Yoshida - 1981 - Fundamentals of Rice Crop Science

202 FUNDAMENTALS OF RICE CROP SCIENCE
(b) For droopy leaves with a k value of 0.8:
[1n (0.05/l.00)]
F = = 3.7 . (5.10)
(– 0.8)
Thus, 95% of the sunlight is absorbed by the leaves in an erect-leaved rice canopy
and 5% reaches the soil surface when LAI is 7.5. In a droopy-leaved canopy, a LAI
of 3.7 is sufficient to intercept 95% of the sunlight. In other words, erect leaves
allow the sunlight to penetrate deeper into the canopy. Because both canopies
intercept the same amount of sunlight (95%), and because the erect-leaved canopy
requires a greater LAI than does the droopy-leaved canopy, the erect-leaved
canopy receives weaker sunlight per unit of leaf area. Consequently, the erectleaved
canopy achieves greater photosynthesis.
Erect leaves allow deeper penetration of incident light because they are oriented
with the incident sun beams when the sun is high. For the same reason, erect leaves
receive light at lower intensities than droopy leaves. The intensity of light received
by the leaf surface is determined by the angle between beams of direct radiation
and the normal to the leaf surface. If the angle is q° , the light intensity at the leaf
surface ( I ) is related to the intensity of incident light ( I 0 ) as:
(5.11)
When the sun is high, flat leaves receive a higher light intensity per unit of leaf
surface but for a smaller leaf area because q is close to zero. Erect leaves, however,
receive a lower light intensity per unit of leaf surface but for a larger leaf area
because q is close to 90°. A rice canopy in which leaves are very erect near the top
and become more horizontal toward the ground reduces the foliar absorption
coefficient of the upper leaves, leaving more light for the lower leaves. A
mathematical crop photosynthesis model indicates that a combination of erect
upper leaves and droopy lower leaves in a plant canopy gives the maximum crop
photosynthesis (Duncan 1971).
The LAI values necessary to intercept 95% of the incident light in rice canopies
suggest that a LAI of 4–8 is needed for good rice photosynthesis.
When light intensity reaches the light compensation point, there is no net gain
by photosynthesis. Assuming that the light compensation point is 400 lx for a leaf
temperature of 25°C and a CO 2 concentration of 300 ppm, and incident sunlight is
20 klx, at what cumulative leaf area per unit ground area ( F ) is the light
compensation point reached? By equation 5.8:
(5.12)
(5.13)
Thus, when the incident sunlight is moderate (about one-fifth of the full sunlight),
the upper 10 layers of leaves in the erect-leaved rice canopy are above the light