Effects And Mechanisms Of Exogenous Silicon On Physiological And Biochemical Metabolisms In Garlic Under Cadmium And Mercury Stresses

In recent years, heavy metal pollution in farmland is becoming more and more serious,especially cadmium (Cd) and mercury (Hg) pollutions, which affect crop yield, quality andthreaten food safety. Exogenous silicon can alleviate injury and decrease heavy metalaccumulation of plants under heavy metal stress. However, the mechanisms forsilicon-mediated alleviation of heavy metal stress are not fully understood. Moreover, littleresearch has been conducted on vegetables, and more work is needed so as to provide atheoretic basis for deceasing heavy metal accumulation in production. In this study, potexperiments were conducted to investigate the effects and mechanisms of exogenous siliconon physiological and biochemical metabolisms in garlic under cadmium and mercury stresses;meanwhile, a field trial was done to explore the effect of foliar spay of silicon on garlicquality and yield. The main results are as follows:1. Application of silicon could increase the plant height of garlic. Under Cd or Hg stress,the growth of shoot and root of garlic was significantly decreased, while silicon supplyimproved the growth. The net photosynthetic rate and transpirational rate of garlic weresignificantly decreased under Cd or Hg stress, while they were increased by addition ofsilicon, suggesting that silicon could improve the photosynthetic system of heavy metalstressed plants. Meanwhile, the stomatal limitation (Ls) was not regularlly altered, suggestingthat the decrease of photosynthesis under Cd or Hg stress was caused by non-stomatal factors.Silicon application could mitigate the decrease of maximum photochemical quantum yield(Fv/Fm) and actual photochemical efficiency (PSⅡ) under Cd or Hg stress, which furtherconfirmed the protective role of silicon. Under Cd or Hg stress, the content of malonaldehyde(MDA), a product of membrane lipid peroxidation, was increased; while addition of siliconcould increase the activities of antioxidant enzymes such as superoxide dismutase (SOD) andcatalase (CAT), and decrease the content of malonaldehyde in stressed plants.2. The soil pH was not changed by added silicon at a rate of50mg/Kg; whereas it wassignificantly increased when silicon was applied at500mg/Kg, and the pH was lower after garlic planting than before planting. There was no difference in the contents of soil availablesilicon, garlic shoot and root silicon among the CT, Si1, Si2, Cd, Cd+Si1and Cd+Si2treatments. Compared with Hg treatment alone, Hg+Si2had higher available silicon contentin the soil. The activities of catalase, urease and sucrose in the soil were significantlydecreased, and added silicon increased the activities of these activities.3. Under Cd stress, the order of Cd content in different parts of garlic was as follows:root> shoot> bulb. Compared with Cd treatment alone, Cd+Si1treated plants had higher Cdcontents in the shoot and bulb of garlic plants, and the plants grew better. A possiblemechanism of detoxification is the formation of Si-Cd complex within garlic plants. Additionof silicon at a higher rate (Si2) significantly decreased the Cd contents of garlic and alleviatedthe toxic symptom. The main reason for this might be due to silicon-induced increase in soilpH, which decreased the exchangeable Cd content in the soil. Under Hg stress, the order ofHg content in different part of garlic was as follows: shoot> root> bulb, and addition ofsilicon inhibited Hg uptake, resulting in decreased Hg content and toxic symptom in plants.This was attributed to silicon-mediated decrease in exchangeable Hg content in soil. Thealteration of Cd or Hg from exchangeable to residual form in soil and/or formation of Cd-orHg-complex in plants are among the reasons for silicon-mediated inhibition of Cd or Hgtoxicities in plants.4. In a field trial, it was observed that foliar application of silicon fertilizer couldobviously increase the yield of garlic bulb without influence on the quality.In conclusion, silicon could alleviate the injury of garlic by toxic mercury and cadmiumby protecting the photosynthetic system, increasing antoxidant defense of plants, increasingthe activities of soil enzymes, and decreasing the contents of Cd or Hg in the soil. Foliarapplication of silicon fertilizer could increase the yield of garlic without influence on thequality in field fertilizer experiment. This study may provide a theoretical basis forapplication of silicon fertilizer in production.