Translocation of NH 4 + in oilseed rape plants in relation to glutamine synthetase isogene expression and activity

• Published in: Physiologia plantarum Vol. 105; no. 3; pp. 469 - 477
• Main Authors: Finnemann, J., Schjoerring, J. K.
• Format: Journal Article
• Language: English
• Published: 01-03-1999
• ISSN: 0031-9317; 1399-3054
• DOI: 10.1034/j.1399-3054.1999.105311.x

Translocation of NH
4
+
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 NH
4
+
in the stem xylem sap of NO
3
−
‐fed plants was 0.55–0.70 m
M
, which was ≈60% higher than that in plants deprived of external nitrogen for 2 days. In NH
4
+
‐fed plants, xylem NH
4
+
concentrations increased linearly both with time of exposure to NH
4
+
and with increasing external NH
4
+
concentration. The maximum xylem NH
4
+
concentration was 8 m
M
, corresponding to 11% of the nitrogen translocated in the xylem. In the leaf apoplastic solution, the NH
4
+
concentration increased from 0.03 m
M
in N‐deprived plants to 0.20 m
M
in N‐replete plants. The corresponding values for leaf tissue water were 0.33 and 1.24 m
M
, respectively. The addition of either NO
3
−
or NH
4
+
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 NH
4
+
translocation to the shoots.