Only the adults survive – A long-term resistance of Isoëtes lacustris to acidity and aluminium toxicity stress in a Bohemian Forest lake

Abstract The aquatic quillwort Isoetes lacustris L. survived five decades of severe acidification in Cerne Lake (Bohemian Forest, central Europe), but failed to reproduce. To detect which ontogenetic stage is more affected by water acidification and to identify water chemistry thresholds for successful recruitment, experiments were carried out on quillwort sporelings grown in the laboratory, using different pH (4–8) and aluminium (Al) concentrations (0–1000 μg/L). Growth-related symptoms and Al accumulations in tissues were compared with those observed in sporelings collected in the lake. In the laboratory, the threshold Al concentrations ≥100 μg/L and pH ≤ 5.0 reduced absorptive organs (macrogametophyte rhizoids, roots, and root hairs), so that the ratio of below-ground to above-ground sporeling biomass decreased to