Tipburn, a leaf marginal apex necrosis, is a serious problem in vegetable production under controlled environments (Cox et al., 1976), such as in closed plant production systems equipped with artificial light (Son and Takakura, 1989). Tipburn is generally considered a calcium-associated physiological disorder (Bangerth, 1979; Thibodeau and Minotti, 1969). The necrosis results from the rupturing of laticifer cells (Olson et al., 1967), and develops as a consequence of decreased calcium concentrations in the leaves (Barta and Tibbitts, 1991; Struckmeyer and Tibbitts, 1965). Many researchers have reported that tipburn is associated with rapid growth rate and high calcium demand in leaves (Collier and Tibbitts, 1982; Gaudreau et al., 1994; Saure, 1998). The rate of plant growth can be increased by controlling the cultivation environment, and light intensity is an important environmental factor in such stimulation. In a plant factory, which is a closed system equipped with artificial light and a controlled environment system to produce high-quality crops all year-round (Kozai, 2013; Kozai et al., 2016), plants are continuously cultivated at a very high rate to maximize the total efficiency of the production process. The stimulation of growth is important for optimizing production (Higashi et al., 2015; Murase et al., 2015). In particular, extension of the lightning period to continuous lightning has been used to produce crops at a higher rate under artificial light (Goto and Takakura, 1988; Oda et al., 1989). However, rapid plant growth under favorable environmental conditions leads to decreased calcium concentrations in leaves. In plant factories, butterhead lettuce (Lactuca sativa var. capitata L.) is considered as an important vegetable because it can be produced with high yield and has a high market value. However, the rapid growth leads to calcium deficiency and tipburn in the enclosed leaves. Therefore, the development of tipburn is frequent in butterhead lettuce production in plant factories (Lee et al., 2013; Son and Takakura, 1989) and can result in severe economic loss.
There are numerous reports of methods to prevent tipburn development, such as the use of tipburn-resistant varieties (Cox and McKee, 1976; Koyama et al., 2012), foliar spraying of calcium salts (Thibodeau and Minotti, 1969), supply of air to inner leaves (Goto and Takakura, 1992), reducing the length of the light period (Tibbitts and Rao, 1968), shortening the day/night cycle (Goto and Takakura, 2003), and altering the spectral characteristics of the light source (Kleemann, 2004). However, although many factors have been implicated in tipburn development, simultaneous measurement of the growth rate and calcium concentration in the leaves is rare.
The objectives of the present study was to determine the relationship between light intensity and tipburn development, in relation to plant growth rate and leaf calcium concentration to minimize tipburn while maintaining rapid growth in a plant factory. Butterhead lettuce was grown in a plant factory equipped with artificial light and was subjected to different light intensities to change plant growth rate. In addition, the rate of calcium absorption by roots and calcium concentration in leaves were determined to characterize the dependence of tipburn development on leaf calcium concentration and plant growth rates at varying light intensities.
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