P_I Efflux and Influx by P-Adequate and P-Deficient Spirodela and Lemna

Abstract

Net P_i influx by P-adequate (+P) and P-deficient (-P) Spirodela and Lemna plants was determined by gross P_I influx and P_I efflux. Rates of P_I influx (gross) and P_I efflux were sensitive to both internal [P] and external [P_I]. The different responses of these two fluxes to external [P_I] resulted in their rates converging at low external [P_I], from P_I efflux being 9-10% of gross P_I influx at 1000 µM P_I to being over 70% at low (1 µM) external P_I in + P plants. At very low external Pi (0.34 µM, 0.22 µM P_i for + P Spirodela and Lemna, respectively), rates of gross P_i influx and P_i efflux were assumed to be either equal or zero (referred to as the equilibrium [P_i]_ext), such that net P_i influx was zero. There was assumed to be insufficient P_i diffusing to the carrier site on the root surface at very low external [P_i] (i.e. equilibrium [P_i]_ext) for the phosphate pump to operate and P_i influx was zero. P deficiency resulted in a simultaneous reduction in rates of P_i efflux and an increase in gross P_i influx such that the equilibrium [P_i]_ext was reduced 3-4 times (0.8, 0.07 µM P_i for -P Spirodela and Lemna respectively) compared with +P plants. Some of the limitations of P_i diffusion to the carrier sites in P-adequate plants are overcome as internal [P] decreases during P deficiency. As a consequence the ability of Spirodela and Lemna to extract P_i from solution during P deficiency was considerably greater than for plants with adequate P status.

Inhibitors of metabolism (low temperatures, arsenate, azide, cyanide and CCCP) reduced rates of gross P_i influx but increased rates of P_i efflux such that net P_i influx was reduced. These findings are discussed in relation to previous work on the kinetics of P_i influx in - P versus + P plants and the role of changes in membrane potentials in the enhancement of P_i influx and reduction of P_i efflux capacity in P-deficient Spirodela and Lemna.