Yoshida - 1981 - Fundamentals of Rice Crop Science

126 FUNDAMENTALS OF RICE CROP SCIENCE
3.6. Uptake of H 2 O and
other nutrient elements as
influenced by H 2 S (Mitsui
1960).
stimulate late tillering and result in disturbed growth patterns (Okajima et al
1958).
b. Temperature. Low temperatures decrease the respiratory rate and retard
nutrient uptake. The percentage reduction in nutrient uptake caused by low
temperatures is not the same for different nutrients (Table 3.9). The inhibitory
effects of low temperatures on nutrient uptake follows the order: P 2 O 5 > H 2 O >
NH 4 -N > SO 4 > K 2 O > MgO > CaO. Note that this order resembles that for the
inhibitory effects of hydrogen sulfide on nutrient uptake.
High temperature also retards the absorption of NH 4 -N, P 2 O 5 , and K 2 O by rice
(Baba et al 1953).
3.4.4. Selective ion absorption
Changes in the solution pH provide information about the plant’s relative rate of
absorption among different ionic species. When a rice plant absorbs a cation from
solution, a hydrogen ion is released from the roots to maintain electrical neutrality
in the solution:
(3.9)
As a consequence, the solution pH goes down. When an anion is absorbed from the
solution, a bicarbonate ion is released from the roots:
(3.10)
The bicarbonate ion combines with a hydrogen ion to form undissociated carbonic
acid, freeing a hydroxyl ion:
(3.11)
Thus, the solution pH rises.
As shown in Table 3.10, the rice plant absorbs NO 3
-
faster than it does Ca 2+ ; as
a result, the solution pH increases. The rice plant absorbs NH 4
+
faster than it does