Exogenous application of silicon alleviates boron toxicity in rice seedlings

Purpose Boron toxicity in soil and plants affects crop yield; it can be alleviated by silicon application. The effects of compounding factors (alkalinity and counterions) of silicon compound on plants have been ignored. We examined the boron accumulation and distribution, and the ultrastructure of mesophyll cells in rice seedlings exposed to boron. Methods Rice seedlings were grown in hydroponic solutions with different concentrations of boric acid and sodium silicate; nitric acid was added to neutralize the alkalinity of sodium silicate; sodium nitrate was added to eliminate the difference of sodium ion level in different treatments. Growth parameters, ultrastructure of mesophyll cells, chlorophyll content, and boron content were determined. Results Silicon decreased boron content, accumulation, and transfer coefficient in seedlings exposed to boron. In boron-treated rice seedlings, the chloroplasts in the mesophyll cells had narrow, disorganized grana and low chlorophyll content, and mitochondria had fewer cristae and an altered shape. After silicon application, the chloroplasts had wider stroma and orderly arranged grana lamellae, chlorophyll content was increased, and the mitochondrial shape was restored, with more cristae. Conclusion Silicon alleviated boron toxicity in rice seedlings. Silicon-mediated alleviation of boron toxicity in rice was associated with boron content in seedlings and restoration of the ultrastructure of mesophyll cells.