Agmatine and N-hydroxypipecolic Acid Synergistically Enhance Salt Tolerance in Wheat Through Antioxidant Defense, Proline Metabolism, and Photosynthetic Protection

Salt stress severely limits wheat growth by disturbing photosynthesis, osmotic balance, and cellular redox homeostasis. This study evaluated whether agmatine and N-hydroxy pipecolic acid, applied alone or in combination, could improve wheat tolerance under 200 mM NaCl stress. Wheat plants were grown under greenhouse conditions and treated with agmatine at 100 µM, N-hydroxypipecolic acid at 0.01 µM, or their combined application. Salt stress reduced growth, leaf area, leaf relative water content, chlorophyll content, gas exchange, Rubisco activity, and biomass accumulation while increasing hydrogen peroxide, lipid peroxidation, and electrolyte leakage. Both compounds improved wheat performance under salinity, but their combined application produced the strongest response across the measured growth, photosynthetic, osmotic, and antioxidant traits. The combined treatment improved root and shoot growth, maintained higher chlorophyll content and Rubisco activity, supported gas exchange, and increased soluble sugar accumulation. It also reduced oxidative injury by strengthening catalase, peroxidase, superoxide dismutase, and glutathione reductase activities. In addition, the combined treatment increased proline accumulation and activated key enzymes involved in proline metabolism, indicating improved osmotic adjustment under salt stress. These findings suggest that agmatine and N-hydroxy pipecolic acid act through complementary physiological and biochemical pathways to improve wheat salt tolerance. The combined treatment may offer a useful foliar strategy for improving wheat performance under saline conditions, although field validation and dose optimization are needed before practical recommendation.