Phosphate Uptake by Spirodela and Lemna during Early Phosphorus Deficiency

Abstract

Growth, internal P concentration and Pi uptake was investigated in sterile cultures of Spirodela oligorrhiza (Kurz) Hegelm. and Lemna major L. plants during early P-deficiency. Within 12 h of transfer to a P-deficient medium, Pi uptake rates by P-deficient (- P) plants were enhanced 30-120% compared with P adequate (+ P) controls at 1-1000 �M external [Pi]. 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] and appearance of visual symptoms. Enhanced Pi uptake rates in - P compared with +P plants resupplied with Pi was more closely correlated with a fall in the internal [Pi] (r = -0.93 to -0.98) than with a fall in the concentration of three other P fractions (i.e. ester P, lipid P, and residual P). The role of tissue [Pi] in Spirodela and Lemna plants as a possible determinant of Pi uptake rates is discussed. Kinetic analysis showed that enhanced Pi uptake in -P compared with + P plants resupplied with Pi was the result of a 2-4-fold increase in V*max of two first- order systems and not by an increased affinity (i.e. reduced K*m) of the carrier for the phosphate ion.