female parent alone was lower while thet inherited from the male parent alone was higher than the control. According to the inheritance of cytosine methylation patterns, M1, Hland HM1patterns of the female parent served as the main patterns at control, D pattern of parents played a major role. The variation frequency from the female and both parents was lower than that of the control, but was higher than the control from the male parent. According to the variation of cytosine methylation patterns, the D pattern of parents played a major role.