Nutrient uptake by intact mycorrhizal Pinus sylvestris seedlings: A diagnostic tool to detect copper toxicity

Abstract

We developed a nondestructive method for detecting early toxic effects of sublethal copper (Cu) concentrations on ectomycorrhizal and non-mycorrhizal (NM) Scots pine (Pinus sylvestris L.) seedlings. The fungal symbionts examined were Paxillus involutus (Fr.) Fr., Suillus luteus (Fr.) S.F. Gray and Thelephora terrestris (Ehrh.) Fr. The accumulation of Cu in needles and fungal development (ergosterol) in roots and substrate were assessed. Inorganic phosphate (P(i)) and ammonium (NH4/+) uptake capacities were determined in a semi-hydroponic cultivation system on intact P-limited plants that were exposed for 3 weeks to 0.32 (control), 8 or 16 μM Cu2+. Short-term effects of a 1-hour exposure to 32 μM Cu2+ on nutrient uptake rates were also determined. None of the Cu2+ treatments affected plant growth or root ergosterol concentrations. The active fungal biomass in substrate invaded by S. luteus was reduced by 50% in the 16 μM Cu2+ treatment compared with the control treatment; however, colonization by S. luteus prevented an increased accumulation of Cu in the needles. In contrast, the 16 μM Cu2+ treatment caused a 2.2-fold increase in needle Cu concentration in NM plants. Ergosterol concentrations in the substrate colonized by P. involutus and T. terrestris were not affected by 16 μM Cu2+. Although P. involutus and T. terrestris were less sensitive to Cu2+ than S. luteus, T. terrestris did not prevent the accumulation of Cu in needles of its host plant in the 16 μM Cu2+ treatment. Mycorrhizal plants consistently had higher P(i) and NH4/+ uptake capacities than NM plants. In the control treatment, specific P(i) uptake rates were almost 10, 4 and 3 times higher in plants associated with P. involutus, S. luteus and T. terrestris, respectively, than in NM plants, and specific NH4/+ uptake rates were about 2, 2 and 5 times higher, respectively, than those of NM seedlings. Compared with the corresponding control plants, a 3-week exposure to 8 μM Cu2+ had no effect on the nutrient uptake potential of plants. In contrast, the 16 μM Cu2+ treatment significantly reduced P(i) uptake capacity of all plants and decreased NH4/+ uptake capacity of seedlings colonized by S. luteus or T. terrestris. The 32 μM Cu2+ 1-h shock treatment reduced specific NH4/+ and P(i) uptake rates of roots colonized by S. luteus to 39 and 77%, respectively, of the original rates. The Cu2+ 1-h shock treatment reduced the NH4/+ uptake rate of NM plants by 51%.