Silicon Alleviates Alone As Well As Combines Toxicities Of Cadmium And PH Stresses In Wheat(Triticum Aestivum L.)Plants

Among various heavy metals,cadmium（Cd）is a highly lethal element because of its relative mobility,high water solubility,and phytotoxicity Due to its dynamic fates,Cd in an ionic form enters into plants through the root and being shifted to shoot xylem and phloem through various active transporters and passive transporters.Higher uptake of Cd reduces photosynthesis by hindering root Fe（III）reductase,affects the activity of the enzyme involved in CO₂ fixation,affects mineral absorption causing a nutritional imbalance（such as N and K⁺）and results in stomatal closure.An intensified oxidative process occurred in cells of Cd-stressed plants and produced a massive amount of reactive oxygen species（ROS）,which caused a remarkable reduction of anti-oxidative enzymes activities in plants.Additionally,Cd accumulates in the human body via the food chain,causing kidney,bone,and pulmonary damage.Soil pH is the most critical factor among various abiotic stressors,influencing plant growth,and development.Soil pH can be assumed as a crucial variable due to its central role in many soil processes and properties,which may affect plant growth,microbial diversity,and solubility and availability of various nutrients.These effects might be further complicated by the interaction of Cd in an acidic environment.Cd bioavailability to plants not only associated with its chemical form and total contents into soils but also affected primarily by soil pH.In ecotoxicological studies,soil pH meaningfully effect on Cd bioavailability;therefore,it has great significance,because acidic soils show a greater threat than other soils.Silicon（Si）,as a beneficial element,shows a crucial part in the growth and enlargement of various crops.To use Si fertilizers or Si-rich soil amendments is important for balanced plant nutrition,optimal growth,and development.Silicon plays a significant role in the alleviation of various symptoms of biotic and abiotic stresses.It is known that Si can decrease the negative influence of excessive concentrations of hazardous elements,including heavy metals.The primary aim of the present study was to find the constructive impact of Si in two Si application approaches and to identify the optimum way of Si supplementation for plant physiological traits,chlorophyll contents,anti-oxidative enzyme activities,proline,H₂O₂,and MDA content in Cd-stressed wheat plants grown hydroponically.Moreover,the present study was to explore the beneficial role of Si on membrane stability and anti-oxidative defense system under alone acidity and alkalinity stresses as well as pH along with Cd stress in hydroponic conditions.Results of the first study showed that all levels of Cd significantly enhanced membrane permeability measured as malondialdehyde（MDA）content,oxidative stress measured as hydrogen peroxide（H₂O₂）content,and triggered the enzymatic and non-enzymatic antioxidant system,resulting in a severe reduction in the growth of wheat seedlings.Besides,Si application either as root（SiR3）or foliar spray（SiF3）remarkably encountered Cd toxicity by further improving antioxidative enzymes activities,and by hindering Cd uptake,but more significant results were recorded for SiR3.The results of the second study showed that the combined effects of Si and Cd in acidic pH on wheat roots depend on the concentrations of Si and Cd.The combined or single effects of low or moderate concentrations of Si（1 or 3 mM）and acidic pH（5）enhanced the growth of wheat roots,and the combined effects were more significant than those of the single treatment.The combined effects of low or high concentrations of Cd（50 or 200μM）and acidic pH significantly reduced root and shoot growth and biomass while increased enzymatic and non-enzymatic antioxidants,osmoprotectants,and H₂O₂ and MDA contents.The incorporation of Si（1 or 3 mM）in Cd-contaminated acidic nutrient solution promoted the wheat root and shoot growth,decreased H₂O₂ and MDA contents,and further increased catalase（CAT）,superoxide dismutase（SOD）,peroxidase（POD）,ascorbic acid（AsA）,glutathione（GHS）,total soluble sugar（TSS）,and proline contents,maintaining the balance of mineral nutrients in the wheat plants compared with Cd single treatments in acidic pH.The demolishing effects were better with a high concentration of Si（3 mM）than the low concentration of Si（1 Mm）.The alleviatory effects were due to Si traps more cadmium in the root cell wall.Results of the third study showed that acidic,as well as alkaline stresses significantly reduced plant physiological traits and photosynthetic rates by creating deficiencies accompanied by toxicities of various macro and microelements.Moreover,acidic and alkaline stresses reduced membrane stability through lipid peroxidation measured as malondialdehyde contents,enhanced membrane injury and damage through electrolytic leakage contents,induced oxidative stress through H₂O₂ contents and triggers enzymatic and non-enzymatic antioxidative defense system through oxidative burst comparing with control（pH;7）.While the addition of Si at the concentrations of 1 and 3 mmol L^-11 significantly encountered adverse effects of acidic and alkaline p Hs through further strengthening the antioxidative defense system by enhancing the activities of enzymatic and non-enzymatic antioxidants and suppressing reactive oxygen species（ROS）contents.The results of the fourth study showed that all levels of Cd toxicities significantly alter the morphological traits of wheat roots by overproduction of ROS,hinder essential nutrient availability,and accumulation of high contents of Cd in root cells.While the application of Si with the concentration of 1and 3 mmol L^-1 in Cd-contaminated alkaline solution significantly demolished Cd-induced oxidative stress by shifting the balance between antioxidant enzymes and oxidants of ROS.Moreover,Si significantly increased cations concentrations in root tips that compete with Cd ions for exchange sites and reduced Cd uptake and accumulation by roots.In conclusion,the addition of Si has been proved by high-quality non-corrosive beneficial elements under the hostile conditions of alone acidic stress,alone alkaline stress,alone Cd stress,combine acidic and Cd stress,and combine alkaline and Cd stress in wheat seedlings as the studies herein.Moreover,we can be strongly emphasized that Si supplementation with the preference of root-application must be used as an optimum approach to reduce Cd toxicity in wheat plants...