Yoshida - 1981 - Fundamentals of Rice Crop Science

MINERAL NUTRITION OF RICE 119
Table 3.4. Effect of respiratory inhibitors on the iron-excluding power of rice roots and
translocation percentage of iron. a
Respiratory
rate
Trans-
Concn (µl O 2 / 100mg Water Iron Iron- location
Inhib- of inhibitor dry roots absorption absorption excluding percentage
itor (M) per hour) (ml/plant) (mg/plant) power b (%) of iron c
None – 39.3 49 1.93 87 19.1
KCN 10 -4 26.6 36 3.34 69 21.3
10 -3 2.6 30 5.73 36 34.6
NaN 3 10 -4 5.2 24 4.51 37 44.3
10 -3 1.1 23 5.23 24 44.7
DNP 10 -5 35.5 51 5.45 64 16.5
10 -4 2.8 27 5.69 30 23.4
a Tadano (1976). b lron-excluding power (%) = ( a – b)/a × 100: where a is the amount of
iron in milligrams contained in the same volume of culture solution as that of water
absorbed by the plant and b is the amount of iron in milligrams absorbed by the plant.
c Translocation percentage of iron = the amount of iron translocated to the shoot relative
to the total amount of iron absorbed by the plant.
exchange of H + released from plant roots with cations from soil colloids. This
process may occur to some extent but the degree of its importance in nutrient
transport is questioned (Mengel and Kirkby 1978).
The soil solution theory, now widely accepted, proposes that soil nutrients are
dissolved into solution and transported to root surfaces by both mass-flow and
diffusion. Mass-flow occurs when there is a gradient in the hydraulic potential, the
sum of gravitational and pressure potential, and nutrients move along with water
flow from higher to lower hydraulic potential. The amount of nutrients reaching
the root is thus dependent on the rate of water flow or the plant’s water consumption
and the average nutrient concentration of the water.
Diffusion occurs when there is a gradient in the ion concentration between the
root surface and the surrounding soil solution. The ions move from the concentrated
to the diluted region. The rate of diffusion is governed by Fick’s first law,
which states that the rate of diffusion is proportional to the gradient in concentration
between the two regions:
(3.2)
If two soils of different nutrient levels are considered, the soil with the higher
level has the steeper concentration gradient and, therefore, the diffusion rate to the