Figure 4.
Thiol content in the phloem sap of Brassica napus. The contents of (a) cysteine (Cys), (b) γ-glutamylcysteine (γ-EC), (c) glutathione (GSH), (d) phytochelatins (PCs), (e) GSH-like peptides, and (f) PC-like peptides were measured in the phloem sap of non-treated and Cd exposed plants (see Results).
(a, c) Cysteine and GSH levels remained constant independently of the length of Cd treatment.
(b) The level of γ-EC in phloem sap samples increased dramatically upon Cd exposure. The GSH-like (sum of γ-glutamyl-cysteinyl-serine and γ-glutamyl-cysteinyl-glutamine, γ-ECQ) and PC-like (sum of hydroxymethyl-PC2 [(γ-EC)2-S] and glutamine-PC2 [(γ-EC)2-Q]) peptides were also induced only after Cd exposure (d–f). Levels of PCs and their homologs are expressed in thiols mg−1 of protein. The phloem sap contained on average 0.174 ± 0.09 mg protein ml−1 (mean ± SE, n = 4); this value was used to express GSH in concentration (196 µm after 2 weeks of Cd exposure), allowing direct comparison with the xylem sap measurements.
The asterisk in the content of PC-like peptides after 2 weeks of Cd exposure denotes a significant difference compared with the levels of PC-like peptides after 1 week of treatment. Values shown are n = 3–4 experiments; two or three samples were measured per experiment.