Nano-silicon protects sugar beet plants against water deficit stress by improving the antioxidant systems and compatible solutes

Silicon (Si) can mitigate the deleterious impacts of various types of stresses on field crops. However, the potential of nano-silicon (nano-Si) in improving water stress and the relevant mechanisms remain unclear. Therefore, here, we examined the combined impacts of nano-Si and various irrigation regimes on antioxidant systems, osmolytes, photosynthesis-related parameters, and growth of sugar beet in a field trial. Treatments included three supplemental irrigation rates (I1, I2, and I3) arranged based on the crop evapotranspiration (100% ETC, 75% ETC, and 50% ETC) and three doses of nano-Si: 0, 1, and 2 mM. Irrigation regime treatments were performed at the six-to eight-leaf stage (49 days after sowing), which continued until the harvest (180 days after sowing). Water stress brought about a detrimental impact on the sugar beet growth, the relative water content of leaves (LRWC), leaf area index (LAI), and photosynthetic performance. In contrast, Water deficiency enhanced hydrogen peroxide (H2O2) and malondialdehyde (MDA) contents, which were followed by increasing antioxidant activities and osmolytes. Supplementation of nano-Si at low dose (1 mM) significantly increased chlorophyll contents, net photosynthesis (PN), glycine betaine (GB), flavonols (quercetin and rutin), and enzymatic antioxidants (superoxide dismutase, catalase, and guaiacol peroxidase). Furthermore, nano-Si at low dose (1 mM) decreased the amount of H2O2 and MDA. Instead, the higher dose (2 mM) of nano-Si exerted toxic effects on severe water-stressed (50% ETC) plants. The parallel increase in MDA and proline contents in sugar beet plants treated with two mM nano-Si along with severe water stress supports the view that proline augmentation presumably is a sign of stress injury instead of stress resistance. Overall, our results imply that nano-Si can play a protecting role in sugar beet plants during water stress by enhancing antioxidants, GB, and flavonols (quercetin and rutin). However, the concentration of nano-Si must be chosen with care.