Effect of Watering Regimen on Secondary Metabolites, Physiology, and Yield of Amaranthus cruentus

Abstract

Water availability is one of the most important factors that affects leafy vegetables' growth and physiological stability and accumulation of metabolites. Amaranthus cruentus is an economically and nutritionally valuable vegetable crop that is grown in arid and semi-arid regions. This study evaluated the comparative effect of four watering regimens; T1 (120% crop water requirement (CWR); overwatered/waterlogged), T2 (100% CWR; control/optimal), T3 (70% CWR; moderate stress), and T4 (40% CWR; severe stress); on the secondary metabolites, physiology, and growth parameters of A. cruentus. Optimal irrigation (T2) resulted in the maximum vegetative growth (P < 0.05), with the highest leaf area (870.34 cm2), fresh shoot weight (71.23 g), dry shoot weight (9.32 g), relative water content (93.28%), chlorophyll content (2.91 mg/g FW), carotenoid content (2.91 mg/g FW), and stomatal conductance (392.34 mmol H2O m-2s-1). Secondary metabolite accumulation significantly increased when exposed to moderate drought stress (T3) with the highest accumulation of total phenolics (33.09 mg GAE/g DW), total flavonoids (22.44 mg QE/g DW) and ascorbic acid (95.34 mg/100g FW) and moderate reduction in biomass (P < 0.05). Severe drought stress (T4) caused maximum accumulation of proline (45.33 µmol/g FW) and increased root-to-shoot ratio (0.34), but significantly reduced stomatal conductance (38.43 mmol H2O m-2s-1), leaf area (120.11 cm2), fresh shoot weight (13.12 g) and ascorbic acid content (20.34 mg/100g FW) (P < 0.05). Overwatering/waterlogging (T1) caused a decrease in the total phenolics content (11.52 mg GAE/g DW), total flavonoids content (6.41 mg QE/g DW), chlorophyll/carotenoid content (0.81 mg/g FW) and root to shoot ratio (0.06) (P < 0.05). In total, moderate stress (T3) will be an effective irrigation protocol to produce high levels of bioactive secondary metabolites in A. cruentus in a controlled manner.