Thus, as leaves age and senesce, their capacity for photosynthesis declines, with a correspondingly neg-
ative effect on assimilate supply and yield.
Many studies have shown a concurrent loss of photosynthetic activity and organic N from the leaves,
especially during seed development [155–158]. An example of this relationship for maize leaves is shown
in Figure 1: the losses in leaf N and photosynthesis were initiated at or near pollination and declined
nearly linearly during the grain-filling period. Although it is clear that the loss of N from the leaf impairs
photosynthetic activity, management practices that increase the N supply (such as supplementary side
dressing or foliar sprays of N) do not automatically increase leaf N status and photosynthetic activity
[159–162]. The absence of these effects is probably attributable to several key photosynthetic enzymes
(the large subunit of RuBPCase) that are encoded for and synthesized by the chloroplast [163]. After full
leaf expansion, the chloroplast loses much of its ability to synthesize these proteins, regardless of the
availability of N [148,163]. This phenomenon indicates that the application of supplementary N to main-
tain photosynthetic activity may be of limited value until a technique is found that will reactivate protein
synthesis in the chloroplast.
Another important role for N in assuring high productivity of crop plants is establishment of repro-
ductive sink capacity. Sink capacity of a cereal plant is a function of the number and the potential size of
grains. Grain number is dependent on the number of ears per unit area, the number of florets per ear, and
the proportion of florets that develop into grain [149,164,165], and the potential size of individual grains
depends on the number of endosperm cells and starch granules [166–169]. In either case, reproductive ini-
tials, like all growing tissues, are characterized by high concentrations of N and high metabolic activities.
This need could indicate that sufficient amounts of both C and N assimilates are required for full expres-
sion of the genetic potential for initiation and early development of grains.
For cereal crops, grain number is usually more closely related to yield than other yield components
[149,164,165]. Consequently, many studies have shown that N-induced yield increases are the result of
more grains per plant [170–173]. For wheat, this enhancement is related to an increase in tiller produc-
tion and survival [174,175] and to a lesser extent to a decrease in floret abortion [176]. In contrast, for
maize, N supply affects kernel number primarily by decreasing kernel abortion [172,177]. An example of
the effect of N supply on kernel number and kernel abortion of maize is shown in Figure 2: kernel num-
ber increases as the N supply is increased from a deficient to a sufficient level, which is associated with
a decrease in kernel abortion. Other studies, however, have indicated that N supply can also affect indi-
vidual grain weights [178,179], perhaps by means of a change in endosperm cell number [180].
Although the number of ears and grains is usually the yield component most affected by N sup-
ply, increases in kernel weight can also affect yield [149,164]. Because vegetative development in ce-
real crops is negligible after flowering, the N subsequently acquired, or remobilized from the vegeta-
tion, is used exclusively for grain development. This need for N is demonstrated by the fact that
adequately fertilized cereal crops typically contain from 9 to 13% protein in the grain. Indeed, some
392 BELOW
Figure 1 Changes in photosynthesis and N content of a selected leaf (the first leaf above the ear) of maize
during the grain filling period. Values presented are for adequately fertilized plants (200 kg N ha^1 ) averaged
over two hybrids at the University of Illinois research farm in 1985 and 1986.