Yoshida - 1981 - Fundamentals of Rice Crop Science

158 FUNDAMENTALS OF RICE CROP SCIENCE
3.24. Effect of pH and moisture content of synthetic soil media on the chlorophyll
content and air dry weight of lowland rice Norin 16 (Takagi 1960).
Impaired translocation of iron from root to shoot in high-pH rooting media is
considered one of the causes of iron deficiency (Tanaka and Navasero 1966b).
b. Flooding-induced iron deficiency. When rice is grown on a mixture of river
sand and bentonite with no organic matter, acute iron deficiency is induced by
flooding. Plants grown under upland moisture conditions remain green (Fig.
3.24). This indicates that excess moisture weakens the ability of rice seedlings to
absorb iron. At lower pH, however, chlorosis does not occur on the flooded
artificial soil. In submerged soils, under most conditions, microbial reduction
increases the concentration of ferrous iron in the soil solution, and rice absorbs the
iron easily. In the artificial soil there is no reduction and, hence, no increase in the
availability of iron. As a consequence, rice suffers from iron deficiency. That
suggests that even in natural soils, if soil reduction proceeds slowly due to low
temperatures or when an upland field low in organic matter is converted to a paddy
field, rice seedlings may suffer from iron chlorosis.
c. Critical plant iron content. Chlorosis is a typical symptom of iron deficiency.
It suggests a low chlorophyll content in leaves. In chlorotic leaves, there is a
linear relationship between chlorophyll and iron within a range of iron concentrations
from 10 to 70 ppm (Fig. 3.25). Thus, the critical iron content in rice leaves is
about 70 ppm on a dry matter basis. Similar relationships exist for many other
upland crops (Oertli and Jacobson 1960). These results indicate that the internal
iron requirement is about the same for rice and other crops.
3.11.4. Iron toxicity
lron toxicity occurs when the rice plant accumulates iron in its leaves. Its occurrence
is associated with a high concentration of ferrous iron in the soil solution
(Ponnamperuma et al 1955). The critical iron concentration in the soil solution