Vergara - 1976 - Physiological and morphological adaptability of ri

262 CLIMATE AND RICE
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DISCLTSSION
A. TANARa (Chairman): The paper of Dr. Ponnamperuma demonstrates to us how complicated
are the interactions among chemical kinetics of flooded soils. nature of soils. and
temperature. We now have substantial evidence from his paper that various processes of
flooded soils are influenced by soil temperature, and that the changes of kinetics caused by an
increase or a decrease of temperature are favorable to some processes and unfavorable to
others.
1 would like to add that if we consider the interactions between plant and soil, the influence of
temperature on plant growth is much more complicated. Soil temperature. as he mentioned. has
direct influence on the physiology of roots: tolerance of the roots to some harmful substances,
and their ability to absorb nutrient elements. arc affected by temperature. This direct effect
results in difTerences in growth of the shoot as well as the roots. Such differences will have
secondary effects on the condition of the soil. The interaction of direct and indirect effects will
produce cumulative effects on the growth process. We should also remember that soil temperature
is not independent from air temperature, fluctuates seasonally and diurnally. and also
has a gradient liom topsoil to lower soil.
He concluded that soil temperatures lower than 25°C or at 45“C are chemically tlnfavorable
for the growth of rice on flooded soils. and both growth and Avield of rice were best under the
35°C and 38° " 20°C regimes. This result is very intlmnativc, however. 1 feel that it is somewhat
difficult to understand from his result why" we can get a good yield ofriee in the northern pan of
Japan where soil temperatures are lower than 25°C during most stages of growth.
Pot experiments are indispensable in obtaining fundamental knowledge, because we can
have perfect control of various thetors. Hotvcvcr. there are certain critical differences bcttveen