Effects of Day Length and Air Temperature Fluctuation on the Occurrence of Leaf Browning in Sesame Seedlings Cultured in a Plant Factory under Artificial Light

Abstract

Sesame (Sesamum indicum), one of the most important oil seed crops, is widely cultivated in Asia and Africa, especially in the high temperature regions (Anilakumar et al., 2010), and its seed is also utilized as a cooking ingredient around the world. The seeds contain the functional component sesamin, a type of lignan. Sesamin is reported to possess health benefits such as a cholesterol-lowering effect (Ogawa et al., 1995; Hirata et al., 1996), preventing high blood pressure and increasing vitamin E supply (Yamashita et al., 1992; Kamal-Eldin et al., 1995). Recently, sesamin has also been found in sesame leaves, and although the content is less than 1/5000 that in seeds (Hata et al., 2010), it has been found that cultivation using plastic pot filled with commercial soil-mix under continuous lighting at constant 28°C increases sesamin content in the leaves up to 71.5 mg gDW , about 30 times compared to plants cultured under photoperiod of 12, 16 and 20-h with fluorescent lamp (Hata et al., 2012). Plant factories, especially the enclosed type using wholly artificial light, create the optimum cultivation conditions for year-round production of vegetables because all environmental factors can be managed precisely. Furthermore, plant factories are ideal for producing vegetables with enhanced functionality (such as high antioxidant activity), since “unnatural” environments (such as continuous lighting and minus value of difference between day and night temperature) can be provided. However, since managing and operating fully enclosed plant factories is expensive, it is necessary to cultivate high value crops (such as highly functional vegetables) to recoup the costs (Shimizu, 2014). On the above evidence, we considered that sesame seedlings could become a new functional vegetable crop with high sesamin content in the leaves if cultivated in a plant factory under artificial light. Thus we tried to cultivate sesame plants hydroponically in plant factory under the environment of temperature (constant 28°C) and photoperiod (24-h) reported by Hata et al. (2012). However, we recognized that leaf browning was a problem in sesame seedlings cultured hydroponically in a plant factory, progressing from lower leaves to upper leaves (unpublished). Thus, it is necessary to establish the means to avoid the occurrence of leaf browning of sesame cultivated in plant factory. In this study, we investigated the effects of photoperiod and temperature regimes, based at 28°C of average