A glimpse into the symplastic and apoplastic Cd uptake by Massai grass modulated by sulfur nutrition: Plants well-nourished with S as a strategy for phytoextraction

To date, there have been no studies demonstrating the influence of sulfur (S) on the cadmium (Cd) uptake kinetics, which limits the understanding of mechanisms involved in the uptake of this element. Therefore, this study was carried out in order to quantify the contribution of symplastic and apoplastic uptakes of Cd (0.1 and 0.5 mmol L−1) by Massai grass (Panicum maximum cv. Massai) grown under low and adequate S-supply (0.1 and 1.9 mmol L−1) by measuring Cd concentration in the nutrient solution (Vmax, Km, and Cmin) along the plant's exposure time (108 h) and determining Cd concentration in root symplast and apoplast. The Vmax of Cd influx in Massai grass exposed to higher Cd and S concentrations was 38% higher than that plants supplied with lower S concentration. The Km and Cmin of plants exposed to the highest Cd concentration was higher than that plants subjected to the lowest Cd concentration, although values were not affected by S supply. Symplastic influx of Cd in plants subjected to the lower Cd and S concentrations was 20% higher as compared to plants supplied with the higher concentration of S, whereas the apoplastic influx of Cd was higher when there was a higher supply S, regardless of Cd concentration in the solution. This result indicates that an adequate supply of S decreases the contribution of the symplastic Cd uptake and increases the contribution of the apoplastic Cd uptake when the toxicity caused by Cd is lower.