Boron Accumulation and Toxicity in Hybrid Poplar (Populus nigra x euramericana)

Poplars accumulate high B concentrations and are thus used for the phytomanagement of B contaminated soils. Here, we performed pot experiments in which Populus nigra x euramericana were grown on a substrate with B concentrations ranging from 13 to 280 mg kg(-1) as H3BO3. Salix viminalis, Brassica juncea, and Lupinus albus were grown under some growing conditions for comparison. Poplar growth was unaffected at soil B treatment levels up to 93 mg kg(-1). Growth was progressively reduced at levels of 168 and 280 mg kg(-1), None of the other species survived at these substrate B levels. At leaf B concentrations < 900 mg kg(-1) only < 10% of the poplar leaf area showed signs of toxicity. Neutron radiography revealed that chlorotic leaf tissues had B concentrations of 1000-2000 mg kg while necrotic tissues had > 2000 mg kg(-1). Average B concentrations of up to 3500 mg kg(-1) were found in leaves, while spots within leaves had concentrations > 7000 mg kg(-1), showing that B accumulation in leaf tissue continued even after the onset of necrosis. The B accumulation ability of P. nigra x euramericana is associated with B hypertolerance in the living tissue and storage of B in dead leaf tissue.