Uptake and Metabolism of [C]Salicylic Acid in Lemna gibba G3

When the long-day plant Lemna gibba L., strain G3 is grown under continuous light on ammonium-free half-strength Hutner's medium (NH₄⁺-free 0.5 H medium) there is virtually no flowering, but addition of 10 micromolar salicylic acid (SA) to the medium results in substantial flowering. Using this system, the uptake and metabolism of [¹⁴C]SA in L. gibba G3 has been examined. SA uptake is rapid and linear for at least the first 24 hours. After 30 minutes, nearly 90% of the radioactivity in the plants is present as free SA. Part of this is rapidly converted to one or more bound forms of SA that appear either in the acidic butanol fraction or in the aqueous residue, and after 12 hours an equilibrium is reached between the free and bound forms of SA. When plants receive SA for 6 days and then are switched to control medium, both the free and bound SA remain nearly constant for at least 5 days. However, there is virtually no transfer of SA from mother fronds to daughter fronds, indicating that the SA is apparently sequestered within the cell. Cell fractionation studies show that nearly 95% of the SA remains in the supernatant even after a 2-hour centrifugation at 300,000 g. Thus, it is unlikely that SA is confined within a specific organelle, but rather is probably secreted into the vacuole.