Effect of air temperature during light and dark periods on the iridoid glycoside content of Hedyotis diffusa.

Plant factories are year-round production systems that produce a high-quality, safe and steady supply of vegetables in a technologically controlled and automated environment. These systems are costly and consume considerable energy, which lowers their profitability. However, this weakness can be improved by cultivating medicinal plants which are one example of value-added crops. Since medicinal plants are used for medicinal purposes, high quality and safety are required. However, Higashiuchi et al. (2016) previously stated that medicinal plants are imported from foreign countries and quality and cost of the medicinal plants are not stable. Therefore, there is a real demand to promote the domestic cultivation of these plants by an improvement of cultivation technique. Hedyotis diffusa (family Rubiaceae) is used in Chinese herbal medicine to treat cancer, urinary infections, appendicitis and bronchitis (Takagi et al., 1982). This species is naturally distributed in the subtropical and tropical zones of East Asia, and its availability depends on the harvesting of wild crops. However, H. diffusa is cultivated with other similar cultivars (Lin et al., 1987; Wee and Keng, 1990), which can result in contamination and thus a lower-quality product. Therefore, it is considered that the cultivation of this crop in a plant factory would be beneficial in order to prevent from the contamination. However, to allow the year-round cultivation of high-quality H. diffusa in plant factories, it is first necessary to analyse the effects of environmental factors on its growth and the biosynthesis of its medicinal properties. The main medicinal properties of H. diffusa is the iridoids, which exist in the form of iridoid glycosides. It is considered that iridoid glycosides exist in the vacuole or cytoplasm of plant cell (Kamata, 2009). It has been reported that H. diffusa contains various types of iridoid glycosides and each of them has a unique medicinal effect (Rui et al., 2016). The Iridoid glycosides begins with glucose (Shioi et al., 2009; Takaishi et al., 2010). Therefore, it is expected that an increase in glucose output through the promotion of photosynthesis will improve the synthesis of iridoid glycosides. Photosynthesis is affected by several environmental factors, such as light intensity, humidity, CO2 concentration and temperature. Higashiuchi et al. (2016) previously showed that a high light intensity and long light period promotes the production of asperuloside, which is one kind of iridoid glycoside. However, Martz et al. (2009) found that high temperatures had a negative effect on the synthesis of iridoid glycosides in Menyanthes trifoliate L. leaves, indicating that a high production of glucose at high temperatures is not directly correlated with high levels of iridoid glycoside synthesis. In a preliminary study, Mori et al. (2015) analyzed the effects of three different air temperatures on plant growth