Yoshida - 1981 - Fundamentals of Rice Crop Science

GROWTH AND DEVELOPMENT OF THE RICE PLANT 39
(1.8)
In other words, the number of intercepts gives an estimation of the total root length
in centimeters. This method can also be used for estimating the length of any
irregular shape of lines.
Root length per unit volume of soil is called root density:
(1.9)
Thus, the root density has a dimension of cm -2 . It is used as a measure of root
growth in a field-grown crop. The root density of rice crops ranges from less than 1
cm -2 in a deep soil horizon to more than 30 cm -2 around the plant base.
1.7.7. Root characteristics related to drought resistance
The rice root system can be studied using several techniques: 1) root box, 2) core
sampling in the field, and 3) monolith. Since drought occurs when there is an
imbalance between water absorption and transpiration, greater root growth can
help the plant perform better under a limited water supply. The root-shoot ratio of
rice varieties can be studied by the root box technique. The ratios range from
0.05 to 0.3 at heading, and average about 0.1. Under drought conditions, the soil
starts drying from the surface but the deep soil horizon may remain wet and able to
supply water to the plant’s roots. Consequently, deep root portions may be more
meaningful than shallow root portions when the drought resistance of a variety is to
be examined. For this reason, the deep root-shoot ratio is considered a better
measure for drought resistance in the field. The deep root-shoot ratio of rice
varieties is defined as the weight, in milligrams, of roots that are deeper than 30
cm/gram shoot. It ranges from less than 10 to greater than 80 mg/g.
Table 1.10. Vertical root density distribution of 7 rice varieties grown on an upland
field, IRRI, 1977. a
Variety
Depth (cm)
0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80
Lowland
IR20 14.4 2.8 0.9 0.4 0.1
IR2035-117-3 22.7 5.8 0.8 0.1 0.1
IR442-2-58
Upland
16.8 7.1 1.2 0.3 0.1
OS4 12.6 1.4 0.8 0.9 0.8 0.5 0.5 0.2
Moroberekan 11.8 2.3 0.9 0.8 0.6 0.8 0.4 0.2
Salumpikit 16.2 5.5 1.9 1.4 0.8 0.6 0.3 0.1
20 A
19.8 2.6 0.9 0.8 0.9 0.9 0.6 0.4
a IRRI (1978).