Vergara - 1976 - Physiological and morphological adaptability of ri

TENIPERATLTRE AND CHEMICAL KINETICS OF FLOODED SOILS 263
pot and actual field conditions, and it is necessary‘ to conduct experiments to fill the gaps between
the two conditions.
‘1".~\.\-1.~\NE2 I guess you use Pc-o: on the basis of the idea that CO; is poisonous. But I don‘t
think that CO; is harmful to the growth of rice plants.
Ponnomperrlma: Chang and Loomis and also Vlamis and Davis have shown that excess CO;
retards the growth of rice.
‘rZuraNc: Even with the same values of PCO_., the amount of total carbon dioxide varies
greatly with pH. Figure 2 gives us a different impression about CO; formed or present in the
soil and in the soil solution. I think it is suitable for this lcjnd of discussion to use total CO;
in stead of Poop. How do you feel about it?
Ponnampenlma: PCO; is a measure of H_CO.. As pH increases HLCO, decreases because of
l-ICU? formation. PCO: is preferable because it is a direct measure of l-I_.C(J,-_ which is believed
to be the harmful species.
T. YOSHIDAI Usually the increase in PCO; is accompanied by a decrease in P0, Concerning
the C0_ injury you mentioned in your paper. what is the relation between them’?
Ponmrmperirma; In upland soils Poo; and P0; may vary reciprocally. In submerged soils
P01 is virtually zero while PCO- may range from 0.1 to 0.7 atm. According to some plant physiologists.
excess CO.- ean injure plants even in the presence of c1.
T. YOSHIDAC I guess your values of NFL-N in Fig. 5 are thirty high. Did you add some
nitrogen source in your incubation experiment’? And did rice plants stand in this experiment‘?
If so. was not injury observed‘?
Ponnamperuma: The values are high but not uncommon in lowland tropical rice soils. No
nitrogen was added and there were no rice plants.
NIX: If the reactions described are biochemical. then presumably some of the differences
between soils from different regions are due to differences in soil microtlora. It would be interesting
to seed sterilized soil with cultures from different soil sources in order to examine this
question. It could be expected that rnicrotlora from different climatic environments would
have different thermal requirements for maximum activity.
Ponnamperuma: ‘res. some of the differences may be due to dittcrenccs in mieroflora. But I
would add that some soil properties such as organic matter content. kind and content of clay‘,
and presence of enzyme inhibitors or activators. etc.. could also influence the kinetics.
T. YOSHIDAI Dr. Nishiyamas presentation this morning indicated that temperature affects
the root development and activity of rice. l|1 your experimental results under greenhouse conditions.
the direct temperature effect on rice roots. other than chemical kinetics in soil. might be
a major factor in the poor rice growth and yield at the temperature below 20°C or at 4ST.
Ponnamperuma: There is no question that temperature affects root metabolism. but it may
not be the major factor.
GHILDYAL: D0 you think that nitrification inhibitors in ceflain soils are responsible for
reducing nitrogen losses?
Pomwmperuma: Yes. in test tubes and pots, not in the field.
T. ‘rostrum: You discused various chemical factors to account for the detrimental effect on
rice growth at certain temperature regimes. Do you have any figures on the quantities of these
chemicals in soil that were detrimental to rice growth under your experimental conditions such
as the rice variety that you used?
Pormamperztma: I agree. But plants grew in ltilaahas clay" both at 15"‘ and 45°C. while on
other soils they died. So there is a temperature-induced soil effect in addition to the direct
effect of temperature on the roots. Yes. there are varietal differences in tolerance to injurious
soil conditions. The levels that physiologists have given for toxicities of CO, organic acids, or
iron serve only as a guide.
PHUNG: If I saw your first slide correctly, the Eh's were markedly different among temperature
treatments.
Ponnamperuma: No. only at 38°C was the Eh markedly different from those at 15'“. 20"‘. and
30°C. This may be due to the higher rate of O_ consumption at 38° than at the other temperature
during the hour or tvro that elapsed between submergence and Eh measurement.
PHUNG: In Fig. 4. is NO,-'\' (ppm) expressed on a soil basis or as nitrate-N in the soil solulion‘?
Pormamperirma: NO; is ppm N in the soil solution.