More extensive early-season N uptake may also be the result of a larger supply of N in the soil, be-
cause available soil N often exhibits a marked decline coincident with rapid vegetative growth, with the
lowest N levels occurring around anthesis [128]. However, whereas N applications made during the early
stages of reproductive development can increase protein percentage [129,130], there is often little or no
response in terms of grain yield [131,132]. Similarly, foliar N sprays have the least impact on increasing
plant N levels when applied around anthesis because the additional N interferes with the metabolism of
indigenous N [124]. Collectively, these findings suggest that there is a level of genetic control over N ac-
cumulation and distribution that is independent of the availability of N. Complicating the understanding
of how N availability and genetics determine plant growth, however, is the inability to control stringently
the supply of N in the soil.
In the few cases of the use of hydroponic culture to deprive maize plants of N at anthesis, yield
either has been unaffected [133] or has decreased only modestly [134,135]. Similarly, grain yield in
soils is generally affected more by the N supply before anthesis than after [136,137]. This evidence,
and the lack of ability to increase yields of most maize cultivars with postanthesis foliar N sprays
[138,139], suggests that N has its main impact on yield before anthesis. However, redistribution of pre-
viously accumulated N from vegetative to reproductive plant parts could minimize the need for postan-
thesis N uptake. For both maize and wheat, the grain typically contains about 70% of the total N in the
plant at maturity, with more than half of it coming from remobilization from other plant parts
[119,120,140,141]. Thus, because of extensive changes in the distribution of N among plant parts, it is
difficult to separate the effects of the timing of N accumulation from the contribution of N to grain de-
velopment and yield.
III. PHYSIOLOGICAL ROLES FOR NITROGEN IN CROP
PRODUCTIVITYA. Importance of Nitrogen to Plant Growth
Crop growth and productivity involve the integrated effect of a large number of components and
metabolic processes that act, with variable intensity, throughout the life cycle of the crop. The interde-
pendence of N and C metabolism creates additional problems in describing an independent role for N in
achieving maximum crop productivity. Nevertheless, four major roles for N have been proposed for at-
taining high yields of rice [142] and maize [122], and these roles appear to be valid for many crops:
Establishment of photosynthetic capacity
Maintenance of photosynthetic capacity
Establishment of sink capacity (the number and potential size of seeds)
Maintenance of functional sinks throughout seed developmentEach of these roles is discussed briefly with reference to the potential impact on crop productivity.
The objective in establishing photosynthetic capacity is to ensure that the supply of N does not limit
development of the photosynthetic apparatus (enzymes, pigments, and other compounds needed for pho-
tosynthesis). Within limits, and if no other restrictive factors are present, an increase in N supply increases
the growth, the composition of N and chlorophyll, and the photosynthetic capacity of leaves [143–145].
Nitrogen supply has also been shown to regulate the synthesis of photosynthetic carboxylating enzymes
by affecting transcription and/or the stability of messenger RNA [146,147]. Collectively, these effects re-
sult in greater light interception, higher canopy photosynthesis, and higher yield. However, because little
N is accumulated by the leaf after it has reached full expansion [148], a sufficient supply of N must be
available throughout the development of each leaf if the individual leaves are to attain their full genetic
potential for photosynthetic capacity.
To achieve high yields, plants must not only establish photosynthetic capacity but also continue pho-
tosynthesis throughout the grain-filling period. Thus, once established, sufficient N must be available to
maintain the photosynthetic apparatus. This role is particularly important because dry matter accumula-
tion in cereal grains is dependent on current photosynthesis [119,149,150]. Most of the N in the leaf is as-
sociated with proteins in the chloroplast—60% in C 4 plants and up to 75% in C 3 plants [122,151,152]—
and these proteins are subject to breakdown and remobilization of the resultant amino acids [153,154].
NITROGEN METABOLISM AND CROP PRODUCTIVITY 391