Exogenous spermidine enhanced the water deficit tolerance of Anoectochilus roxburghii by modulating plant antioxidant enzymes and polyamine metabolism

Water stress is an important factor affecting herb quality and yield and exogenous spermidine (Spd) addition can enhance dehydration tolerance of plants. Anoectochilus roxburghii (Wall.) Lindl is a perennial herb widely used in medicine, health care, and beauty industry, yet, its dehydration resistance remains largely unknown. In this view, the current research aimed at elucidating the drought resistance mechanism of A. roxburghii under PEG-induced water stress at different levels (10%, 20%, and 30% PEG6000). In particular, the morphological, physiological, and biochemical changes occurring in leaves treated with (0.5 mM, 5 mM) or without Spd were discussed under moderate water stress (20% PEG6000). Results showed that water stress significantly inhibited the growth of A. roxburghii by decreasing the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), arginine decarboxylase (ADC), S-adenosylmethionine decarboxylase (SAMDC), and polyamine oxidase (PAO). The contents of soluble protein, soluble proline, hydrogen peroxide (H2O2), malondialdehyde (MDA), and the relative conductivity (REC) of leaves increased by 27.32%, 31.37%, 68.84%, 103.61%, and 63.64%, respectively. Under dehydration condition, foliar spraying Spd effectively improved plant growth with the contents of soluble protein and soluble proline increased, while the H2O2, REC, and MDA contents decreased. The SOD activity of 0.5 mM and 5 mM treatments were 1.13 and 1.01 times higher than only 20% PEG treatment, respectively. Similarly, the POD activity of 0.5 mM and 5 mM treatments increased by 52.82% and 34.11%, while the CAT activity of 0.5 mM and 5 mM treatments increased by 34.39 and 33.15 U/mgprot, respectively. In addition, exogenous Spd application significantly increased the key enzyme activities of polyamine metabolism (ADC, SAMDC, and PAO) under moderate water stress. In conclusion, exogenous Spd addition with low concentration (0.5 mM) improved the resistance of A. roxburghii against water stress by increasing the antioxidant enzymes activities and endogenous polyamines contents. This study helps to the understanding of dehydration resistance of A. roxburghii and provide guidelines for its cultivation under water deficit condition.