Yoshida - 1981 - Fundamentals of Rice Crop Science

MINERAL NUTRITION OF RICE 117
3.4. Relation between time of nitrogen
application and production of S 2
in media (Okajima 1958).
culture with ammonium nitrogen produce a root mat but those grown with nitrate
nitrogen do not (Soezima and Kawata 1969).
The reduction of the rice rhizosphere is not necessarily related to particular
growth stages but is largely dependent on nitrogen nutrition (Fig. 3.4). Appreciable
amounts of hydrogen sulfide can be found in the culture solution when a rice
plant becomes nitrogen deficient. No hydrogen sulfide is found when the plant
receives ample nitrogen regardless of the growth stage. In the field, however, rice
absorbs large quantities of nitrogen, depletes available soil nitrogen, and grows
vigorously at around panicle initiation. As a consequence, rice tends to be lower in
nitrogen content afterwards. Therefore, the beginning of rice rhizosphere reduction
tends to coincide with panicle initiation to heading.
3.2.4. Anaerobic respiration
The existence of an oxygen transport system from shoot to root in rice and other
bog plants suggests that these plants avoid suffocation of root tissues in anaerobic
root environments. While there is some evidence of anaerobic respiration in rice
(John et al 1974, John and Greenway 1976), it appears to be of minor importance.
The cells of excised rice roots are highly sensitive to oxygen deficiency. When
excised rice roots are exposed to a nitrogen atmosphere for 4–5 hours, destructive
changes take place in the tissue’s cellular organelles (Vartapetian 1973). The
respiratory rate of excised roots declines very rapidly in a nitrogen atmosphere,
while intact roots maintain a respiratory rate as high as in an aerobic atmosphere.
The respiratory rate of excised roots in an aerobic atmosphere is as high as that of
intact roots (Arikado 1959). Apparently, rice roots have only a slight tolerance for
oxygen deficiency but avoid suffocation when they are connected to shoots.
3.2.5. Iron-excluding power
In submerged soils, ferric iron is reduced to the ferrous form. As a consequence,
the level of ferrous iron in the soil solution may increase to 300 ppm or higher.
Although rice benefits from increases in ferrous iron, it is often injured by