Phosphorus transport and phosphorus nutrition in Lemna (Lemna major L.) and Spirodela (Spirodela oligorrhiza (Kurz.) Hegelm.)

Abstract

This study investigated Pi uptake and P efflux in sterile cultures of Spirodela (Spirodela oligorrhiza (Kurz.) Hegelm.) and Lemna (Lemna major L.) plants during the early stges of P-deficiency. Within 12 hours of transfer to a P-deficient medium Pi uptake rates by P-deficient (-P) plants were enhanced 30 to 120% compared with P-adequate (+P) controls when Pi was resupplied at 1 to 1000 M. The enhancement in Pi uptake rates with P-deficiency normally preceded and was more pronounced than other effects of P-deficiency such as reduced growth, reduced internal P concentration and appearance of visual symptoms. The enhanced Pi uptake rates in -P compared with +P plants resupplied with Pi was more closelv related with a fall in the con- centration of internal Pi than with a fall in the concentration of three other r fractions (i.e. ester-P, lioid-P and residual-P). The role of tissue Pi concentration in Spirodela and Lerrma plants as a possiblle determinant of Pi uptake rates is discussed. Kinetic analysis showed Pi uptake by Spirodela and Lerrma plants from 1 to 1000 uM external Pi was best described by two first-plants from 1 to 1000 J.JM external Pi was best described by two first- order systems operating simultaneously and having differing affinities for the phosphate ion. The high-affinity (HA) system was responsible for most of the Pi uptake (77-96%) at low external Pi (1 µH), whereas the contribution co uptake b_- t·ne low-affinity system (LA) increased with increasing external Pi concentration . The enhanced Pi uptake in -P compared with +P plants resupplied with Pi was facilitated by a 2-4 fold increase in Vrnax of both systems and not by an increased affinit · (i.e. reduced Km) o: the carrier for the phosphate ion. Analysis of J_p efflux from labelled Spirodô€€¢Za and Lemna revealed the presence of two phases; one a rapidly effluxing phase of short half-times of exchange ( t 1/2 = 15-18 minutes) assumed to be of extracellular or 'free space' origin and one a slowly exchanging phase of long half-times of exchange t 1/2 =1300 h) assumed to be of cellular origin. There was little evidence for the presence of a third more rapidly effluxing phase. Problems associated with the use of 'compartmental analysis' of 32P efflux from Spirodela and Lemma are discussed. Rates of Pefflux from the slow phase were shown to be large with respect to gross Pi uptak rates and therefore contribute significantly to reducing net Pi uptake (net Pi uptake gross Pi uptake - P efflux). At low external Pi concentration (0.34 and 022 uM for +P Spirodela and lemma respectivel_·) rates of P efflux and gross Pi uptake were equal ('equilibrium point concentration'). P-deficiency resulted in a 3-4 fold reduction in the equilibrium point concentration since P efflux rates were reduced whilst gross Pi uptake rates were increased in -P plants resupplied with Pi. Inhibitors of metabolism (low temperature, CCCP, arsenate, cyan, e and azide) reduced Pi uptake; however, rates of P efflux were increased by the inhibitors, This is discussed in terms were increased by the same inhibitors. This is discussed in terms of the effect of these inhibitors on membrane permeability and intracellilar P fluxes. lular P fluxes. Thin layer chromatography of P efflux material from Spirodela and Lemma plants showed little evidence for the release of organic P indicating that under non-stressed conditions plants exert a close control over the chemical nature of released P.