Although both NH4+ and NO3- are important sources of N for plant growth, NH4+
preference is common in plants occupying habitats with restricted nitrification (Garnett et al. 2001; Kronzucker et al. 2000; Kronzucker et al. 1998; Kronzucker et al. 1995; Tylova- Munzarova et al. 2005; Wang et al. 1993; Yan et al. 2011). Anaerobic conditions cause substantial reduction in net NO3- uptake by roots of higher plants (Glass 1988) due to
denitrification and leaching (Buwalda & Greenway 1989). However, they can also reduce NH4+ uptake by ~ 50% (Bradley & Morris 1990). Nitrogen uptake was reduced by 16% in
wheat grown in a nutrient solution containing a mixture of NH4+ and NO3- under hypoxia
(Buwalda et al. 1988; Buwalda & Greenway 1989). Nitrogen uptake under hypoxia is sometimes limited by H2S and elemental toxicity or salinity (Morris 1980).
2.7.1 Ammonium uptake under hypoxia
There is ample evidence that NH4+ can be absorbed and utilized directly by many plant
species of higher plants (Arnon 1937; von Wirén et al. 2000). However, NH4+ uptake at
concentrations less than 500 µM occurs against an electrochemical gradient (Taylor & Bloom 1998), which is reduced by hypoxia (Buwalda & Greenway 1989). A time- dependency study of NH4+ influx into rice roots after the onset of hypoxia showed an
initial increase in NH4+ uptake followed by a 50% decline (Kronzucker et al. 1998).The
importance of NH4+ as a N source under hypoxic conditions is vital due to: adsorption of
NH4+ to soil colloids, while NO3- is readily leached; slow mineralization of organic N and;
cessation of nitrification and denitrification of NO3- (Buwalda & Greenway 1989). The
HATS induced in N starved roots (von Wirén et al. 2000) plays an important role in N acquisition under flooded anaerobic conditions when NH4+ is dominant and significant
nitrification occurs on the root surface (Feng et al. 2011).
2.7.2 Nitrate uptake under hypoxia
NO3- is a major source of N for the vast majority of plants. It is reduced to nitrite (NO2-) by
nitrate reductase (NR); the NO2 is further reduced to NH4+ by nitrite reductase (NiR) under
hypoxia (Bailey-Serres & Voesenek 2008; Daniel-Vedele et al. 1998). This makes NH4+
by the roots (Colmer 2015). NO3- is used as an alternative electron acceptor by some soil
microorganisms during periods of partial anaerobiosis allowing the electron transport system (ETS) and oxidative phosphorylation to continue (Colmer 2015; Drew 1991). In O2
deficient roots, NO3- ions may enter passively and translocate to the shoot in sufficient
amounts to be of temporary benefit (Trought & Drew 1981). This temporary benefit stimulates NADH-dependent NR activity thereby diverting NADH from the reduction of acetaldehyde to ethanol, whose accumulation could damage sensitive tissues (Drew 1991).
Understanding the uptake of N and how this varies along the different physiological root zones under hypoxia requires a detailed study of nutrient fluxes. While a few studies have looked at the flux of NH4+ and NO3- ions in wheat and barley; most paid no attention to
NH4+ and NO3- uptake under hypoxia and how this varies along the root axis. This research
investigated to what extent NH4+ and NO3- uptake is affected by hypoxia along the root
axis of selected wheat and barley varieties using the non-invasive microelectrode ion flux estimation (MIFE) technique. The MIFE technique offers a unique temporal and spatial pattern of the ions uptake, permitting detailed examination of inorganic N acquisition and its component ionic interactions (Henriksen et al. 1990).
2.7.3 The inhibitory effect of NH4+ on NO3- uptake in plants
The rate of NO3- uptake can be suppressed in the presence of NH4+ ions (Criddle et al.
1988; Tylova-Munzarova et al. 2005; Youngdahl et al. 1982). This suppression can be both short term and long term (Crawford & Glass 1998; Glass 2003; Orsel et al. 2002a). The short term effect is apparent within minutes of exposure to NH4+ while the long term
can range from hours to days. The short term effect is attributed to the direct inhibition of NO3- uptake and stimulation of its efflux (Glass 2003; Kronzucker et al. 1999). This
conversion of absorbed NH4+ to glutamine, a potent feedback inhibitor of transcription of
the putative iHATS transporter gene NRT2.1 (Glass 2003).
NH4+ markedly inhibits the uptake of NO3- in barley and wheat roots; it generally exceeds
NO3- uptake from equimolar solutions (Minotti et al. 1969; Newman 2001; Taylor &
Bloom 1998). This creates a high concentration of NH4+ and H+ adjacent to the cellular
boundary membranes, which modifies the permeability of the membranes to NO3- (Minotti
et al. 1969). A study using 13NO3- in barley showed that influx diminishes and efflux
increases within minutes of exposure to NH4+ due to depolarisation of the plasma
membrane. The depolarisation reduces the proton motive force (PMF) for active NO3-
uptake by the 2H+/1NO3- symport mechanism (Crawford & Glass 1998). In rice,
compartmental analysis by efflux and 13N radiotracer showed that NO3- influx and
metabolism are strongly repressed by NH4+ (Kronzucker et al. 1999).
The extent of NH4+ suppression on NO3- uptake depends on the ionic concentration in the
solution (Criddle et al. 1988). For instance, NH4+ of 100-500 µM inhibited net NO3- uptake
in barley seedlings at a NO3- concentration of 10 µM but not at 100 µM (Criddle et al.
1988; Deane-Drummond & Glass 1983). Exposure to 1 mM NH4+ strongly reduces the
influx of NO3- in uninduced plants particularly when the external concentration of NO3- is
low (Kronzucker et al. 1999). At higher [NO3-] and in induced plants the inhibitory effect
of NH4+ diminishes an indication that NH4+ inhibition of NO3- uptake is mediated via
effects on the iHATS rather than the cHATS or the LATS (Kronzucker et al. 1999).
The severity of NH4+ restriction of NO3- uptake can be moderated in the presence of K+
(Pan et al. 1985; Rufty et al. 1982), which facilitates the flow of H2O and NH4+ assimilates
(potential negative assimilates) through the root symplasm to the xylem (Pan et al. 1985). Plants efficient in translocating negative effectors away from sites of NO - uptake or
reduction exhibit minimal inhibition of NO3- uptake by NH4+ (Pan et al. 1985). The influx
of NO3- can also be improved by induction through pre-treatment with NO3- (Newman
2001), though NO3- uptake is strongly reduced by prior accumulation of NO3- in the roots
(Breteler & Siegerist 1984; Deane-Drummond & Glass 1983). Of note, the inhibition of NO3- accumulation in the root tissue and translocation via xylem vessels varies with
genotype and root age (Criddle et al. 1988; Pan et al. 1985).