PHOSPHATE-UPTAKE, EFFLUX AND DEFICIENCY IN THE WATER FERN, AZOLLA

High phosphorus status (High-P) Azolla mexicana plants (P content 15.5-mu-moles g fr wt-1, doubling time ca. 2.2 d) and Low-P plants with early signs of P-deficiency (P content 6.2-mu-moles g fr wt-1, doubling time ca. 3.2 d) were used to study Pi uptake, efflux and deficiency. When High-P plants were transferred to medium lacking Pi, uptake capacity increased 1.5-fold within 12 h and before any detectable change in growth rate (24-48 h). When High-P and Low-P plants were compared, uptake rates from 0.3-10000 mmoles m-3 Pi were 2.6-1.7 times higher in Low-P than High-P plants (18-1150 vs 7-665-mu-moles g fr wt-1 h-1). The relationship of uptake rate to concentration was interpreted as arising from a combined operation of a high- and a low-affinity uptake system. Higher uptake in Low-P plants involved a 3-4-fold increase in V(max) (high affinity), no change in K(m) (high affinity), and a 1.5 to two-fold increase in both V(max) (low affinity) and K(m) (low affinity). Rates of P efflux into 1-1000 mmoles m-3 Pi were 1-7 to two times higher from High-P than Low-P plants (12-22 vs 7-11-mu-moles g fr wt-1 h-1). Below 1 mmole m-3 Pi, uptake and efflux rates were similar: the equilibrium concentration, at which net uptake was zero, was 0.22 mmoles m-3 for High-P plants and 0-05 mmoles m-3 for Low-P plants. Similar results were obtained with A. filiculoides. P transport characteristics of Azolla, a fern, are closely comparable with those of higher plants. Its high P requirement in the field arises from its ecological rather than physiological behaviour. We interpret the field behaviour by exploring the relationship between Azolla growth rate in the field, plant P concentration in the field, Pi transport rates required to support such growth, and Pi concentrations in pond waters. The transport characteristics which must operate in the field match those of Low-P plants in the laboratory, not High-P plants. Thus, Pi uptake in High-P plants should be interpreted as repressed from the normal state, instead of that in Low-P plants being induced.