Vergara - 1976 - Physiological and morphological adaptability of ri

260 CLINIATE ma) RICE
Luisiana clay and least in Pita clay loam, suggesting an association between iron
reduction and the release of silica.
Water-soluble sulfate
The two temperature regimes with a high initial temperature favored sulfate
reduction in all soils, but low temperature retardation was least in the soils high
in organic matter (IRRI. 1968a). After 6 weeks’ submergence no sulfate was
detectable in any soil, regardless of temperature in the range 15° to 38°C (Cho
and Ponnamperuma. 197 1 ).
Reducing substances
The concentration of substances in the soil solution that reduce potassium
pennanganate may be regarded as a measure of the reduction stress on rice
roots (Ponnamperuma. 1965).
Temperature markedly influenced the kinetics of reducing substances (IRRI,
1967). The concentrations were highest and remained high longest at 45°C in
all soils in contrast to those of water-stiluble Fez‘; at other temperatures they
ran parallel to water-soluble Fe“.
High concentrations of reducing substances may be one cause of physiological
disorders of rice attributable to high soil temperature.
IMPLICATIONS FOR RICE
The chemical kinetics of flooded soils indicates that the chemical environment
in the root zone is an important factor determining the influence of temperature
on the groxvth and yield office.
A soil temperature lower than 25°C is chemically unfavorable for the growth
of rice on flooded soils because (l) it slows down the release of ammonia. phosphate.
and silica. (2) it retards the increase in pH of acid soils, and (3) it favors
the later build-up of high and persistent concentrations of carbon dioxide,
volatile organic acids. reducing substances. and xvater-soluble iron. The unfavorable
effects of low temperature are most severe on acid soils and least in
neutral soils. The adverse effects of a low initial temperature are reduced by a
later increase in soil temperature. High initial temperatures (30° to 38°C) hasten
and shorten the peaks of water-stiluble carbon dioxide, organic acids, and
water-soluble Fez’. and accelerate the release of ammonia. phosphate. and
silica. So such temperature regimes should favor the growth and yield of rice.
Four greenhouse experiments with a total of l6 soils at 15°. 25°, 35°. and 45°C
or at 38° " 20°C. 30°C, 18° " 31°" 20°C. and 20°C showed that growth and
yield ofriee were best at 38° " 20°C (Fig. 8) and at 35°C; it was worst below 20°C
and at 45°C. The ill effects of a low soil temperature were more severe in acid
soils than in neutral soils and were partially counteracted by prolonged submergence
and by the addition of nitrogen and phosphate fertilizer (IRRI. 1968a).