Optimization of light and air temperature conditions to produce grafted transplants of fruit vegetables in a closed transplant production system with white LEDs

Abstract

Transplant production in plant factories has increased to overcome unfavorable environments, including high temperatures in summer and a lack of sunlight in winter. To successfully apply plant factory using artificial light (PFAL) for the production of high-quality grafted transplants of fruit vegetables should establish optimal environmental conditions. The effects of environmental factors, such as light intensity, light quality, photoperiod, and temperature, on plants have been studied individually to control plant growth in a closed cultivation system. However, a few studies have investigated the application of more economical white light-emitting diodes (WLEDs) and the combined effects of environmental factors on plant growth and morphology changes. In chapter one, lighting systems using WLEDs were established, and the effects of warm and cool WLEDs at various ratios on the growth of cucumber, tomato, and watermelon seedlings that would be used as scions and root stocks for grafted transplant production were examined. Four WLEDs were specifically manufactured for this study by adjusting the numbers of installed warm- and cool-white light chips: W1C0 (only warm-white light chips), W3C1 (warm-:cool-white light chips = 3:1), W5C2 (warm-:cool-white light chips = 5:2), and W1C1 (warm-:cool-white light chips= 1:1). The seedlings cultivated under treatment W1C1 had the shortest hypocotyls among the three tested vegetables. The hypocotyls of scions and rootstocks of tomato and watermelon were shortened in the order of W1C0, W3C1, W5C2, and W1C1, that is, in the same order with an increased proportion of cool-white light. The stem diameters were not significantly different except in tomato scions and watermelon rootstocks. In chapter two, the growth and morphology of tomato, red pepper, cucumber, gourd, watermelon, and bottle gourd seedlings that would be used as scions and/or root stocks for grafted transplant production were examined under different supplemental far-red-enriched WLED lights. The specifically manufactured WLED lighting fixtures consisted of different numbers of cool-white and far-red light chips adjusted at ratios of 5:0, 5:1, 5:2, and 5:3. The phytochrome photostationary state (PSS) values that resulted in the maximum hypocotyl length ranged from 0.69 to 0.77 in tomato and red pepper seedlings. Although the hypocotyl lengths of cucumber and watermelon seedlings were also greatly affected by PSS, the PSS values resulting in the maximum hypocotyl length were lower than those found in tomato and red pepper seedlings. This suggests that far-red-enriched WLEDs could be used to control the growth and morphology of vegetable seedlings with some variations among plant species and cultivars. In chapter three, the growth of tomato and red pepper seedlings was examined under different daily light integrals (DLIs) and day and night temperatures (DIFs) using a selected WLED lighting fixture. The seedlings were grown under different photoperiods with five different photon flux densities (PFDs) at an air temperature of 25/20°C. Increasing the DLI from 4.32 to 21.6 mol·m2·d1, either by increasing the photoperiod or PFD, improved seedling growth in both species. Under the same DLI conditions, tomato seedling growth was significantly enhanced with increased photoperiod and decreased PFD. Under higher DLI conditions, the reduced growth due to higher PFD indicated that excessive light energy was a limiting factor. The seedlings were also grown under three different air temperatures, 23/20, 25/20, and 27/20°C (photo-/dark periods), with five different PFDs. At 23/20 and 25/20°C, tomato seedlings showed a similar correlation between plant growth and an increase in PFD. At 27/20°C, however, the slope of the curve was flatter than those found in other treatments. On the other hand, red pepper seedlings showed similar correlation curves between growth and PFD at all tested air temperatures, and they accumulated more dry weight even at higher air temperatures. The results from this study provide more information and a deeper understanding of the growth characteristics of seedlings and better predict their responses to various cultivation environments, which is crucial for the production of grafted transplants of fruit vegetables in a closed system with artificial light, such as high-power WLEDs.