Effects Of Sulfur And Cadmium Interaction On Uptake And Translocation Of Cadmium And Glutathione-related Enzymes In Brassica Chinensis L.

With the rapid development of industry and agriculture and unreasonable emission of industrial wastes in the past few decades, our environment including soil, water and air in China has been severely polluted by cadmium(Cd). Cd can be absorbed by plants and enters the food chain after a certain crop production, posing a serious risk for human health. In recent years, studies have shown that sulfur plays a vital role in the regulation of plant growth and development under Cd stress. To date, the ways in which the enzymes and metabolites involved in GSH metabolism respond to the interactive effects between Cd and sulfur are poorly known. In the present study, hydroponic experiment was conducted to investigated the effects of sulfur and Cd interaction on growth characteristics, uptake and translocation of Cd and key enzymes activity and metabolites contents of GSH metabolism using 2 pakchoi cultivars, Aikangqing(tolerant to Cd stress) and Qibaoqing(sensitive to Cd stress) according to the previous experiment. The pot experiment was performed to investigate the effects of exogenous sulfur(50 mg·kg-1) on the growth, uptake and transportation of Cd, photosynthetic characteristics and osmotic adjustment substances of pakchoi(Brassica chinensis L.)cultivar Qibaoqing under different Cd concentrations(0, 5, 25, 50, 100 mg·kg-1). Our results are going to further clarify the possible mechanisms of the sulfur-enhanced resistance and/or tolerance of plants to Cd stress, and provide valuable guidance for the reasonable utilization of sulfur and pollution-free cultivating vegetable in heavy metal pollution risk area. The main results of the present experiments were listed as follow:1. In hydroponic experiments, Cd contents in shoots and roots were remarkedly increased when plants were exposed to Cd-containing solution. While addition of sulfur in the solution could enhance Cd accumulatin in roots of pakchoi plants, but inhibit Cd translocation from roots to shoots. As compared with Cd stress alone, exogenous sulfur significantly increased the ATPS activity of leaves and roots in Aikang and ATPS activity of roots in Qibaoqing by 10.34%, 27.82% and 28.23%, respectively. Sulfur addition progressively reduced the OASTL activity of leaves in Qibaoqing by 23.81%, but increased the OASTL activity of roots in Aikangqing by 50.33%. Addition of sulfur into Cd-stressed plants significantly decreased the γ-ECS activity of leaves in Aikangqing and Qibaoqing by 20.84% and 18.35%,respectively, but increased the γ-ECS activity of roots by 55.92% and 25.31%, respectively. When compared to Cd stress alone, exogenous sulfur significantly increased the activity of GST and contents of PCs and GSH in both pakchoi cultivars under Cd stress(except GSH contents in roots of Aikangqing and PCs contents in roots of Qibaoqing). The results indicated that sulfur application could restrain Cd translocation from roots to shoots and promote sulfur assimilation and biosynthesis of GSH and PCs, thus maintaining higher antioxidant capacity and heavy metal chelation and detoxification ability of pakchoi plants.2. In hydroponic experiments, 100 μmol·L-1 Cd stress resulted in a significant reduction in fresh weight, dry weight, plant height and length of shoots and roots of both pakchoi cultivars when compared to control. Meanwhile, MDA and O2·- contents in both leaves and roots of the 2 pakchoi cultivars were remarkably increased after exposure to Cd stress. As compared to Cd stress alone, sulfur addition significantly increased the activity of AAO, APX, DHAR, MDHAR and GR, the contents of GSH and As A(ascorbate) and the ratio of GSH/GSSG and As A/DHA in both pakchoi cultivars under Cd stress. The results indicated that sulfur addition could promote the activity of key enzymes of As A-GSH cycle and the contents of GSH and As A, maintaining the efficiency metabolism of As A-GSH cycle under Cd stress. Thus, sulfur could significantly alleviated growth inhibition and oxidative damage caused by Cd toxicity.3. In pot experiments, exogenous sulfur significantly promoted the growth and photosynthesis of pakchoi under 5 and 25 mg·kg-1 Cd stress, with the plant height increased by 11.14% and 21.16%, shoot dry mass increased by 22.55% and 19.90%, root dry mass increased by 27.83% and 20.29%, chlorophyll contents increased by 11.81% and 15.13%, net photosynthetic rate(Pn) increased by 13.15% and 15.22%, respectively. Exogenous sulfur reduced the translocation factor(TF), shoot bioconcentration factor(BCF) of pakchoi, but improved root BCF. Under 25 mg·kg-1 Cd stress, exogenous sulfur significantly reduced shoot Cd contents by 24.82%, increased root Cd contents by 23.94%, and TF reached the minimum. Under 25 mg·kg-1 Cd stress, exogenous sulfur significantly improved the osmotic adjustment substances contents of pakchoi, with the soluble protein, soluble sugar, proline and ascorbate contents increased by 20.82%, 23.34%, 26.79% and 25.72%, respectively. The results indicated that exogenous sulfur(50 mg·kg-1) could effectively alleviate Cd-induced toxicity and protect growth of pakchoi under 25 mg·kg-1 Cd stress through inhibiting Cd translocation from root to shoot, promoting photosynthesis and osmotic adjustment substances contents in pakchoi.