Manganese toxicity in two varieties of Douglas fir (Pseudotsuga menziesii var. viridis and glauca) seedlings as affected by phosphorus supply.

Manganese (Mn) is an essential micronutrient in all organisms but may become toxic when present in excess. To investigate whether interior and coastal varieties of Douglas fir [Pseudotsuga menziesii (Mirbel) Franco, var. glauca (DFG) and var. viridis (DFV)] differed in Mn tolerance, seedlings were exposed to excess Mn in hydroponic solutions. Root growth, biomass production, Mn concentrations in different tissues and Mn subcellular localisation were determined. Both varieties showed similar whole-plant Mn accumulation and biomass reduction in response to increases in Mn in the nutrient solution. Since excess Mn inhibited root elongation growth more strongly in DFG than in DFV, biomass allocation in DFV was shifted towards a relative increase in root biomass and a relative decrease in DFG. X-ray microanalysis showed that Mn enrichment in cell walls and vacuoles of DFV root cortex and epidermis cells was higher than in DFG. In roots, precipitates were observed in which Mn concentrations correlated with phosphorus (P) and to a minor extent with calcium. We suggest that the higher persistence of root growth under Mn stress in DFV was caused by greater entrapment in the vacuole and in the apoplast and by more efficient Mn detoxification in insoluble complexes with P than in DFG. To investigate whether P supply affected Mn uptake and toxicity, Douglas fir seedlings were exposed to excess Mn under P deficiency. In P-limited seedlings of both varieties, roots growth was less sensitive to excess Mn than under sufficient P-supply, although in planta Mn concentrations were not diminished. In DFG, which maintained P homeostasis under limited P supply, the negative effect of Mn stress was partially reversed, showing that Mn susceptibility is affected by P metabolism.