workers have suggested that the deposition and/or accumulation of storage protein by the kernel is a
factor regulating grain development [181–183]. This suggestion is based on the positive correlation be-
tween storage protein, kernel weight, and grain yield [182] and on genetic studies showing reduced lev-
els of storage protein (zein) and starch in zein-deficient mutants of maize [184]. An alternative expla-
nation, however, is that the availability of N within the plant and to the grain is positively associated
with kernel development, and as such the amount of storage protein deposited is only an accurate re-
flection of the N supply [122].
Other needs for N by developing kernels could include embryo growth and the initial and continued
synthesis of enzymes needed for energy generation and the deposition of storage products in the kernel.
Embryo development could affect the kernel’s hormonal balance because a large portion of kernel phy-
tohormones are produced by the embryo [185,186]. Because several of the key classes of phytohormones
either contain N (auxins, cytokinins, polyamines) or are synthesized from amino acids (auxins, ethylene,
polyamines), an adequate supply of N may be needed for their production. With regard to storage prod-
uct formation, provision of N to developing maize kernels has been shown to increase their capacity to
synthesize proteins and to utilize sugars for the biosynthesis of starch [187]. Nitrogen supply also exerts
a marked effect on endosperm enzymology and on the deposition of storage proteins in the endosperm
[187,188]. Thus, it appears that at least a portion of the yield increase produced by N fertilization results
from a modification of kernel metabolism in response to N supply.
B. Interactions of Carbon and Nitrogen
Grain yield of crops is primarily a function of the plant’s ability to acquire, metabolize, and utilize C and
N assimilates and its genetic potential for maximum grain production. For cereal crops, the relative abun-
dance of C versus N in the plant (approximately 44% C vs. 1.5% N) dictates a predominant role for pho-
tosynthesis in achieving maximum yields. However, as discussed in Sec. III.A, the metabolism of N plays
a major role in the production of C assimilates and in their utilization for reproductive development. In
addition, as evidenced by the use of reduced ferredoxin in NO 2 reduction and NH 4 assimilation (see Sec.
II.B.3), C and N interact at numerous points in plant metabolism [189]. This interdependence in C and N
metabolism creates problems when one is attempting to describe an independent role for either C or N in
achieving maximum productivity.
Grain composition offers a prime example of the complexities involved in understanding how C and
N interact to affect productivity. A negative relationship between grain yield and protein percentage is
widely noted in cereals, especially in cultivars selected for abnormally high or low percentages of grain
protein [190,191]. The higher metabolic cost associated with the synthesis of protein than with carbohy-
drate has been proposed to explain this relationship [192,193]. However, evidence showing that carbo-
hydrate supply does not normally limit kernel development [194–196] and progress toward identification
and breeding of high-protein, high-yielding cereals [197,198] make this explanation seem unlikely. In ad-
NITROGEN METABOLISM AND CROP PRODUCTIVITY 393
Figure 2 The effect of N fertilizer rate on kernel number and kernel abortion of maize. Values are averaged
over two hybrids at the University of Illinois research farm in 1990.