Spermidine-induced improvements in water relations and antioxidant defense enhance drought tolerance in yarrow (Achillea millefolium L.)

L.). Plants were subjected to three drought levels (25 %, 50 % and 75 % field capacity) and foliar sprayed with 0, 1.5 and 3 μM spermidine. Drought significantly reduced relative water content, photosynthetic pigments (chlorophyll, carotenoids), osmolyte (proline, soluble sugars) accumulation and antioxidant enzyme activities such as catalase (CAT), peroxidase (POD) and ascorbate peroxidase (APX), indicating oxidative damage. Spermidine treatment attenuated drought injury by improving the above parameters. Maximum responses were observed at 1.5 μM for photosynthetic pigments and osmolytes, while 3 μM performed best for secondary metabolites (phenolics, flavonoids, anthocyanins) and antioxidant enzymes. Drought also upregulated secondary metabolites like phenolics, while spermidine further augmented their levels. Moreover, spermidine maintained membrane integrity and osmotic adjustment under water deficit. Overall, spermidine enhanced yarrow's drought tolerance by modulating physiological and biochemical processes. Our findings provide insights into spermidine-induced adaptation mechanisms in plants combating water scarcity. Optimization of spermidine concentration may help develop drought-resilient crops.