Studies on Matter Production of Rape Plant : IV. Diurnal changes in apparent photosynthesis and dark resiraton of rape plants under field conditions

Abstract

Diurnal variations in CO₂ exchange of rape plants were observed over 2-4 successive day at about two-week's intervals from their early growth to full maturity. The measurement of CO₂ exchange was conducted using an assimilation chamber in which conditions were regulated as similar as possible to those of the outside. Principal environmental factors, i.e., total solar radiation, diffused solar radiation, air temperature, soil temperature, air humidity and CO₂ concentration of air were recorded simultaneously writh CO₂ exchange. From the data obtained, dependence of the CO₂ exchange rate on the environmental factors was examined. In addition, the after-effects of low night temperature on daytime photosynthesis, the existence of which had been suggested by the result of field observations, were examined under controlled conditions. The results obtained are summarized as follows: 1. The photosynthetic rate depended primarily on total solar radiation throughout daytime on all the days when observations were conducted. 2. Based on the mode of correspondence between the photosynthetic rate and total solar radiation at each time of a day, patterns for diurnal variations in photosynthesis were grouped into the following two types. Type A: In this type, photosynthetic rates in the morning were nearly equal to those in the afternoon provided they were compared at equivalent solar radiation levels. Type B: In this type, photosynthetic rates in the morning were lower than those in the afternoon even compared at equivalent solar radiation levels. 3. The pattern of Type A was observed on cloudy days, or on fine days whose preccding night was not so cold or warm. The pattern of Type B was observed on fine days preceded by a very cold night. 4. Type B predominated in winter while Type A prevailed in other seasons. 5. The dark respiration rate at night varied mainly with air temperature at each stage of growth. 6. It was confirmed in the experiment under controlled conditions that the photosythetic rate in the daytime was affected by the air temperature of the preceding night. Night temperatures below a certain limit brought about depression of photosynthesis during the succeeding daytime. The lower the night temperature, the lager the extent of depression and the longer time required to recover from the depression.