Exogenous putrescine enhances salt tolerance in Populus nigra × maximowiczii: growth, physiological, and biochemical responses

Introduction: Putrescine, a polyamine involved in plant growth and stress responses, has shown potential in mitigating abiotic stress effects. However, little is known about the effects of exogenous addition of putrescine regarding salt tolerance in trees. Methods: , clone NM6) under a short duration of salt stress. Salt stress was induced by irrigating roots with 100 mM and 200 mM NaCl, followed by foliar spraying of putrescine on several days. Measurement of growth including plant height and stem diameter for each plant were recorded in the greenhouse every 15 days throughout the experiment. Gas exchange, total chlorophyll, carotenoids, soluble sugars and proteins, amino acids, polyamines, and relative water content were analyzed in foliage collected 3, 6, 7, 13, 20, 35 days after treatment. Results: Foliar putrescine application significantly promoted growth, increasing stem height by ~20% and stem diameter by ~15% under 100 mM NaCl compared to untreated plants. Foliar spray significantly enhanced fructose accumulation, with ~37% higher levels at day 6 under 100 mM NaCl compared to unsprayed plants and increased sucrose by ~28% at day 13. Based on metabolic responses, plants treated with 100 mM NaCl fared better when sprayed with putrescine than those treated with 200 mM NaCl. Discussion: Exogenous application of putrescine alleviated salt-induced growth inhibition, likely through its role in maintaining osmotic balance and energy metabolism. These findings highlight the potential of exogenous putrescine treatment as a practical strategy to enhance salt tolerance in young poplar trees, with implications for forestry and land reclamation in saline environments.