Yoshida - 1981 - Fundamentals of Rice Crop Science

MINERAL NUTRITION OF RICE 159
3.25. Relationship between chlorophyll content
and iron concentrations in the leaf blades
(LB) (Takagi 1960).
varies with the pH, and is about 100 ppm at pH 3.7 and 300 ppm or higher at pH 5 .0
(Tanaka et al 1966b). Characteristic symptoms of iron toxicity may be observed
when the iron content in leaf blades is higher than 300 ppm (Tanaka et al 1966b).
High concentrations of iron in the soil solution decrease absorption of other
nutrients, such as phosphorus and potassium (Table 3.28).
The production of hydrogen sulfide and ferrous sulfides in flooded soils under
highly reduced conditions may contribute to iron toxicity. Hydrogen sulfide and
ferrous sulfides lessen the oxidizing capacity of rice roots, thereby increasing the
susceptibility of the rice plant to iron toxicity (Tanaka et al 1968, Inada 1966b).
Applications of ammonium sulfate increase the incidence of iron toxicity under
certain conditions (Inada 1966a).
The plant's nutritional status affects its tolerance for iron toxicity (Table 3.29).
Potassium, calcium, magnesium, phosphorus, and manganese deficiencies
decrease the iron-excluding power of rice roots. Since calcium, magnesium, and
manganese deficiencies do not normally occur in irrigated rice, potassium
deserves special attention. Potassium-deficient plants often have a high iron
content and exhibit severe symptoms of iron toxicity.
Table 3.28. Effect of iron supply on growth and nutrient content
of the rice plant. a Nutrient content
Iron concn
in culture Dry wt Fe N P K
solution (ppm) (g/plant) (ppm) (%) (ppm) (ppm)
2 6.8 148 3.22 0.35 3.12
75 2.8 341 3.47 0.30 3.06
150 0.9 785 2.96 0.21 2.42
300 0.2 1602 2.52 0.13 1.30
a Ishizuka and Tanaka (1969).