Hj. Kronzucker et al., Dynamic interactions between root NH4+ influx and long-distance N translocation in rice: Insights into feedback processes, PLANT CEL P, 39(12), 1998, pp. 1287-1293
Ammonium influx into roots and N translocation to the shoots were measured
in 3-week-old hydroponically grown rice seedlings (Oryza sativa L,, cv, IR7
2) under conditions of N deprivation and NH4+ resupply, using (NH4+)-N-13 a
s a tracer. Root NH4+ influx was repressed in plants continuously supplied
with NH4+ (at 0.1 mM), but a high proportion of absorbed N (20 to 30%) was
translocated to the shoot in the form of N assimilates during the 13-min lo
ading and desorption periods. Interruption of exogenous NH4+ supply for per
iods of 1 to 3 d caused NH4+ influx to be de-repressed, This same treatment
caused N translocation to the shoot to decline rapidly, until, by 24 h, le
ss than 5% of the absorbed N-13 was translocated to the shoot, illustrating
a clear priority of root over shoot N demand under conditions of N depriva
tion. Upon resupplying 1 mM NH4+, root NH4+ influx responded in a distinct
four-phase pattern, exhibiting periods in which NH4+ influx was first enhan
ced and subsequently reduced. Notably, a 25 to 40% increase in root influx,
peaking at similar to 2 h following re-exposure was correlated with a 4- t
o 5-fold enhancement in shoot translocation and a repression of root GS act
ivity. The transient increase of NH4+ influx was also observed in seedlings
continuously supplied with NO3- and subsequently transferred to NH4+. Exte
nded exposure to NH4+ caused root NH4+ influx to decrease progressively, wh
ile shoot translocation was restored to similar to 30% of incoming NH4+. Th
e nature of the feedback control of NH4+ influx as well as the question of
its inducibility are discussed.