Abstract
In this paper, is discussed a new method of electronic measurement of water content in plants by means of high sensitive impedance meter. The impedance meter was designed to measure stem-water in plants with a low impedance (capacitance; 10pF∼40pF, resistance; 2kΩ∼15kΩ). The water content is measured by capacitance and the concentration of solution in water is measured by resistance. The most important qualities required of the impedance meter are the sensitivity to capacitance and resistance changes, the stability during a continuous operation for long hours and the insensitivity to any other change in the plant. In the equipment, the capacitance output is insensitive to a resistance change and the resistance output is insensitive to a capacitance change. The measuring range of capacitance is about 0∼100pF and the measuring range of resistance is about 1kΩ∼∞. The maximum resolution of capacitance is 10-3pF and the maximum resolution of resistance is 10-2μ_??_. The stability was observed to be within less than 4mV in terms of zero drift during 24 hours' continuous operation. In order to apply the equipment to plant physiological research, responses of tobacco plant to light inputs, humidity inputs and temperature inputs were examined in a computer controlled growth cabinet. As a result.
(1) By a step input of light (30, 000lux) at the air temperature of 25°C and humidity of 60% R.H., the change of water content in stem was about 1pF in capacitance. (2) By a step input of humidity (60% R.H._??_80% R.H.) at the same temperature and light on, the change of water content in stem was about 1pF in capacitance. (3) By a step input of temperature (25°C_??_30°C) at the air humidity of 60% R.H. and light on, the change of water content in stem was about 0.5pF in capacitance.
The reliability of the relation between the capacitance determined by these results and the transpiration from a usual method was examined by digital simulation. It was made clear that the equipment was a useful instrument in measuring water contents in plants.
