Agriculture Reference
In-Depth Information
Fig. 8.1 Schematic life cycle of an annual plant. Light and temperature determine the potential
growth season. The life cycle of an annual plant starts with germination. Seedling leaf develop-
ment initially depends on resources from the seed, and young leaves act as a sink organs until they
develop sufficient photosynthetic activity to function as source tissue by exporting sugars and
nutrients to other parts of the plant. Here the chlorophyll content of leaves is graphed as a proxy for
photosynthetic capacity in leaves. Individual leaves eventually enter a senescence phase resulting
essentially in a programmed self-destruction. During this phase leaf constituents such as proteins,
lipids and mineral nutrients are degraded and exported to sinks. Each subsequent leaf growing
from the shoot apical meristem follows this developmental progression. During the vegetative
phase, the plant accumulates biomass, establishes a canopy and grows. Changes in light or other
environmental conditions trigger flower formation from the shoot apical meristem, constituting a
switch from a vegetative to generative growth phase and eventually results in seed production and
seed maturation. During the seed developmental phase, leaves undergo senescence and remobilize
necessary nutrients to the seeds. Mature seeds then enter a dormancy phase until germination in the
next growth season
90 % of nitrogen in the seed is remobilised from vegetative tissues (Hirel
et al.
2007
; Masclaux-Daubresse et al.
2010
; Masclaux-Daubresse and Chardon
2011
). As a consequence, in these crops grain yield and quality depend primarily on
pre-anthesis uptake of nitrogen. In contrast, 35-55 % of seed nitrogen in maize is
taken up from the soil during grain filling (Hirel et al.
2007
; Gregersen et al.
2008
).
Starch accumulation is the major factor determining seed yield, but other grain
parameters are equally important for farmers and consumers. For example, nutri-
tional quality is primarily a function of seed protein content and amino acid
balance. Therefore the supply and remobilisation of nitrogen, and to a lesser extent
sulfur for the sulfur-containing amino acids cysteine and methionine, represent
