Translocation of NH4+ in oilseed rape plants in relation to glutamine synthetase isogene expression and activity

Abstract

Translocation of NH4+ was studied in relation to the expression of three glutamine synthetase (GS, EC 6.3.1.2) isogenes and total GS activity in roots and leaves of hydroponically grown oilseed rape (Brassica napus). The concentration of NH4+ in the stem xylem sap of NO3−‐fed plants was 0.55–0.70 mM, which was ≈60% higher than that in plants deprived of external nitrogen for 2 days. In NH4+‐fed plants, xylem NH4+ concentrations increased linearly both with time of exposure to NH4+ and with increasing external NH4+ concentration. The maximum xylem NH4+ concentration was 8 mM, corresponding to 11% of the nitrogen translocated in the xylem. In the leaf apoplastic solution, the NH4+ concentration increased from 0.03 mM in N‐deprived plants to 0.20 mM in N‐replete plants. The corresponding values for leaf tissue water were 0.33 and 1.24 mM, respectively. The addition of either NO3− or NH4+ to N‐starved plants induced both cytosolic gs isogene expression and GS activity in the roots. In N‐replete plants, gs isogene expression and GS activity were repressed, probably due to carbon limitations, thereby protecting the roots against the excessive drainage of photosynthates. Repressed gs isogene expression and GS activity under N‐replete conditions caused enhanced NH4+ translocation to the shoots.