Physiochemical Response And Ultrastructural Alterations In Seedings Of Three Cotton Varieties Under Nickel/Lead Stress

Cotton is one of China’s as well as the world’s primary cash crops, occupying animportant position in Chinese agriculture. Heavy metal contamination of soil not only leads toreduced crop production, but also poses significant health risks due to the high toxicity ofthose elements. In China,20million hectares or nearly1/5of cultivated land suffers fromheavy metal pollution of varying degree. Heavy metal stress can stunt cotton’s growth anddevelopment, resulting in losses of both yield and quality.The present experiment imposed nickel and lead stresses on cotton varieties collected allover China. We examined the accumulation of nickel and lead in, and the resistancephysiology of cotton seedlings, using scanning electron microscopy to investigate theultrastructural changes of stressed cotton tissue, observing the effects of nickel on methylationand acetylation of chromatin in cotton seedlings via immunostaining methods. We alsoevaluated the cotton varieties’ abilities to fight off heavy metal poisoning, as well as thetoxicity of nickel and lead, by comparing their nickel and lead resistance mechanisms. Theresults of this study are summarized as follows:Nickel and lead stresses resulted in elevated concentration of these metals in the leavesof cotton seedlings, impairing root growth, chlorophyll contents, and the accumulation ofbiomass. Furthermore, concentrations of hydrogen peroxide and malondialdehyde wereescalated, causing severe and often irreversible tissue damage. Cotton seedling can combatheavy metal poisoning by increasing its tissues’ osmotic potential via raising soluble proteinand proline contents, and resist heavy metal-induced oxygen radicals by regulating pertinentprotective enzyme systems.Scanning electron microscopy results showed that cells of cotton seedlings under severeheavy metal stresses shrank, deformed, even ruptured, suffering from cell layer breakage. Nickel and lead had similar physiological effects on this regard.Immunostaining results showed that low concentrations of nickel did not significantlyimpact nucleus histone modification, while high concentrations of nickel led to the activationof some normally quiescent genes as well as the silencing of several genes that should beactive. This phenomenon was stronger in roots than in leaves.Under nickel and lead stresses, Zhongzhimian2, which is widely planted in the YellowRiver basin, showed relatively low levels of heavy metal concentrations, significant increasesin osmoregulation substances, and rapid responses from the antioxidant enzyme system.Concentrations of malondialdehyde were relatively low in this variety’s tissues, indicatingexcellent heavy metal resistance. Xinluzao16, a variety favored in the arid north-west inlandregions, was slow to react against nickel and lead poisoning, consequently suffering the mostsevere tissue damage. Zhongmiansuo41was middle-way between the two afore-mentionedvarieties in heavy metal resistance, and exhibited a greater vulnerability to lead than to nickel.Lead poisoning impacted heavy metal accumulation, root length, hydrogen peroxideconcentration, superoxide dismutase content, peroxidase content, and proline content moresignificantly than nickel did. The two elements had similar effects on dry weight, chlorophyllcontent, malonaldehyde content, and soluble protein content. Nickel caused greater decline infresh weight than lead. The results showed that lead had greater toxicity to nickel, which wasalso extremely harmful to plants at high concentration levels.