Abstract
The various causes are considered of the midday depression of plant photosynthesis; the condition that the plants are grown under the extremely high temperature and the low carbon dioxide concentration of air in the plant community at midday is accepted as a cause of it.
The present paper discussed theoretically the midday depression of photosynthesis from the point of view of the diurnal variation of absorptivity of incoming radiation in the plant community.
Under the cloudy sky, that almost all of incoming radiation is the diffused light, the absorptivity of radiation of the plant community is nearly constant in a whole day, So, the total photosynthesis of foliages of a plant community will be proprtioned to the intensity of incoming radiation.
Under the sunny sky, that almost all of incoming radiation is the direct light and the path lengthes of the radiation within the plant community are varied with the solar angles, the absorptivity of radiation of the plant community changes diurnally and differs with the growth stages of plant Particularly at the middle stage of growth the absorptivity shows the remarkable diurnal variation which has the minimum. value at noon, Therefore the total photosynthesis of the foliages of the plant community decr eases around midday.
The conclusion seems to be quite reasonable in compared with Murata's data which observed the diurnal variation of photosynthesis through the various growth stages of paddy rice.
