Response of sugar maple (Acer saccharum) seedlings to manganese

Long-term research in north-central Pennsylvania has revealed high levels (> 2000 ppm) of foliar Mn in seedlings and overstory sugar maples (Acer saccharum Marsh.). To determine thresholds for sensitivity of sugar maple to Mn, 4-week-old seedlings growing in sand were irrigated for 8 weeks with nutrient solution (pH 3.8) containing 0.1 (control), 5, 10, 20, 40, or 80 mg Mn/L. Seedling mortality occurred at 40 (92%) and 80 (100%) mg Mn/L. At lower Mn, seedling dry mass decreased with increasing Mn concentrations; root growth was inhibited more than shoot growth. Foliar concentrations of all mineral nutrients except P were significantly reduced by Mn. Symptoms of Mn toxicity included chlorosis and necrosis in leaves, darkened root tips, and loosening of outer cortical cells in roots. Cellular symptoms observed in Mn-treated seedlings (5 mg/L and higher) included irregularities in cell shape, increased vacuolation, and swollen mitochondria in root meristems; leaves showed discrete electron-dense areas in chloroplast thylakoid membranes, increased starch in mesophyll cells, and collapse of phloem in midveins. Investigation of potential Mn toxicity will require simulated field conditions to fully evaluate interactions with other factors.