Influence of Foliar-Applied Humic Acid and Some Key Growth Regulators on Sugar Beet (Beta vulgaris L.) Under Drought Stress: Antioxidant Defense System, Photosynthetic Characteristics and Sugar Yield

Abstract

To assess the impact of humic acid and three potential growth regulators as salicylic acid (SA), gibberellic acid (GA3) and ascorbic acid (AsA) on sugar beet under drought stress, a field experiment as a split plot based on completely randomized block design was performed. The main plots comprised three watering regimes, well-watered, 60% field capacity (mild water stress) and 45% field capacity (severe water stress). Foliar application of humic acid, SA, GA3 and AsA was allocated into sub-plots. Both drought stress regimes caused a decline in root yield, sugar content, chlorophyll content, relative water content and leaf area index. In contrast, drought stress resulted in a marked rise superoxide dismutase, peroxidase and catalase activities as well as the levels of proline, glycine betaine, malondialdehyde and hydrogen peroxide in sugar beet plants. However, foliar application of humic acid, SA, GA3 and AsA reasonably increased the activities/levels of enzymatic and non-enzymatic antioxidants and vita organic osmolytes. The current findings suggest that exogenous application of humic acid is effective in tailoring the sugar plants to thrive well on drought-prone areas.