Chemical nature of P efflux from P‐adequate Spirodela and Lemna plants

The chemical nature of P released (efflux) from healthy (P‐adequate) Spirodela oligorrhiza (Kurz) Hegelm, and Lemna major L. plants growing in sterile solution culture was investigated using thin‐layer chromatography and autoradiography. Ester patterns of P efflux material from [ 32 P i ] labelled plants was compared with P ester patterns in the tissue of the same plants. Nearly all of the released P was P i (99.8% in Spirodela , 99.6% in Lemna ) there being little consistent evidence for the release of significant amounts of organic P (0.2% and 0.4% of released P – Spirodela and Lemna respectively). Efflux of P predominantly as organic P (hexose or nucleotide P) would have resulted in an important loss of energy for the plant. The corresponding proportion of organic P in the tissue of the labelled plants from which the P efflux material arose was 15% ( Spirodela ) and 8% ( Lemna ). Consequently there was no evidence for the preferential release of phosphorylcholine, a compound implicated in P efflux because of its high membrane permeability. Autoradiographic resolution of the chromatogram was improved by labelling plants at high 32 P specific activity [185 GBq (mol P i ) −1 ] compared with levels considered safe [37 GBq (mol P i ) ‐1 ] without any detrimental effects. Spirodela and Lemna plants appear to exert a close control over both the quantity (rates of P efflux) and quality (proportion of P i to organic P) of their P efflux. The possible role of externally located phosphatases in minimising the release of organic P is also discussed.