The elucidation of a natural strategy for metal hyperaccumulation enables the rational design of technologies for the clean-up of metal-contaminated soils. Organic acid has been suggested to be involved in toxic metallic element tolerance, translocation, and accumulation in plants. The impact of exogenous organic acids on cadmium (Cd) uptake and translocation in the zinc (Zn)/Cd co-hyperaccumulator Sedum alfredii was investigated in the present study. By the addition of organic acids, short-term (2 h) root uptake of ¹⁰⁹Cd increased significantly, and higher ¹⁰⁹Cd contents in roots and shoots were noted 24 h after uptake, when compared to controls. About 85% of the ¹⁰⁹Cd taken up was distributed to the shoots in plants with citric acid (CA) treatments, as compared with 75% within controls. No such effect was observed for tartaric acid (TA). Reduced growth under Cd stress was significantly alleviated by low CA. Long-term application of the two organic acids both resulted in elevated Cd in plants, but the effects varied with exposure time and levels. The results imply that CA may be involved in the processes of Cd uptake, translocation and tolerance in S. alfredii, whereas the impact of TA is mainly on the root uptake of Cd.

Copper accumulation and intracellular distribution in Elsholtzia splendens, a native Chinese Cu-tolerant and accumulating plant species, was investigated by transmission electron microscope (TEM) and gradient centrifugation techniques. Copper concentrations in roots, stems and leaves of E. splendens increased with increasing Cu levels in solution. After exposure to 500 micromol/L Cu for 8 d, about 1000 mg/kg Cu were accumulated in the stem and 250 mg/kg Cu in the leaf of E. splendens. At 50 micromol/L Cu, no significant toxicity was observed in the chloroplast and mitochondrion within its leaf cells, but separation appeared at the cytoplasm and the cell wall within the root cells. At >250 micromol/L Cu, both root and leaf cell organelles in E. splendens were damaged heavily by excessive Cu in vivo. Copper subcellular localization in the plant leaf after 8 days' exposure to 500 micromol/L Cu using gradient centrifugation techniques was found to be decreased in the order: chloroplast>cell wall>soluble fraction>other organelles. The plant root cell wall was found to be the site of highest Cu localization. Increase of Cu exposure time from 8 d to 16 d, increased slightly Cu concentration in cell wall fraction in roots and leaves, while that in the chloroplast fraction decreased in leaves of the plants grown in both 0.25 micromol/L and 500 micromol/L Cu. TEM confirmed that much more Cu localized in cell walls of E. splendens roots and leaves, but also more Cu localized in E. splendens' chloroplast when the plant is exposed to Cu levels>250 micromol/L, as compared to those in the plant grown in 0.25 micromol/L Cu. Copper treatment at levels>250 micromol/L caused pronounced damage in the leaf chloroplast and root organelles. Copper localization in cell walls and chloroplasts could mainly account for the high detoxification of Cu in E. splendens.

In the current study, caffeic acid was an important metabolite in the highly copper-tolerant plant Elsholtzia splendens. Preparation and purification of caffeic acid were performed on the dried biomass of the plants by means of sonication/ethanol extraction, followed by purification using a macroporous resin (D101 type) column and silica gel chromatography. The faint-yellow caffeic acid product was yielded with a purity of 98.46%, and it was chemically identified from spectra of Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (¹H NMR)/carbon nuclear magnetic resonance (¹³C NMR), and electrospray ionization mass spectrometry (ESI-MS). Caffeic acid is a possible product from the post-harvest processing of Elsholtzia splendens biomass.