Abstract
The present paper deals with the effects of water deficits on physiological activities. Sunflower plant was used as the experimental material. The plant was grown in a glasshouse without temperature control. When 5 to 6 weeks old, the plant was transferred to a controlled growth cabinet for experiments. The results of the experiments can be summarized as follows :
1) It is difficult to measure the absolute internal water status of plants directly and continuously. For that reason, an apparatus was built to estimate the relative internal water status of the plant. The apparatus which enables the measurement of changes in the diameter of the stem has been developed by the use of an eddy current element. This apparatus has no contact with the plant surface, so that it can measure the changes in the stem diameter without interfering with the conditions of the tissue surface.
2) Immediately after the plant had been transferred from the glasshouse to the growth cabinet, oscillations of both the stem diameter and the leaf temperature were induced. The periods of the two oscillations were of the same length, but there were 7 min time lag between the peak of the osillations of leaf temperature and that of the stem diameter. This time lag was not observed between petiole and stem ca. 50 cm below the petiole. Therefore, it can be concluded that the resistance to water movement in the xylem vessels of the stem is relatively small compared with the resistance in the other parts.
3) When a high status of internal plant water deficiency was induced and then the plant was watered, the stem diameter increased to the original value within a few hours, however the air-leaf temperature difference did not recover for a long time.
4) Immediately after rewatering, the respiration rate of the roots decreased by about 30 percents compared with the control plant. However, the respiration rate had recovered to a normal value not later than the following day.
5) Abscisic acid (ABA) in leaves was estimated by electron capture gas chromatography. ABA content increased exponentially with increasing water deficit in leaf. After watering, ABA decreased exponentially with time. The decrease-rate of ABA was very small, and a half-life period of it was 17 hr in this experiment. It is considered that the large after-effect of water deficit on transpiration rate have close connections with the ABA created in plants.
