Response of miscanthus to toxic cadmium applications during the period of maximum growth

Plants of miscanthus were grown in a Cd-free solution up to 1 month before heading and then were exposed to 0, 0.75, 1.5, 2.25 and 3 mg l(-1) cadmium for 36 days. All cadmium levels were toxic to miscanthus. Growth response was not dose-dependent and two toxicity thresholds were identified: one between 0 and 0.75 mg l(-1) Cd, the other between 2.25 and 3 mg l(-1) Cd. The former caused a biomass decrease by about 50%, whereas the latter completely inhibited growth and disrupted the mechanisms that restricted Cd translocation to the shoot. Growth of the aerial part was affected by cadmium more than that of the hypogeal one. Cadmium did not change the N concentration of different plant parts, but markedly reduced the N uptake of the plant, the N net uptake rate (NUR) and the N net translocation rate (NTR) from the rhizome to the aerial part. These two indexes equalled zero when plants ceased to grow. Otherwise, the Cd-NUR increased with Cd supply and the Cd-NTR from rhizome to aerial part showed the highest increment when plants did not grow at all. This suggests different uptake pathways for the two elements, active for nitrogen and passive for cadmium. The Cd concentration and the Cd content markedly increased with all Cd levels, following the order roots >> rhizome > culms > leaves. The Cd concentration and the Cd content of aerial organs increased with Cd supply, but increments were highest between 2.25 and 3 mg l(-1) Cd. The highest Cd concentrations were recorded in plants grown with 3 mg l(-1) Cd and were 41 and 122 mg kg(-1), respectively, for the aerial and the hypogeal plant parts. The hypogeal plant part retained most of the cadmium taken up from solution, accounting for approximately 87% of total plant cadmium with the three lower Cd levels, and for 73% with the highest one. The maximum Cd content of the entire plant was achieved with the two higher Cd levels and was approximately 4.7 mg, while the Cd content of the aerial part was highest with 3 mg l(-1) Cd (1.2 mg Cd per plant) and that of the hypogeal one with 2.25 mg l(-1) Cd (4 mg Cd per plant). The highest aerial content achieved in this experiment was 10-fold that obtained in a previous research when small-sized plants were exposed to the same Cd level.