Beyond maximizing harvest index,
ideal crop plant architecture should minimize
the highest photon flux density at an individual
leaf level while at the same time maximize the
total solar energy absorbed by the canopy per
unit ground area. Ideally, the plant architecture
and leaf biochemical properties should be designed
so that the light levels match the photosynthetic
capacity at different layers within the
canopy (52, 90). It is not fully clear how well nitrogen
distribution within a canopy tracks light
distribution, although the photosynthetic apparatus
show clear differentiation under different
light conditions (121–123). What is the optimal
plant architecture?
When leaf area index
(LAI) is lower than 2, canopies with horizontal
leaves will enable the greatest interception of
daily incident solar irradiance (73).
For a canopy
with a higher LAI, however, an ideal plant architecture
will have a more vertical leaf angle at
the top of the canopy that gradually decreases
with depth into the canopy (72).
This will ensure
that light is spread more evenly through
the canopy and that a high proportion of leaves
will fall on the high-efficiency left-hand side of
Figure 3.
Theoretical analysis suggested that
compared to a canopy with horizontal leaves,
a canopy with a gradual decreased leaf angle
can increase the daily intergral of carbon uptake
as much as 40% on a sunny day at midlatitude
(72).
A season-long improvement of εc of
∼20%could result from the avoidance of severe
light saturation at the top of the canopy and severe
light limitation within the canopy due to
the improved canopy architecture.
Substantial progress has been made in elucidating
the genetic basis of plant architecture
determination (85, 112). In rice, the dwarf
stature is caused by loss of function of brassinosteroid
insensitive1 ortholog, OsBRI1 (85).
One allele of OsBRI1, d61–7, confers important
agronomic traits—semidwarf stature and
erect leaves—and led to 30% more grain yield
than wild type at high planting densities (85).
Genes for the erect leaves likely exist in most
current crops (107, 10
; if so, searching for
d61–7 like alleles may be an important way forward
in improving εc.
Additionally, engineering
or selecting plants with gradually decreasing
leaf angles at different layers of canopy has
the potential to further increase εc compared to
either a uniform horizontal leaf angle or a uniform
erect leaf angle (72). Theoretically, optimal
architecture in plant monocultures will
differ among species that vary in plant stature,
leaf chlorophyll content, canopy albedo, and
other species-specific features. Additionally, geographic
location and time of the year matter
because canopies with higher LAI and more
erect leaves show the greatest advantage with
high solar elevation,such as during summer or
at low latitude (26).