18O enrichment in phosphorus pools extracted from soybean leaves

The objective of this study was to investigate the isotopic composition of oxygen bound to phosphate (delta 18O-PO4) in different phosphorus (P) pools in plant leaves. As a model plant we used soybean (Glycine max cv Toliman) grown in the presence of ample P in hydroponic cultures. The leaf blades were extracted with 0.3 M trichloroacetic acid (TCA) and with 10 M nitric acid. These extractions allowed measurement of the TCA-soluble reactive P (TCA P) that is rapidly cycled within the cell and the total leaf P. The difference between total leaf P and TCA P yielded the structural P which includes organic P compounds not extractable by TCA. P uptake and its translocation and transformation within the soybean plants lead to an 18O enrichment of TCA P (delta 18O-PO4 between 16.9 and 27.5 parts per thousand) and structural P (delta 18O-PO4 between 42.6 and 68.0 parts per thousand) compared with 12.4 parts per thousand in the phosphate in the nutrient solution. delta 18O values of phosphate extracted from soybean leaves grown under optimal conditions are greater than the delta 18O-PO4 values of the provided P source. Furthermore, the delta 18O-PO4 of TCA P seems to be controlled by the delta 18O of leaf water and the activity of inorganic pyrophosphatase or other pyrophosphatases.