Effects of Different Irrigation Levels and Phosphate Fertilizer Doses with or without Nano Iron Application on Sugar Beet Growth and Physiology

Despite the importance of phosphate and iron in plant nutrition, their combined effects on sugar beet’s physiological and biochemical responses to drought stress remain unexplored. This study aims to fill this gap and optimize nutrient management for enhanced drought tolerance. A split-plot design with three irrigation regimes (optimal, mild, and severe stress) as main plots and 6 combinations of two iron (F0 and F5 are 0 and 5 kg/ha, respectively) and three phosphorus (P0, P75, and P150 are 0, 75, and 150 kg/ha, respectively) fertilizers as subplots. Medium (P75) and high (P150) phosphate doses, particularly with iron (Fe5), enhanced antioxidant enzymes, catalase and superoxide dismutase, by up to 72%, photosynthetic efficiency (chlorophyll content, F_v/F_m, photosynthetic rate) by up to 107%, and osmolyte accumulation (proline and soluble sugars) by up to 73%. These treatments improved water relations, increasing relative water content by up to 56%, reducing vapor pressure deficit by up to 57%, and making osmotic potential (ψs) up to 57% less negative. However, oxidative stress (malondialdehyde) increased by up to 200%. Despite this, the improved physiological and biochemical functions enhanced white sugar yield by up to 59% and root yield by up to 56%, highlighting the effectiveness of these treatments in increasing sugar beet productivity under drought stress. In conclusion, medium and high phosphate doses, especially when combined with iron, improved drought tolerance and yield in sugar beet under stress by enhancing antioxidant activities, photosynthetic efficiency, osmolyte accumulation, and water relations while reducing oxidative stress and water loss.