An accurate and stable measuring system for net photosynthetic rate is developed, which is a semi-closed system and can be applied for a measuring period longer than a month. In this system, the flow rate of CO2 gas supply to the assimilation chamber is controlled to be equivalent to the measured absorption rate by the plants (Fig. 1). The mass-flow-controller is used for the flow controller, which is reconganized as one of the most accurate flow controllers. CO2 analyzers are used to make a feedback roop by detecting the deviation of CO2 gas supply from the absorption by the plants. Because CO2 gas supply from ventilation air is less than 1% of CO2 gas absorped by plants, the effect of measurement errors caused by CO2 analyzers and air flow meter is negligible. If the zero point which causes the systematic error of measured values is checked frequently, this compentation method can easily achieve a high accuracy and precision.
A new data analysis method for CO2 analyzer outputs is also developed. The signal-to-noise ratio is improved by averaging the outputs excluding outlying ones. The photosynthetic rate is calculated by the eq. (3) or (4) from sampled values obtained by this method. Especially, eq. (4) is thought to be able to estimate more precisely the photosynthetic rate from the precise estimation of the CO2 concentration change in the assimilation chamber (Fig. 3).
The averaging time for the sampled values is selected to 10 seconds from Fig. 3. The sampling period to estimate the photosynthetic rate is determined to 7 minutes from Fig. 2, considering the noise level and frequency of the feedback. At this time, the errors given by the 2nd term of eq. (3) are nearly equal to that of the 3rd term.
An example of measured photosynthetic rate of tomato plants by this system is shown in Fig. 4.
The coefficients of variation of photosynthetic rate are shown in Table 4. These coefficients range from 1 to 3% in daytime. The larger values in the early morning are caused by the various operations for the cultivation and adjustment of instruments. From these results, this system is thought to be able to achieve the high accuracy and stability of measurement.