Effects of magnetic treatment on the ascorbate-glutathione cycle and endogenous hormone levels in Populus x euramericana 'Neva' under cadmium stress

Cadmium (Cd) contamination in soil has become a serious worldwide environmental and health problem. Cd is easily taken up by plants and translocated to aboveground tissues. A pot experiment was carried out to explore the role of the ascorbate-glutathione (AsA-GSH) cycle and endogenous hormones in enhancing Cd tolerance and promoting translocation of Cd in one-year-old seedlings of Populus x euramericana 'Neva'. The antioxidant substances ascorbic acid (AsA), dehydroascorbic acid (DHA), glutathione (GSH), and oxidized glutathione (GSSG); the activities of the antioxidant enzymes ascorbic acid peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), and dehydroascorbic acid reductase (DHAR); the levels of the endogenous hormones indole-3-acetic acid (IAA), abscisic acid (ABA), gibberellin (GA(3)), and zeatin riboside (ZR); and the contents of malondialdehyde (MDA) and hydrogen peroxide (H2O2) were investigated after 30 days of irrigation with half-Hoagland solution containing 0 or 100 mu mol.L-1 Cd(NO3)(2) under magnetic treatment (MT) or nonmagnetic treatment (NMT). The results were as follows. (i) Compared with NMT, MT increased the AsA levels in roots compared with those in leaves under Cd stress, whereas it increased the DHA levels in the leaves but decreased the DHA levels in the roots. The GSH and GSSG levels both increased by 8%-151% under MT. (ii) MT inhibited the APX activity in blades and roots, and a similar effect was observed on the foliar activities of GR and MDHAR, with a decrease of 8%-50%; however, MT increased the activation of DHAR in the blades and GR in the roots. In addition, compared with NMT, MT increased the activities of GR, MDHAR, and DHAR by 19%-285% in Populus (poplar). (iii) With the exogenous addition of Cd, the Cd accumulation and biological transport coefficient of Cd from roots to leaves (S/R) were enhanced in poplar, accompanied by increased levels of H2O2 and MDA due to MT. (iv) The levels of IAA, ABA, GA, and ZR were inhibited by 19%-95% in the leaves following MT. In contrast, the levels of these endogenous hormones were increased by 18%-203% in the roots following MT. (v) MT improved the seedling growth of poplar, with an increase of 0.4%-90%, compared with that of the NMT. The ground diameter and number of root tips showed the greatest increases, with average ratios of 29% and 87%, respectively. These results suggested a large increase in AsA and decreases in poplar antioxidant enzymes, especially in the leaves, with a high GSH level. In this review, we concluded that the antioxidant substance GSH plays an important role in the AsA-GSH cycle following exposure to a magnetic field under Cd stress. Additionally, the roots play a major role in eliminating oxygen free radicals by regulating the levels and ratios of various endogenous hormones. Moreover, magnetization could alleviate Cd-induced oxidative stress by stimulating MDHAR, DHAR, and GR activities; enhance the defense capability of the AsA-GSH cycle; and maintain normal physiological metabolism in poplar.