Phytochelatins are only partially correlated with Cd-stress in two species of Brassica

Synthesis of phytochelatins (PCs) is triggered in response to cadmium (Cd) and Cd-PC complexes are non-toxic. However, the relationship between PCs and Cd-tolerance is not clear. Brassica napus and Brassica juncea were compared to determine if the level of Cd-induced stress (as measured by accumulation of Cd, biomass, chlorophyll content and photosystem II efficiency) is related to the accumulation of PCs. While both species contained similar concentrations of Cd, B. juncea was more Cd-tolerant over the lifetime of the plant. After short-term exposure (4–7 days) to Cd, concentrations of chlorophyll declined in both species but maximum photosystem II efficiency was unaffected. Both species produced PCs in response to the lowest Cd treatment. With 50–200 μM Cd in solution, roots of B. juncea produced about four times more PCs than did B. napus but PCs in the leaves of B. juncea were about half that of B. napus. Concentrations of PCs did not increase with concentrations of Cd in the tissues and the only stress correlated with PC accumulation was reduced chlorophyll content. Differential tolerance cannot be explained in terms of differential Cd-induced stress, differential Cd or concentrations of PCs in the leaves; some other mechanism(s) offers B. juncea increased protection from Cd-toxicity.