Identifying and Modelling the Dynamic Response of Leaf Water Content to Water Temperature in Hydroponic Tomato Plant

Hydroponic culture techniques have several potential advantages over soil culture techniques for cultivation, e.g., technical ease of flexible control of the root-zone environment (Gale and Ben-Asher, 1983; Raviv and Lieth, 2007). Promoting growth and producing high quality plants can also be expected through optimal control of the root-zone environment. In order to realize an effective control such as an optimal control, it is important to make a dynamic model of plant response to root-zone environment and then understand the dynamic and static characteristics between these two variables in more detail. In the root-zone environment in hydroponics, water temperature is one of the major manipulating factors for controlling plant growth and development, because it is easy to control using a heater and a cooler. Optimal control of water temperature brings about good fruit ripening with no reduction in growth or fruit set. Ikeda and Osawa (1984) reported that root temperature is an important factor that directly affects plant growth. Root temperature also affects many physiological processes such as respiration (Jensen, 1960), water absorption (Unger and Danielson, 1967), nutrient uptake (Hussain and Maqsood, 2011) and water movement and transpiration (Gray, 1941). Davis and Lingle (1961) elicited increased growth with warmed roots (25 30°C) and decreased growth when roots were cooled below 15°C (Martin and Wilcox, 1963). Although water temperature has been shown to affect plant growth, the physiological basis for the dynamic response in controlling the plant production system has not been thoroughly investigated. Leaf water content, on the other hand, is one of the most important control factors for optimizing growth in plants, because it significantly affects both the quantity and quality of plants (Nonami, 1998). It is, however, difficult to directly and continuously measure the dynamic change in leaf water content of an intact whole plant, without damaging the plant. Leaf thickness is used as a sensitive indicator for estimating leaf water content of plants. Meidner (1990), Syverrtsen and Levy (1982) and Búrquez (1987) used indirect methods for monitoring water status based on using displacement transducers to measure swelling and shrinkage in a wide range of plant tissue such as in leaves. In this study, therefore, the leaf water content was estimated from leaf thickness. An eddy current-type displacement sensor allows the leaf thickness to be measured in a continuous and non-destructive manner. Many researchers have modelled the process of water movement in plants, including root water uptake, using mathematical approaches (Gardner, 1991; Roose and Fowler, 2004; Doussan et al., 2006; Foster and Miklavcic, 2013). They applied mathematical equations to build models of the static relationships between input factors and output factors. However, it is thought that modeling the dynamic behaviors of the physiologicalecological proc-