Effects of phosphate stress on the rate of phosphate uptake during resupply to deficient tomato plants

Plants of Lycopersicon esculentum Mill. P. I. 120262 show an increased phosphate uptake rate per unit dry weight of root after as little as one day of growth in solutions lacking phosphate. The reversibility of this response in plants experiencing various degrees of phosphate stress was investigated by measuring the depletion of phosphate from solutions over 3-h intervals. Measurements were made at three times in the first 30 h after phosphate was resupplied. Reversibility decreased as the level of phosphate stress increased. The phosphate uptake rate was returned to the level of controls after 30 h of phosphate resupply in plants grown without phosphate for one or three days. Plants grown without phosphate for five or seven days had uptake rates 26 and 40% higher than controls, respectively, after the same period of phosphate resupply. Internal phosphate concentrations after 30 h of phosphate resupply were equal to or greater than the controls in all plants. These results are consistent with a simple reversible feedback of phosphate status on phosphate transport in slightly stressed plants, but such a mechanism seems inadequate to explain the responses observed in more severely stressed plants.