crops is beneficial under production conditions [242–247]. Several reports [243,246,247] have shown
moderate yield increases of 6–11% when maize plants were grown under field conditions designed to pro-
vide mixed-N nutrition. However, not all environments [243] or hybrids [247] are responsive to mixed N,
indicating that factors other than the availability of NH 4 can affect the N use and productivity of maize.
Other work shows that cultivars differ in their physiological strategy for achieving mixed N–induced
yield increases and in the magnitude of response [235,238,247].
In most cases, mixed N–induced yield increases are the result of more grains per plant
[235,237,238,248], although increases in individual grain weight have also been reported [217,233]. For
wheat, the additional kernels are primarily achieved by increasing the number of grain-bearing tillers
[238,240,248] and to a lesser extent the number of grains per tiller [248]. Alteration of the N form at an-
thesis showed that mixed N supplied continuously, or during vegetative growth only, increased yield and
tillering over all NO 3 plants, but mixed N during reproductive growth only did not [240]. Thus, for wheat,
it appears that mixed N–induced increases in yield potential occur during the early stages of plant devel-
opment, when tillers are being formed.
In contrast to wheat, the main effect of enhanced NH 4 on maize is an increase in the number of grains
per plant through more kernels per ear [235,239], although there is also a tendency for increased prolifi-
cacy [247,249]. Additional kernels per ear result primarily from a decrease in kernel abortion [246,250]
and sometimes an increase in ovules per ear [247]. These findings suggest a direct physiological effect of
N form on kernel development, inasmuch as all studies presumably supplied a more than adequate level
of N (either as NO 3 or mixed N) to the plant. These results also suggest that mixed N–induced yield in-
creases are associated with events that occur during ovule initiation and pollination rather than with pro-
cesses occurring during the grain-filling period.
Support for a pre-grain-fill effect of mixed N on productivity has been obtained by transfer experi-
ments in which N was supplied either as all NO 3 or as an equal mixture of NO 3 and NH 4 until anthesis,
whereupon half the plants in each group were switched to the other N form [233,239]. In both sets of ex-
periments, yield was increased over all NO 3 plants when mixed N was available continuously or only be-
fore anthesis but not when it was available only after anthesis. Similarly, Reddy et al. [134] reported that
the N form supplied before, but not after, anthesis affected growth and nutrient status of maize. These
studies and other data [251] suggest that late vegetative and early reproductive development are the most
crucial times to supply mixed N to the plant.
Although the physiological basis for improved productivity with mixed N is not understood, addi-
tional plant N accumulation has been implicated. As in reports for vegetative growth [64,228], cereal
plants grown to maturity with mixed N typically contain more plant N (both content and concentration)
than those grown with NO 3 alone. These results have been observed for plants grown hydroponically
[233,235,238–240] and in soils [241,244,247,252]. Like the results of earlier work with seedlings, these
data suggest that when N is supplied primarily as NO 3 , cereal crops are unable to acquire sufficient N for
maximal productivity.
Although it is unclear exactly how this additional N (from mixed nutrition) enhances productivity, it
is well known that N supply and plant N status affect tillering in wheat [174,175], and kernel abortion and
prolificacy in maize [172,209,253,254]. Alternatively, a certain level of NH 4 may exert a direct effect on
reproductive development, with a corresponding change in plant metabolism. For example, NH 4 nutrition
has been reported to stimulate sucrose uptake by maize kernels, which in turn increased the production
and translocation of assimilates from the leaves [182]. Similarly, NO 3 -N was uniformly assimilated
throughout the plant, whereas NH 4 -N was assimilated in the root and preferentially exported as organic
N to meristematic regions like the ear [255,256]. Indicative of an enhanced supply of N to the ear is an
increase in grain protein concentration under mixed N, compared with plants grown on all, or predomi-
nantly, NO 3 [134,239,247,257].
In addition to enhanced N accumulation, mixed N–induced increases in reproductive development
and yield may be related to energy status. Because assimilation of NH 4 requires a third as many ATP
equivalents as does NO 3 [258], plants acquiring a large percentage of their N as NH 4 may expend less
total energy, especially if NO 3 is assimilated in the root [259]. Although the physiological impact of
this energy saving is unclear, it seems possible that any effect would be largest for crops such as maize,
which require high levels of N. However, based on cost estimates for NO 3 assimilation [173], Alexan-
der et al. [233] concluded that mixed N–induced increases could not be explained solely on the basis
of energetics.
NITROGEN METABOLISM AND CROP PRODUCTIVITY 397