Dynamic Interactions between Root NH4+ Influx and Long-Distance N Translocation in Rice: Insights into Feedback Processes

Ammonium influx into roots and N translocation to the shoots were measured in 3-week-old hydroponically grown rice seedlings (Oryza sativa L., cv. IR72) under conditions of N deprivation and NH4+ resupply, using 13NH4+as 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 loading and desorption periods. Interruption of exogenous NH4+ supply for periods 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, less than 5% of the absorbed 13N was translocated to the shoot, illustrating a clear priority of root over shoot N demand under conditions of N deprivation. Upon resupplying 1 mM NH4+, root NH4+ influx responded in a distinct four-phase pattern, exhibiting periods in which NH4+ influx was first enhanced and subsequently reduced. Notably, a 25 to 40% increase in root influx, peaking at ˜2 h following re-exposure was correlated with a 4- to 5-fold enhancement in shoot translocation and a repression of root GS activity. The transient increase of NH4+ influx was also observed in seedlings continuously supplied with NO3− and subsequently transferred to NH4+. Extended exposure to NH4+ caused root NH4+ influx to decrease progressively, while shoot translocation was restored to ˜30% of incoming NH4+. The nature of the feedback control of NH4+ influx as well as the question of its inducibility are discussed.