Concentrations of metals and potential metal‐binding compounds and speciation of Cd, Zn and Cu in phloem and xylem saps from castor bean plants (Ricinus communis) treated with four levels of cadmium

Abstract

We examined the concentrations of metals (Cd, Zn, Cu, Fe and Mn) and potential metal-binding compounds [nicotianamine (NA), thiol compounds and citrate] in xylem and phloem saps from 4-week-old castor bean plants (Ricinus communis) treated with 0 (control), 0.1, 1.0, and 10 μM Cd for 3 weeks. Treatment with 0.1 and 1 μM Cd produced no visible damage, while 10 μM Cd retarded growth. Cadmium concentrations in both saps were higher than those in the culture solution at 0.1 μM, similar at 1.0 μM and lower at 10 μM. Cd at 10 μM reduced Cu and Fe concentrations in both saps. NA concentrations measured by capillary electrophoresis-mass spectrometry (MS) in xylem sap (20 μM) were higher than the Cu concentrations, and those in phloem sap (150 μM) were higher than those of Zn, Fe and Cu combined. Reduced glutathione concentrations differed in xylem and phloem saps (1-2 and 30-150 μM, respectively), but oxidized glutathione concentrations were similar. Phloem sap phytochelatin 2 concentration increased from 0.8 μM in controls to 8 μM in 10 μM Cd. Free citrate was 2-4 μM in xylem sap and 70-100 μM in phloem sap. Total bound forms of Cd in phloem and xylem saps from 1 μM Cd-treated plants were 54 and 8%, respectively. Treatment of phloem sap with proteinaseK reduced high-molecular compounds while increasing fractions of low-molecular Cd-thiol complexes. Zinc-NA, Fe-NA and Cu-NA were identified in the phloem sap fraction of control plants by electrospray ionization time-of-flight MS, and the xylem sap contained Cu-NA.

Cadmium accumulation is enhanced by ammonium compared to nitrate in two hyperaccumulators, without affecting speciation

“…This indicates that Cd was transported as either the free ion or complexed with carboxyl groups before being chelated by S-containing compounds, which may explain the low Cd-S proportion in stems. Moreover, the importance of free hydrated Cd 2+ ions and/or Cd-O/N complexes for Cd translocation through xylem sap has also been found previously in other Cd-hyperaccumulating and non-hyperaccumulating species (Salt et al , 1995; Ueno et al , 2008; Hazama et al , 2015). Thirdly, young leaves tended to have a higher proportion of Cd bound to S-containing ligands than did old leaves.…”

Plant Nutrient Dynamics in Stressful Environments: Needs Interfere with Burdens

“…Such transport processes and their regulation allow an accumulation of nutrients in harvested vegetative [16] or reproductive plant parts [17,18]. The mobility of an element or of certain forms of an element in the phloem is crucial for redistribution processes within the plant [13,19,20]. Such redistribution processes are crucial for heavy metal homeostasis [21], hyperaccumulation [22], and toxicity [11].…”

Heavy Metals in Crop Plants: Transport and Redistribution Processes on the Whole Plant Level

“…Another possibility is the transfer to the phloem before the xylem sap reaches mesophyll cells [2,68,69]. Several types of metal-binding compounds including nicotianamine and phytochelatins were reported to be relevant for the transport of heavy metals in the phloem [75][76][77]. A transporter for the copper-nicotianamine complex was found to be located in the phloem of rice leaves and was proposed to be important for the translocation of Cu from leaves to developing organs and maturing seeds [78].…”