Pi suppression and resupply effects were studied on nitrogen metabolism by means studied of evaluating on nitrogen biological fixation (N2), GS, Fd-GOGAT and NADH-GOGAT activities in alfalfa cv. Florida 77 plants inoculated with Sinorhizobium meliloti in different vegetative (V3 and V4) and reproductive (R6 and R8) stages. The experiment was conducted in a greenhouse and the plants cultivated in nutritive solution. The Pi ten days suppression period decreased Pi levels in leaves and nodules in all growth stages, whereas in the roots this decrease was observed only in the vegetative stages.After Pi resupply to the nutritive solution during a similar period, ten days, it was observed Pi levels recovery in stages R6 and R8 for leaves, V3 and V4 for roots and V3 for nodules. Pi suppression significantly reduced total amino acids levels in leaves and roots in all growth stages, indicating that there was some alteration in the N2 fixation process. After Pi resupply to the nutritive solution, total amino acids levels in roots in all growth stages reached similar values when compared to the control. Whereas in leaves this only happened in the vegetative stages. Xylem sap aminoacids composition did not alter as a function of either the Pi suppression or resupply. GS and Fd-GOGAT in the nodules decreased only in stages V3 and R8, whereas with the resupply there was no Fd-GOGAT recovery. Nonetheless, there was recovery of GS activity in R8. In the leaves, GS activity was significantly altered in all stages, while Fd-GOGAT was altered only in V3 and V4. After the resupply there was no GS and Fd-GOGAT recovery in V3. The NADH-GOGAT in leaves and nodules was significantly reduced in tratments that suppressed Pi in all stages evaluated. Therefore, there was no recovery after resupply. NADH-GOGAT was quite sensible to low Pi availability. Pi suppression seems to exert control over those enzymes activities throughout all different growth stages.
Medicago sativa; phosphorus suppression; phosphorus resupply; aminoacids; GS/GOGAT
