Evaluation of Silicon and Proline Application on the Oxidative Machinery in Drought-Stressed Sugar Beet

Drought stress deleteriously affects growth, development and productivity in plants. So, we examined the silicon effect (2 mmol) and proline (10 mmol) individually or the combination (Si + proline) in alleviating the harmful effect of drought on total phenolic compounds, reactive oxygen species (ROS), chlorophyll concentration and antioxidant enzymes as well as yield parameters of drought-stressed sugar beet plants during 2018/2019 and 2019/2020 seasons. Our findings indicated that the root diameter and length (cm), root and shoot fresh weights (g plant^-1) as well as root and sugar yield significantly decreased in sugar beet plants under drought. Relative water content (RWC), nitrogen (N), phosphorus (P) and potassium (K) contents and chlorophyll (Chl) concentration considerably reduced in stressed sugar beet plants that compared with control in both seasons. Nonetheless, lipid peroxidation (MDA), electrolyte leakage (EL), hydrogen peroxide (H₂O₂) and superoxide (O₂^●-) considerably elevated as signals of drought. Drought-stressed sugar beet plants showed an increase in proline accumulation, total phenolic compounds and up-regulation of antioxidant enzymes catalase (CAT) and superoxide dismutase (SOD) activity to mitigate drought effects. Si and proline individually or the combination Si + proline considerably increased root and sugar yield, sucrose%, Chl concentration and RWC, MDA and EL were remarkably reduced. The treatments led to adjust proline and total phenolic compounds as well as CAT and SOD activity in stressed sugar beet plants. We concluded that application of Si + proline under drought stress led to improve the resistance of sugar beet by regulating of proline, antioxidant enzymes, phenolic compounds and improving RWC, Chl concentration and Nitrogen, Phosphorus and Potassium (NPK) contents as well as yield parameters.