Abstract
In earlier research, we developed a system to measure the volumetric water content in the branches and trunks of satsuma mandarin (Citrus unshiu Marcow.) by portable Time Domain Reflectometry (TDR). However, there were some problems, such as temperature dependency or the unevenness of the values at the beginning of the measurement because of errors in the insertion of the probe. Therefore, in this research, a revised equation was used to adjust for the temperature dependency; and to remove the unevenness of the values, a relative evaluation based on the annual point time at which the stem water content became stable was examined. Concerning the revised equation, we recognized a strong correlation between the temperature and the TDR value through which we obtained the revised equation. During the measurement period, from early and mid-July to mid-September, when the temperatures were above 30 degrees and the soil was wet, the standard value was determined to be the point at which the stem water content reached an annual peak. Thus, the relative value of TDR (Rrev) could be fixed, and the correlation between that value and the leaf water potential (LWP, φmax) was demonstrated month by month. The correlation coefficient in July was 0.888, and that in August and September was about 0.7, but, in October and thereafter, the coefficient decreased to 0.435. However, the TDR value converted into volumetric water content (VWC) maintained its correlation coefficient with LWP 0.3 and under every month. The predicted value of LWP from Rrev and temperatures showed a strong correlation (r2 = 0.712) with the observation LWP value. In this way, the method used in this research seemed to be able to measure the water status of the stem with high precision.
