Responses of tobacco (Nicotiana tabacum) suspension cells to Cd and Zn were studied in the presence and absence of ligand of Cd-peptide in order to understand the role of this peptide versus other mechanisms in Cd and Zn accumulation and accommodation in plants. With 45 micromolar Cd and 300 micromolar Zn (non-growth-inhibiting levels), metals appeared rapidly within cells, and intracellular Cd and Zn reached medium concentrations after 6 to 10 hours. Cd-peptide was observed in response to Cd after 2 hours, but this form only accounted for -30% of soluble Cd after 24 hours. Peptide was not observed in cells exposed to 300 micromolar Zn for up to 7 days. Organic acid-to-metal stoichiometry indicated that endogenous organic acid content of cells was more than sufficient to complex absorbed metals and no evidence was found for stimulation of organic acid biosynthesis by Cd or Zn. Metal-complexing potential of organic acids for Cd and Zn versus endogenous cations is discussed as is vacuolarextravacuolar distribution of metals. The absence of Cd-peptide does not limit Cd-accumulation in the system studied. Results suggest that tobacco suspension cells accommodate the presence of non-growth-inhibiting and growth-inhibiting levels of Cd and Zn by sequestration in the vacuole as complexes with endogenous organic acids and that this may be a principal means for accommodation of Cd as well as Zn in the presence and absence of Cd-peptide.Recent interest in unique (-y-Glu-Cys), Gly Cd-binding peptides, also called cadystins, phytochelatins, etc., centers on their structure, biosynthesis, and possible role in tolerance of plants and plant cultured cells to challenge with high levels of this metal (4,6,9,16,20,21,24,31 Prior to the demonstration that plants exposed to high levels of Cd form Cd-peptide, models to explain the mechanisms of heavy metal tolerance in plants focused on Zn and Cu and possible roles of the cell wall and vacuolar organic acids in metal sequestration (32). Ernst was first to postulate vacuolar accumulation of Zn organic acid complexes as a mechanism for Zn tolerance in naturally tolerant ecotypes (32). Considerable, but not entirely consistent, evidence exists for a correlation between Zn content and organic acid content in various plants and cultured cells (2,8,13,15,25,26,32 Other extraction media and methods were used as noted. A previously described procedure was used to determine Cdpeptide content (18). Homogenates were centrifuged (4°C) at 16,000g for 5 min and the resulting supernatants were centrifuged at 100,000g, 30 min. Pellets were combined and washed with growth media less metal; insoluble fractions were dried and digested with 9:1 (v/v) HNO3:HClO4 and the digest was evaporated and analyzed for Cd and Zn content in 1 % HCI using flame atomic absorption spectroscopy (with background correction for Zn). For determination of K+, Mg2+, Ca2+, NO3-, C1-, S042-, PO34, and H+, water washed cells were homogenized in freshly boiled, distilled, deionized H20. The K+, Mg2+, and Ca2+ contents ...