Phosphate Absorption, Fluxes, and Symplasmic Transport in Osmotically-Shocked ZeamaysRoots

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Abstract
Osmotic shock with sequential 30 min treatments in ice-cold saline solutions and distilled water inhibited both the subsequent uptake of orthophosphate (Pi) and its transport into the xylem of excised corn (Zea mays L.) roots. Measurements of Pi fluxes with 32P indicated that the decrease in net Pi uptake over a 24 h period caused by osmotic shock was due primarily to delayed recovery of Pi influx rather than to increasing efflux. Despite complete recovery of Pi absorption within 2–6 h after shocking with 150–200 mM NaCl, transport to the xylem during the subsequent 24 h only partially recovered. Leucine uptake and incorporation into protein was also markedly inhibited by osmotic shock but both almost completely resumed control rates within 24 h after shocking with up to 150 mM NaCl. Tetracycline inhibited recovery of Pi uptake after NaCl treatment whereas puromycin did not. These results with corn roots are consistent with the hypothesis that recovery of Pi uptake activity after moderate osmotic shock requires de novo synthesis of membrane proteins. Incomplete recovery of Pi transport to the xylem suggests that osmotic shock may damage plasmodesmata.