Vergara - 1976 - Physiological and morphological adaptability of ri

as a function of solar elevation in Fig. 2.
IMHCROCLISLATE or THE RICE CROP 119
The decrease of value of lo, with increasing
solar elevation implies that direct solar radiation can penetrate deeper
when the solar elevation is higher. The dependence of k, on solar elevation
presented in Fig. 2 resembled that for a model canopy" consisting of leaves with
an inclination of70° to 75° (eg. Isobe. 1969).
Diffuse radiation
As already described, diffuse radiation flux in plant canopies consists of two
components of diffuse solar radiation tSDR) and complementary diffuse radiation
due to scattering of light by plant elements (CDR). It is reasonable to
assume that the extinction of both diffuse radiation fluxes in plant canopies is
given by Beefs law. The intensity of SDR penetrating to a leaf area index of L
is described by
ML) = $.40) EXPFRAL) (5)
where S10) is the SDR intensity at the top of plant cantipy, and k, is the extinction
coefficient for diffuse radiation. On tl1e basis of research by Monsi and
Saeki (i953). the following values of kd were determined:
Manryo canopy k,, = 0.67-0.70‘
[R8 canopy k,, = 0.60
The hatched area in Fig. 3 denotes vertical profiles of CDR emanating from
each unit leaf lajver (B(z) and C(z)). As shown in Fig. 3. when solar elevation is
lovi“. the flux of CDR emanating from each unit leaf layer is not large and its
peak value is about '7 vvlml. With increasing solar elevation, the generation of
CDR is observed throughout the whole foliage layer of the rice crop. mainly
because of deeper penetration of direct solar radiation. The leaf area depth at
tvhich the CDR was maximum moved gradually dowmvard with increasing solar
elevation and reached 0.6-0.7 of canopy} height. Pltnvever, the depth at which
CDR was maximal was consistently higher than the layer with the maximum of
leaf area density (LAD)
The upward flux of CDR decreased monotonically downward. The upward
flux of CDR at the top of crop canopy characterizing the radiation flux reflected
from rice canopies increased with increments of solar elevation. The profiles of
doo/nn-"ard flux of CDR are characterized by the curve with a rapid increase in
the upper half layer of the canopy and a slower decrease in the bottom cantipv
layer. The slower decrease of downward flux of CDR in the bottom half layer
of the canopy was mainly because of the steep decrease of leaf area density in
this region.
Radiation intensity on leaf surfaces
The radiant energy impinging on leaf surfaces within a plant canopy is known
to play a decisive role in plant evaporation and canopy photosynthesis. The
irradiance ofleafsurfaces (S) can be expressed as