Studies on Matter Production in Wheat Plant : I. Diurnal changes in carbon dioxide exchange of wheat plant under field conditions

Abstract

Diurnal changes in CO2 exchange of wheat plant and the relationships of these diurnal changes to environmental factors were investigated under field conditions. CO2 exchange was measured by using an assimilation chamber and at the same time, the principal environmental factors, such as intensity of solar radiation, air temperature, soil temperature, air humidity, CO2 concentration of air, etc. were also measured simultaneously. These measurements were carried out at intervals of 2 weeks from the early stage of growth to the full maturation. The results obtained are summarized as follows: 1. The photosynthetic rate during the daytime varied generally corresponding to the solar radiation intensity. 2. Basing upon the relationship of photosynthetic rate to solar radiation intensity, the modes of diurnal changes in photosynthesis were classified into the following three types. Type A: Under this type, the photosynthetic rate in the morning is equivalent to that in the afternoon, provided that the solar radiation is much the same in intensity. Type B: Under this type, the photosynthetic rate in the morning is lower than that in the afternoon even if compared with each other under the condition with the same intensity of solar radiation. Type C: Under this type, the photosynthetic rate in the morning is higher than that in the afternoon, even if compared with each other under the condition with the same intensity of solar radiation. 3. In the depth of winter when it was critically cold in the night, the diurnal change of type B was observed. However, in case of warm night the diurnal change of type A was observed. These facts suggest that, on ordinary days in the depth of winter, the photosynthetic mechanism of wheat plant is inhibited by the intense cold during the night, while it is relieved from the inhibition due to the warmth in the daytime. 4. From the beginning of April onward, the diurnal change of type C was observed predominantly, though the diurnal change of type A was observed as well on such days when the solar radiation was not so intense. 5. In the transitional period from autumn to winter, the types of diurnal changes in photosynthesis were irregular. Immediately after the invasion of the first severe cold little or no CO2 absorption was observed during daytime in spite of fine weather with bright sunshine. 6. The dark respiration of wheat plant changed corresponding to the air temperature during the night irrespective of seasons or of weather conditions.