Efficacy of Humic Acids and Chitosan for Enhancing Yield and Sugar Quality of Sugar Beet Under Moderate and Severe Drought

Seeking for effective and easy–to–implement tactics to mitigate the negative impacts on crop growth as a result of water shortage will remain the main objective in irrigation water rationalization programs. Therefore, along two growing seasons of 2018/19 and 2019/20, field trial was executed to assess the potentiality of humic acids and chitosan for diminishing the unwanted effects of drought in sugar beet yield and quality. The study involved three irrigation regimes (60, 80, and 100% of actual crop evapotranspiration, denoted ETc60, ETc80, and ETc100, respectively). Three humic acids rates (0, 15, and 30 L ha −1 ) and two chitosan levels (without and with 200 mg L −1 ) were applied. The trial implemented in a split–split plot design with three replicates. Enzymes activity, anatomical, agronomic, and quality traits have been estimated. Findings revealed that catalase (CAT) and glutathione peroxidase (GPX) activity substantially increased by increasing water deficit degree. There was insignificant difference between ETc80 and ETc100 in root and sugar yields ha −1 in both seasons. ETc60 recorded the highest values of sucrose %, potassium content, and extracted sugar % in both seasons, in addition to α–amino nitrogen in the first season and sugar lost to molasses in the second one. Humic acids markedly increased CAT activity in both seasons and GPX activity in the first one. Application of humic acids at a rate of 30 L ha −1 resulted in the maximum increases in root length, root and top fresh weights plant −1 , top/root ratio, leaf area, and root and sugar yields ha −1 . Except for sodium content, all other sugar quality traits showed the maximum increases with application of 30 L ha −1 humic acids. Chitosan-treated plants had higher activity of CAT and GPX and produced increases of 1.8, 4.2, 11.7, 7.5, 3.5, and 4.2% in root length, root fresh weight, top fresh weight, top/root ratio, leaf area, and root yield ha −1 , respectively, compared to the untreated plants. Also, sucrose %, extracted sugar %, and sugar yield ha −1 showed significant increases with chitosan-treated plants higher than that of untreated ones. Application of humic acids (30 L ha −1 ) + chitosan (200 mg L −1 ), compared to no application, under ETc80, reduced the stomatal closure % from 48.86 to 31.06% with promising improvement in root and sugar yields and quality. In conclusion, the interactive effect of humic acids and chitosan exhibited favorable changes in antioxidant defense and sto