No abstract

A new type of water stress sensor for plants is recommended for appropriate water irrigation. This sensor simply consists of a strain gauge (base dimensions of 3.5 mm width × 8.5 mm length) and a thin sheet of neoprene rubber (rubber hardness: 65). The sensor is wound around a plant stem with suitable tension and can measure a physical quantity corresponding to the change in circumference of the plant stem. In this paper, the sensor output characteristics (the relationship between the change in the sensor output and the change in the length of rubber) upon loading and unloading were investigated in experiments using a universal testing machine. The mechanical behavior of the sensor output upon loading was different from that upon unloading in response to the change in length of the rubber. The sensor output characteristics were unaffected by the rubber width (varying from 5 to 10 mm) and the length of rubber (varying lengths above 20 mm) but were strongly affected by the initial tensile tension. Sensor sensitivity increased with the initial tension according to a quadratic function. The characteristic differences between the sensor and a conservative diameter sensor were experimentally clarified by measuring changes in the stem of a citreous tree over a period of a week. The tendency of the behaviors of the outputs of the two sensors accorded well (correlation coefficient of 0.95). Consequently, it is shown that the sensor can measure simply and finely the changes in diameter of a stem and is available for estimating plant water stress.

Effects of sheet mulching on auraptene (AUR) and polymethoxyflavone (PMF) contents in the peel of mid-late maturing citrus 'Kawachibankan' (Citrus kawachiensis hort. ex. Y. Tanaka) fruit were investigated. Six vigorous nine-year-old trees were used, and three of them were mulched with a permeable plastic sheet on the ground from September to January. The results showed that no significant difference was observed in fruit development between mulch and control. Mulching cultivation caused higher rind color a* and b* values than in the control. Soluble solid content in the juice of mulched fruit markedly increased after treatment, which retained higher levels even in May. The juice citric acid content in treated fruit was also higher than in untreated fruit until the end of March, although no significant difference was observed thereafter. Peel AUR in mulched fruit showed a significantly higher content in December and April compared with the control. Peel AUR in whole fruit had the highest content in April. The juice AUR content in mulched fruit was also higher than in the control in April and June. No difference in the PMF content in peel was found between the treatment and control. Mulching cultivation caused rapid increases in total sugar, glucose, and fructose contents in the peel in October. These results indicate that peel AUR in whole fruit had the highest content in April and May, whereas mulching cultivation is effective to increase the amount of AUR in the peel and juice, suggesting that these were caused by drought stress and cold temperature.