Phosphorus homeostasis in Populus alba L. under excess phosphate conditions, assessed by 31P nuclear magnetic resonance spectroscopy and X-ray microfluorescence.

The phosphates (Pi) are nowadays recognized as pollutants. We studied the effect of Pi (0.625-12.500 mM KH2PO4) in the culture medium on in vitro grown 2-month-old Populus alba trees. The levels of sugar phosphates and vacuolar and cytoplasmic Pi in cell compartments of roots and stems were determined using 31P NMR, while tissue-specific micro- and macroelements mapping on stem cross-sections were performed using synchrotron-based X-ray microfluorescence. Plants grown on 0.625 mM Pi (MS/2 medium) showed a survival rate of 70%. With the increase in Pi concentrations up to 6.250 mM, plant growth and survival increased, without changes in total P content per mass or in the levels of cytoplasmic and vacuolar phosphates, in both stems and roots, while the levels of Fe, Cu, Zn, Ca and Mn in stems increased. Further increase in Pi to 9.375 and 12.500 mM in the medium resulted in inhibited growth comparable with plants grown on MS/2, with the increase in total P content per mass up to 50%, in both stems and roots, but with no changes in cytoplasmic and vacuolar phosphates; 12.500 mM Pi affected even plant survival (70%) and thus might be considered as mildly toxic. 31P NMR results indicate that the high tolerance of P. alba to increased Pi could result from its ability to maintain an intracellular P homeostasis, despite P accumulation up to 50%, in both stems and roots, indicating P. alba as a promising wood species for dendroremediation.