Yoshida - 1981 - Fundamentals of Rice Crop Science

PHOTOSYNTHESIS AND RESPIRATION 201
When the amount of daily incident solar radiation is constant, the intensity of
light energy per unit of time weakens as day length increases. Consequently, as the
days lengthen, daily photosynthesis increases (Montieth 1965). This long-day
effect on photosynthesis is considered an advantage of the temperate climate over
the tropical.
c. Leaf area index and leaf orientation. A large leaf area index (LAI) is
necessary to intercept incident solar radiation. However, the size of LAI needed to
give maximum crop photosynthesis depends on the leaf orientation of the canopy.
The leaf orientation affects photosynthesis because it determines the light environment
within a canopy. For a comprehensive description of the light environment
and rice photosynthesis, the reader is advised to refer to a more specialized
paper (Uchijima Z. 1976).
A simplified model will explain the relationship between LAI, leaf orientation,
and photosynthesis. The model assumes that decreases in light intensity are due to
light absorption by the leaves in a manner analogous to Beer’s law.
In their pioneering work on photosynthesis and dry matter production in a plant
community, Monsi and Saeki (1953) stated that light absorption by the plant
community can be adequately described by Beer’s law:
In I
=-kF (5.8)
I o
where I o = light intensity incident on the leaf canopy;
I = light intensity in the plant
community where the LAI is F;
F = average cumulative total leaf area
per unit of ground area,
F is zero at the top of canopy and takes
its maximum value at ground level
(a value generally referred to as the LAI); and,
k = foliar absorption coefficient (dimensionless).
k is related to leaf angle or orientation. In rice varieties, k ranges from about 0.4
for erect leaves to about 0.8 for droopy leaves (Hayashi and Ito 1962).
The cumulative leaf area per ground area at which the incident light would be
reduced by 95% for erect and droopy rice canopies can be calculated with equation
5.8:
(a) For erect leaves with a k value of 0.4:
[1n (0.05/l.00)] [2.30 3 log (0.05/1.00)]
F =
=
(–0.4) (–0.4)
= [–2.303 log (1.00/0.05)]
(-0.4)
= 7.5 (5.9)