Mechanisms for tolerance of very high tissue phosphorus concentrations in Ptilotus polystachyus

Study of plants with unusual phosphorus (P) physiology may assist development of more P-efficient crops. Ptilotus polystachyus grows well at high P supply, when shoot P concentrations ([P]) may exceed 40mg Pg(-1) dry matter (DM). We explored the P physiology of P.polystachyus seedlings grown in nutrient solution with 0-5mM P. In addition, young leaves and roots of soil-grown plants were used for cryo-scanning electron microscopy and X-ray microanalysis. No P-toxicity symptoms were observed, even at 5mM P in solution. Shoot DM was similar at 0.1 and 1.0mM P in solution, but was approximate to 14% lower at 2 and 5mM P. At 1mM P, [P] was 36, 18, 14 and 11mg Pg(-1) DM in mature leaves, young leaves, stems and roots, respectively. Leaf potassium, calcium and magnesium concentrations increased with increasing P supply. Leaf epidermal and palisade mesophyll cells had similar [P]. The root epidermis and most cortical cells had senesced, even in young roots. We conclude that preferential accumulation of P in mature leaves, accumulation of balancing cations and uniform distribution of P across leaf cell types allow P.polystachyus to tolerate very high leaf [P].