A low-cost device for accurate and continuous measurements of fruit diameter

This work describes a fruit gauge based on a low-cost linear potentiometer interfaced to a data-logger, which allows continuous measurement of fruit diameter. The sensor is supported by a custom-built stainless steel frame designed to be easily applied to different size fruits. Highly significant linear relationships between tension (mV) and displacement (mm) have been found during sensor calibration. The average slope of this relationship, 5.4 · 10 -3 with a standard deviation of 5 · 10 -5 , was used as conversion coefficient. Temperature stability was tested by submitting both the sensor and the whole gauge to a temperature range of 5-60 8C. The maximum deflection found was ±1 mV. Measurements performed during the three developmental stages of peach (Prunus persica (L), Batsch.) fruit showed high sensitivity of the gauge which allowed clear detection of diurnal patterns of fruit diameter changes and precise monitoring of minute variations in fruit growth rates during 24 h. Fruit growth dynamics has received little attention over the years. Knowledge of the daily variations in fruit size in response to environmental and physiological conditions is highly desirable, but it is hampered by the technical difficulty of measuring fruit diam- eter changes accurately and at a sufficiently low cost to allow proper replication of the experiments. In recent decades, continuous and precise monitoring of stem and fruit diameter changes have been carried out by automatic gauges, most often custom-built by the authors for the specific studies they intended to carry out (Araki et al., 2000; Berger and Selles, 1993; Garnier and Berger, 1986; Jones and Higgs, 1982; Peramaki et al., 2001). However, either the high cost of the components or technical difficulties in con- struction could hinder their use both for research and practical purposes. The measurement of fruit diameter change is of great interest both for whole- tree and single-fruit physiology research and for commercial applications, as it may pro- vide useful information for orchard manage- ment. Several papers report fruit or stem diurnal shrinkages in response to environ- mental conditions and tree water status (Berger and Selles, 1993; Garnier and Berger, 1986; Jones and Higgs, 1982; Tromp, 1984). Studies on fruit growth mechanisms were made possible by precise monitoring of fruit diameter changes over time, such as the daily in/outflows of phloem, xylem, and transpiration to/from the single fruit in apple (Lang, 1990) and peach (Morandi, 2006). Likewise, models to manage orchard irriga- tion have been developed (Huguet, 1985) and patented, complete with field systems for automatic data collection and analysis (patent numbers: US2004088916; WO02084248; WO0235193; US20020170229). Many devices for accurate measurement of fruit growth have been developed in the past (Beedlaw et al., 1986; Higgs and Jones, 1984; Tukey, 1964). In most cases, a sensor, supported by a frame, is placed in contact with the epidermis of the growing fruit. Although supporting frames have been pro- gressively modified and improved in their shapes and structures, the most adopted sensors are either linear variable differential transducers (LVDTs) or strain gauges. LVDT technology, though precise, accurate, and stable under field conditions, is expensive; strain gauges are cheaper than LVDTs, but they require some very specific electronic sensor arrangements (Wheatstone bridge) and need to be mounted on flexible frames (Beedlaw et al., 1986; Link et al., 1998). Commercial units are also available, but their price seems to hinder their widespread adop- tion, as a large number of units would be needed for both research and orchard man- agement purposes. One aspect that bears considerably on the choice of sensor and of the materials used in constructing field probes is their ruggedness and accuracy under varying environmental conditions, including large temperature, precipitation, and air moisture changes. This is more so when the degree of accuracy required of the