Lead tolerance mechanism in sterilized seedlings of Potamogeton crispus L.: subcellular distribution, polyamines and proline.

The effects of increasing concentrations of lead (Pb) on malondialdehyde (MDA) content, soluble protein, Pb accumulation, nutrients, polyamines (PAs) and proline metabolism were investigated in sterilized seedlings of Potamogeton crispus L. after 5d exposure. Significant oxidative stress was not caused, indicated by a little induction of MDA content and soluble proteins. Pb accumulation increased in a concentration-dependent manner and most of Pb was stored in the cell wall. Total P, Mg, Na and Zn rose and total Fe fell; total Ca increased at 25 μM Pb but then declined. The nutrients in cell wall fraction changed in the same pattern as total nutrients, whereas those in soluble and organelle fraction declined. Total putrescine (Put) decreased markedly, while total spermidine (Spd), spermine (Spm) and (Spd+Spm)/Put ratio increased progressively but then declined. The trends for free, perchloric acid soluble conjugated (PS-conjugated) and perchloric acid insoluble bound (PIS-bound) PAs were similar to those on total PAs, except that PIS-bound Spm increased significantly. Arginine decarboxylase (ADC) and ornithine decarboxylase (ODC) activities rose gradually, while diamine oxidase (DAO) initially increased but then declined. Proline content increased initially only to decline later, due to the increase of r-glutamyl kinase (GK) activity. Meanwhile, ornithine-d-aminotransferase (OAT) activity gradually reduced, while no significant change was observed in proline dehydrogenase (PDH) activity. Our results indicated that the tolerance of P. crispus to Pb stress was based on cell wall compartmentalization combined with increase of nutrients, alterations of PAs, and induction of proline.