Yoshida - 1981 - Fundamentals of Rice Crop Science

34 FUNDAMENTALS OF RICE CROP SCIENCE
1.32. Relationship between total dry weight and root dry weight of 4 rice varieties
(IRRI 1978).
where W R is root dry weight, W T is total dry weight (i.e., shoot dry weight plus
root dry weight), and H and h are constants. Equation 1.4 can be converted into
a logarithmic form:
Log W R = log H + h log W T . (1.5)
Thus, log W R is a linear function of log W T . The above relationship has been tested
for different rice varieties under different environmental conditions. One such
study gives the following formula:
W R = 0.212 W T
0.936
(1.6)
where the weight is expressed in milligrams (Fig. 1.32). Thus, when the plants are
small (substitute 1 for W T ), W R /W T is about 0.2; W R /W T value approaches 0.1 as the
plant grows larger (substitute 10 5 for W T ). In other words, the ratio of root dry
weight to total dry weight ranges from about 0.2 at the seedling stage to about 0.1
at heading.
The above relationship between root and total dry weight gives an estimate of
root mass left in the soil if shoot weight is known. For instance, when a crop
produces a shoot dry weight of 3 t/ha at heading, a root dry weight of about 330
kg/ha is left in a recoverable form in the soil. Since it is known for other cereal
crops that a considerable amount of organic substances is released into the soil
from roots as root exudates (Barber and Martin 1976), the actual amount of