Acid phosphatase activity (E.C.3.1.3.4.1) measured in leaf extracts of field-grown wheat was used to indicate the phosphorus status of the plants. Plants responded to phosphorus applications at both experimental sites, and were sampled on a number of occasions during crop development. Significant (P < 0.001) negative correlations between phosphatase activity and phosphorus status, either phosphorus content or phosphorus concentration, were obtained during the early stages of development (r > - 0.87, R2 > 79 %, and r > - 0.87, R2 > 86 % respectively). The activity at those times was also correlated significantly with final grain yield (r > 0.79, R2 > 73%, P < 0.001). The relations were less useful in the later harvests. The evidence indicates reasons for this, and suggests that even at those harvests leaf phosphatase activity continued to be a very sensitive measure of plant phosphorus status. This sensitivity should make it a useful research tool, but as a field measure, renders the assay subject to interactions with the biological environment that affect growth and yield. Under these circumstances, no single 'critical value' for activity relative to plant performance could be derived to suit all situations. Nevertheless, there seems to be scope for determining local phosphatase standards for particular crops and regional environments which would make the assay a useful diagnostic tool. The assay is simple, direct and rapid, and has the advantage, even under poor seasonal conditions, that the relation with grain yield is best for young plants, the stage at which fertilizer applications have the greatest benefit on final yield.
ELECTRON HISTOCHEMICAL LOCALIZATION OF ACID PHOSPHATASE ACTIVITY IN THE SMALL INTESTINE OF MOUSE
