Alleviate Effect And Mechanism Of Silicon On Oryza Sativa L.under Acid Rain Stress

Acid rain is a serious environmental problem throughout the world,it decreases the growth and yield of crops and damages the agricultural ecosystem.Silicon?Si?is widespread in soil and plant,as a a beneficial macroelement,many studies have shown that can enhance the resistance of plants exposed to abiotic and biotic stresses.Rice?Oryza Sativa L.?is one of the main economic crops in acid rain area,repeated mono-cropping with rice might greatly decrease the amount of Si in soil available to plants.As a consequence,Si might subsequently become a yield-limiting element for rice cultivation.Therefore,it is great practical significance to explore the influence of Si on rice under acid rain stress.Hydroponic experiment was carried out to study the alleviation effects of Si on rice seedlings under simulated acid rain?SAR?stress,from the perspective of physiological,biochemical,nutrient absorption and organizational structure.the main results from our experiments were shown as follows:?1?The combined or single effects of Si and/or SAR on rice growth depend on the concentration of Si and the pH of the SAR.For the single treatments of Si?1,2 or 4mM?,with the increase of the Si concentration,the growth indicators were the first increased then decreased,and the highest value was obtained in the 2 mM Si treatment.Si?1,2 or 4 m M?could promote the above-ground growth of rice seedlings,but the addition of a high concentration of Si?4 mM?restricted root growth.For the single treatments of SAR?pH 4.0,3.0,2.0?,SAR spraying affected rice seedlings,light SAR?pH 4.0?improved the rice growth,moderate SAR?pH 3.0?did not obviously decrease the rice growth,severe SAR?pH 2.0?significantly inhibited the growth of rice seedlings.The incorporation of Si?1.0 or 2.0 mM?into SAR at pH values of 4.0 or 3.0obviously promoted rice growth compared with the single treatment with SAR,at the same time,the incorporation of Si?1,2 or 4 mM?with SAR pH 2.0 alleviated the damage of SAR.Under the same pH values of SAR,with the increase of the Si concentration,the growth indicators were the first increased then decreased,and the highest value was obtained in the 2 mM Si treatment.Light and moderate SAR could alleviate the inhibition effect of high concentration of Si on root growth of rice.?2?incorporation of Si into SAR-treatment interfered with the absorption of mineral elements,and maintained the balance of mineral element concentrations in rice seedlings compared with SAR alone.SAR?pH 4.0,3.0,2.0?spraying affected the absorption of mineral elements in rice,moderate and severe SAR?pH 3.0,2.0?spraying decreased the Si content in rice seedlings,the addition of Si?1,2 or 4 mM?significantly alleviated the decrease in the content of Si in rice,with the increase of the Si concentration,the trend of the mitigation effect was the first increased then decreased.The correlation analysis showed that the aboveground and underground biomass of rice were significantly positively correlated with the Si content of corresponding sites.Severe SAR?pH 2.0?resulted in rapid changes in the content of K,Ca,Mg,Cu,Zn and Fe,and the incorporation of Si into SAR-treatment alleviate dramatic change of the mineral elements compared with the treatment with SAR alone.To a certain extent,Si can maintain the balance of mineral elements in plants under the SAR stress.?3?Effects of Si on the physiological and biochemical characteristics of rice seedlings under acid rain stress were investigated.When rice seedlings exposed to moderate or severe SAR?pH 3.0 or 2.0?,the hydrogen peroxide?H₂O₂?was significantly increased,the malondialdehyde?MDA?and soluble protein conent were obviously decreased.Moreover,the incorporation of Si?1,2 or 4 mM?into SAR increased CAT,POD,SOD and APX activities,decreased H₂O₂ content,increased MDA and soluble protein content.When rice seedlings treated with moderate or severe SAR,the net photosynthetic rate?P_n?,chlorophyll content,stomatal conductance?G_s?,the maximum quantum efficiency of PSII photochemistry?F_v/F_m?and the actual photochemical quantum efficiency of PSII photochemistry?Yield?were significantly decreased,the intercellular CO₂ concentration?C_i?was significantly increased,and chloroplast ultrastructure was damaged.The incorporation of Si into SAR increased P_n,chlorophyll content,G_s,F_v/F_m and Yield,decreased C_i,improved chloroplast ultrastructure in rice seedlings,alleviated the damage of chloroplast structure.The moderate concentration of Si was more effective than both the low and high concentrations.The above conclusion showed that Si improved the activity of antioxidant enzyme,reduced the oxidative damage,increased the photosynthesis of rice.?4?The roles of Si in the effects on phenotype,micromorpology and anatomy of the leaves of rice seedlings under acid rain stress were elucidated.The combined or single effects of the moderate concentration of Si?2.0 mM?and light SAR?pH 4.0?enhanced the growth of the rice leaves and the development of the mesophyll cells,and the combined effects were stronger than those of the single treatments.The high concentration of Si?4.0 mM?and severe SAR?pH 3.0 or 2.0?exerted deleterious effects.The incorporation of Si?2.0 or 4.0mM?into SAR at pH values of 3.0 or 2.0promoted rice leaf growth,decreased necrosis spots,maintained the structure and function of the mesophyllcells,increased the epicuticular wax content and wart-like protuberance?WP?density,and improved the stomatal characteristics of the leaves of rice seedlings more than the SAR only treatments.The alleviatory effects observed with a moderate concentration of Si?2.0 mM?were better than the effects obtained with the high concentration of Si?4.0 mM?.The alleviatory effects were due to the enhancement of the mechanical barriers in the leaf epidermis.Under acid rain stress,moderate concentration of silicon improved the epidermis structure of rice leaf,maintained the relative balance of nutrient absorption,increased the activity of antioxidant enzymes,reducing the oxidative damage,protected the photosynthetic effect,improved the resistance of rice to acid rain stress,promoted plant growth.