Enhancing Citrus aurantium Tolerance to Waterlogging: Effects of Various Elicitors

Climate change-induced waterlogging negatively impacts citrus crops. This study examined the effects of waterlogging stress and elicitor treatments on Citrus aurantium seedlings' leaf traits. Elicitors, including γ-aminobutyric acid (GABA) (5 and 10 mM), melatonin (50 and 100 µM), and potassium phosphite (KPhi) (2 and 3 g/l), were applied to seedlings under field capacity (FC) and waterlogging stress (WS) conditions for four weeks. The results showed that WS significantly reduced relative water content, but 10 mM GABA preserved the highest levels under FC conditions. WS decreased lignin content by 20%, while 10 mM GABA boosted lignification under stress. Total phenol content peaked with 2 g/l KPhi and control treatments under WS, inversely correlating with lignin content. Phenylalanine ammonia-lyase activity dropped 19% in waterlogged controls but was highest with 10 mM GABA under stress. Cell wall-bound peroxidase activity declined under WS, yet 100 µM melatonin showed maximum activity under FC. Superoxide dismutase activity also decreased with WS, while 100 µM melatonin maintained the highest activity. Catalase activity remained stable but increased with 10 mM GABA. Hydrogen peroxide and malondialdehyde levels were highest in waterlogged controls, but 10 mM GABA significantly reduced them under both conditions. Overall, 10 mM GABA and 2–3 g/l KPhi enhanced the resilience of C. aurantium seedlings under WS.