In the course of investigations on photosynthesistranpiration-CO2 concentration relations, cyclic changes of transpiration rate with approximately 30 minute periods were found under conditions of 0.75 cal⋅cm-2⋅ min-1 light intensity, 25°C air temperature and 65% relative humidity in normal CO2 concentration of air. The amplitude of oscillation was affected by wind speed and increased with increase of wind speed. The average transpiration rate increased with increase of wind speed and showed the maximum value at 150 cm·sec-1 wind speed.
In high CO2 concentration, the period and the amplitude of oscillation became longer and narrower respectively and the cyclic changes disappeared at about 3, 000 ppm, but when air temperature was raised to 30°C, the cyclic changes continued up to 20, 000 ppm.
The average transpiration rate reduced with increase of CO2 concentration and showed minimum value at about 1, 000 ppm. Over that concentration, the transpiration rate increased up to approximately 20, 000 ppm, and also in darkness, the transpiration rate increased over 1, 000 ppm. This phenomenon depended on air temperature, i.e., the depression of transpiration rate disappeared when air temperature was raised to 35°C. Similar phenomena were observed on pet-sai (Brassica pekinensis PUPR), cabbage (Brassica oleraces L., var. capitata L.), turnip (Brassica ropa var. Mana) and radish (Raphanus sativus L.) .
The causes of increase of the transpiration rate in high CO2 concentration were not clarified, but the transpiration rate was reduced by spraying ABA solution on the plants. This means that CO2 causes the stomata to open again in high CO2 concentration.