Infuluence Of Sulfur On The Contents And Distribution Of Iron, Cadmium And Related Complexes In Rice

Cadmium (Cd) pollution in rice has been a serious environmental issue all around the world. It is very important to eliminate the Cd absorption and transport in rice. Sulfur (S) is an essential macronutrient of rice and plays critical roles in PCs and NA synthesis. In the present study, an effective method for thiol compounds quantification of rice was established. Hydroponic experiments with different S treatments were carried out to regulate the contents of PCs and NA, and to investigate the distributions, quantities and correlations among Fe, Cd and their complexes in rice.TCEP, a thiol-free reducing agent was chosen as a reductant to prevent thiol oxidation in the processes of extraction and derivatization. Fluorochrome mBBr was used to derive thiol compounds in samples and the method of derivatization was optimized. Separation of these thiol compounds by was achieved by using RP-UPLC. The method established in this study was simple, stable, sensitive and efficient for simultaneous determination of Cys, GSH and PC2-4 in rice samples. The results showed that, after reduction with TCEP, the above thiols were rapidly separated with binary gradient elution in 15 min. The calibration curve was linear in a range of 1-20 μg·mL-1. The LOD for thiols was 0.14 pmol for 1μL injection (0.10 μg·g-1 F.W.), and the recovery was 91.55% on average.When S was sufficiently supplied, Cd content of rice shoot and root were reduced 23.7% and 19.5% respectively. In addition, Cys, GSH and PC2-4 levels increased significantly, while opposite effects were observed in the S-deficiency treatment. However the thiol compounds distributed differently in rice:PCs, especially PC3, were the main thiol compounds in rice roots; Cys and GSH dominated the thiol pools in rice stems and leaves. In addition, upon H2O2 pretreatment, the amounts of rice thiols and TBARS and the activity of GST were increased, yet the seedling growth was not inhibited. In conclusion, the results in this study demonstrated the importance of adequate S supply, cellular thiol levels and GST activity in Cd detoxification in rice.Normal (S3) and higher (S4) S treatments significantly increased the Fe concentration in rice roots. The effect of S supply on Fe accumulation can be explained by an increase of nicotianamine content. Actually, we found higher NA content in S-deficiency (Si) and S-lower (S2) conditions in roots. In particular, TBARS content were decreased with increasing S supply in rice plants, and it reached its highest value in rice leaf under S-deficiency condition. Results indicated that sufficient S nutrition may lead to proper uptake of Fe by rice roots and translocation to shoots. It is also interesting to note that the adequate S supplement may increase transport rate of NA from roots to leaf and antioxidant capacity in rice plants.The increasing of S level could alleviate Cd stress and increase biomass of rice seedlings. After inducing iron plaque, Fe uptake decreased in rice roots under Cd stress, but there was no significant change in the Fe content of iron plaque. Moreover, Cd was accumulated in rice roots but not iron plaque, suggesting that root tissue rather than iron plaque on the root surface had important roles in Cd detoxification within rice plants. After analyzing the contents and distribution of thiol compounds, NA, Fe and Cd in rice tissues, it may be speculated that Cd might be complexed with thiols and NA in rice.