Application of Biostimulants Alleviated Drought Stress in Sugar Beet (Beta vulgaris L.) by Improving Oxidative Defense System, Osmolytes Accumulation and Root Yield

Purpose Drought stress poses a detrimental impact on the growth and production of sugar beet (Beta vulgaris L.) worldwide. Limited information related to the role of biostimulants in alleviating the adversities of drought stress in sugar beet has been found in the literature. Methods A greenhouse experiment was designed to investigate the relative effectiveness of foliaged-applied biostimulants viz., isabion, proline, methionine, tyrosine and moringa leaf extract (MLE),on photosynthetic pigments and relative water content (RWC), net photosynthetic rate, transpiration rate, stomatal conductance, accumulations of organic osmolytes, and activities of enzymatic antioxidants, and root yield and attributes of sugar beet exposed to two water regimes: (80% water holding capacity: well-watered and 40% water holding capacity: drought stress) at two phenological growth stages (BBCH18 and BBCH31)growth stages. Results The 40% water holding capacity remarkably decreased the root yield, root diameter, leaf relative water contents, photosynthetic pigments, and net photosynthetic rate, transpiration rate, and stomatal conductance compared with the 80% WHC (well-watered regime). However, exogenously applied biostimulants significantly improved these parameters under drought stress conditions in order of effectiveness: tyrosine > proline > methionine > isabion > MLE. Water deficit condition (40% water holding capacity) significantly (p < 0.05) augmented H2O2, malondialdehyde (MDA) contents and electrolytes leakage. However, foliar application of biostimulants under drought stress considerably decreased H2O2, MDA contents and electrolyte leakage. Foliar treatments of biostimulants stimulated enzymatic antioxidant (superoxide dismutase, peroxidase, ascorbate peroxidase and catalase) activities, free proline, soluble sugar, and free amino acid concentration under both well-watered and drought-stressed conditions. Enzymatic antioxidant (superoxide dismutase, peroxidase, and ascorbate peroxidase and catalase) activities, free proline, soluble sugar, and free amino acid concentration were stimulated by foliar treatments of biostimulants under drought-stressed conditions. In addition, foliar application of these biostimulants significantly enhanced root yield, root diameter, top yield, sugar content and sugar recovery in sugar beet under limited water conditions. Conclusion Overall, it was concluded that foliage-applied tyrosine was found to be the most effective biostimulant among all others to alleviate the negative effect of drought stress in sugar beet through decreased the damaging effect of ROS and enhanced enzymatic antioxidant activities and accumulation of organic osmolytes. The current findings suggest that the exogenous application of tyrosine represents a promising approach for promoting the successful cultivation of sugar beet in drought-prone areas.