Vergara - 1976 - Physiological and morphological adaptability of ri

252 CLINIATE AND RICE
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Weeks submevued
2. Influence of temperature on the kinetics of P»... in three soils.
Cho and Ponnamperuma (1971) found that temperature regimes markedly
affected the kinetics of Ppq: in eight submerged soils. In almost all soils, the
38°a 20°C regime produced the lotvest peaks and the smallest overall Pm:
during the 14 weeks ofsubmergence (Fig. 2). In the 30°C regime. the PM values
were slightly-higher than in the 38°" 20°C regime. The 20°C and 15°" 31°" 18°C
regimes gave higher Pm: values than the two high temperature regimes. after 2
weeks" submergence. The differential effect of temperature regime xvas most
pronounced in the sandy loam from Korea (plI: 4.8; organic matter: 1.5%;
free Fe: 0.4%) and least in Maahas clay (pH : 6.6; organic matter: 2.0%; free
Fe; 2.2%). A high temperature during the first 2 weeks of stibmergence prevented
the build-up of high concentrations of carbon dioxide; a low initial temperature
favored it.
Cho and Ponnamperuma (1971) suggested that excess carbon dioxide was one
of the factors retarding the growth of rice on cold acid submerged soils.
Volatile organic acids
The volatile organic acids. formic, acetic. propionic, and butync, are intennediate
products in the decomposition of organic matter i11 submerged soils. The
kinetics of volatile organic acids in eight submerged soils (IRRI, 1968b; Cho and
Ponnamperuma. 1971) under four temperature regimes showed that low" temperature
led to a strong accumulation of organic acids, as Yamane and Sato
(1967) had found.
In all soils, at all temperatures, the conceittratitm of water-stiluble ttolatile
acids increased on flooding, reached a peak. and then decreased to practically
nothing (Fig. 3). The 38%| 20°C regime produced the earliest and smallest peaks.
In the 30°C regime the peaks were higher and broader than in the 38% 20°C.
The heights and breadths of the peaks were generally greatest under the 20°C