Yoshida - 1981 - Fundamentals of Rice Crop Science

22 FUNDAMENTALS OF RICE CROP SCIENCE
1.21. The growth of individual leaves on the
main culm of the rice plant (adapted from
Tanaha 1954).
At early stages of leaf growth, weight increases are associated with elongation.
After the completion of elongation, the weight continues to increase because of an
accumulation of proteins and starch and an increase in cell wall materials such as
hemicellulose. The leaf weight decreases at later stages of growth because proteins,
sugars, and starch are translocated to upper leaves.
1.4.3. Leaf growth of a crop
The leaf growth of a rice crop, as a whole, can be measured in terms of the weight
of whole leaves per unit of ground area, and of leaf area of whole leaves per unit of
ground area. Because leaves intercept incident solar radiation, leaf area index
is widely used in the research of crop photosynthesis and growth analysis.
Leaf area index (LAI) is defined as:
sum of the leaf area of all leaves
LAI = . (1.1)
ground area of field where the leaves have been collected
The same unit must be used for both leaf area and ground area. Even though a leaf
has two surfaces, it is customary to measure only one. In crops such as wheat, all
the exposed green tissues, including leaf blades, leaf sheaths, and culms, are
measured for LAI. In rice, however, it is common to measure only the area of leaf
blades because photosynthesis by sheaths and culms is negligible. LAI values
imply the magnitude of leaf area relative to ground area. Thus, LAI = 2 means that
a crop has a leaf area twice as large as the ground area. If all the leaves are arranged
flat in one plane parallel to the ground with no dead space, the leaves of a crop with
LAI = 1 covers the ground surface completely. But all the leaves are neither flat
nor in the same plane. Hence, LAI values greater than 1 are needed to rover the
ground surface. LAI increases as growth advances and reaches a maximum at
around heading when the rice plant has its 5 largest leaves. After heading,
however, LAI declines as the lower leaves die (Fig. 1.22).