| A series of hydroponics experiments were performed to investigate effects of sulfur on ascorbate（AsA）-glutathione（GSH） cycle and the content of phytochelatins（PCs） in pakchoi（Brassica chinensis L.） leaves under cadmium（Cd） stress. The results showed that application of moderate level of sulfur decreased the contents of peroxide（H2O2） and malondialdehyde（MDA）, but increased key enzyme activities and antioxidant content of AsA-GSH cycle in pakchoi leaves. Under low Cd stress（5 μmol·L-1）, the content of H2O2 were significantly decreased and the content of MDA fell first then rose with increasing concentration of sulfur, while PCs content and the activity of glutathione reductase（GR） were raised. Along with the increase of the sulfur concentration, other key enzymes activities, such as ascorbate peroxidase（APX）, dehydroascorbate reductase（DHAR） and monodehydroascorbate reductase（MDHAR） in AsA-GSH cycle and the content of AsA increased at the beginning, reaching to the peak in the 3 mmol·L-1 sulfur level. Under high Cd stress（50 μmol·L-1）, the contents of H2O2 and MDA decreased first then increased, while the key enzymes activities and antioxidant content of AsA-GSH cycle showed an opposite tendency, and many of them reached to the peak in the 2 mmol·L-1 sulfur level. The results indicated that application of moderate level of sulfur could maintain the efficiency metabolism of AsA-GSH cycle during the stress period, increase the synthesis of PCs and keep the regenerative capacity of antioxidant substances and heavy metal chelating ability. Although sulfur plays an important role in reducing Cd toxicity, overhigh concentration of SO2-4 could cause toxicity of salt stress and thus affect its Cd toxicity relief. |
