Yoshida - 1981 - Fundamentals of Rice Crop Science

118 FUNDAMENTALS OF RICE CROP SCIENCE
excessive levels in the soil solution. A physiological disorder attributable to iron
toxicity occurs in acid sandy, acid sulfate, and acid organic soils.
Rice roots have three functions to counteract iron toxicity (Tadano 1976,
Tadano and Yoshida 1978):
(a) Oxidation of iron in the rhizosphere, which keeps iron concentrations low
in the growth media.
(b) Exclusion of iron at the root surface, which prevents the iron from entering
the root.
(c) Retention of iron in the root tissue, which decreases the translocation of
iron from root to shoot.
When the concentration of iron in the culture solution is low, its absorption by
the rice plant is not directly related to water absorption. However, when the
concentration is high, the iron content in the shoot is increased proportionately
with increases in water absorption, and the total amount of iron absorbed also
increases. That indicates that iron absorption by mass flow is an important
mechanism when the concentration is high in the rooting media.
Respiratory inhibitors that normally retard the absorption of nutrients such as
ammonia and phosphate, also retard iron absorption from a culture solution
containing low concentrations of iron. However, at high iron concentrations,
respiratory inhibitors, such as KCN, NaN 3 , and DNP, decrease the rate of
respiration but increase iron absorption (Table 3.4). The iron-excluding power
shown in Table 3.4 is calculated as:
Iron-excluding power = ___ a–b × l00 (%), (3.1)
a
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, actually absorbed by the plant.
The iron-excluding power of a healthy rice plant is 87%, meaning that 87% of
the iron that has reached the root surface, along with the water absorbed by the
plant, is not absorbed or excluded. A pretreatment of rice roots with respiratory
inhibitors decreases the iron-excluding power. Thus, the iron-excluding power of
rice roots appears to be associated with the roots’ metabolic activity. That suggests
that iron toxicity in rice can be caused by high levels of ferrous iron in soil solution
and also by the decreased metabolic activity of roots.
3.3. NUTRIENT AVAILABILITY IN SOIL
3.3.1. Transport of soil nutrients to plant roots
Two major theories have been proposed to explain how soil nutrients can be made
available to plant roots: contact exchange and soil solution.
The contact exchange theory proposed by Jenny and Overstreet (1938) postulates
that a close contact between root surfaces and soil colloids allows a direct