Wenshuo Xu¹, D.T.P. Nguyen¹, Shunsuke Sakaguchi², Takuji Akiyama², Satoru Tsukagoshi³, A. Feldman³ and Na Lu^3
1 Graduate School of Horticulture, Chiba University, Matsudo, Chiba, Japan
² PlantX Corporation, Wakashiba, Kashiwa, Chiba, Japan
³ Center for Environment, Health and Field Sciences, Chiba University, Kashiwa, Chiba, Japan

SUMMARY Plant factories or vertical farms with LED lighting have been developed to grow fresh and high-quality vegetables inside urban buildings without being restricted by climate and land. Environmental control technologies are developing rapidly for improving productivity of leafy vegetables in plant factories. However, tipburn issues are also rising with the increase of plant growth rate and this reduces the product quality and its marketable value. The tip-burned parts of the vegetable must be removed manually at harvest, which causes more yield loss and labor cost. In this research, the effects of light intensities with different total light integrals (TLI) or with the same TLI on lettuce growth and tipburn occurrence were investigated. In the first experiment, three light intensities: 85, 125, and 187 µmol m-2 s-1 were applied to the rapid growth stage of romaine lettuce plants; and in the second experiment, an average light intensity of 150 µmol m-2 s-1 was applied in four different lighting patterns to the plants. The results show that plant biomass clearly increased with increases of light intensity (or TLI) but was not affected by different lighting patterns under the same TLI. Leaf areas were not significantly affected by different light intensities under the present experimental conditions, indicating that plants firstly expand leaf area (rather than increase thickness) to increase light interception when light intensity is low. Tipburn occurred during 23~26 days after sowing and its occurrence was positively correlated with light intensity and relative growth rate (RGR). Regression analysis indicated that there could be a key threshold value in RGR which induces tipburn. To control an RGR value lower than the threshold value during 23~26 days after sowing would be critical for limiting tipburn occurrence. Keywords artificial lighting, lettuce growth stage, light intensity, RGR value, total light integral, indoor farming

Significance of this study What is already known on this subject? Tipburn issues reduce lettuce quality and its marketable value. Tipburn is usually rising with the increase of plant growth rate. Light is one of the most important factors affecting plant growth rate in plant factories. What are the new findings? By applying different light intensities at different growth stages of lettuce, plant growth rate will be regulated, thus the tipburn occurrence can be controlled. A key relative growth rate (RGR) threshold was indicated for controlling tipburn occurrence during 23~26 days after sowing. What is the expected impact on horticulture? This study will provide important timing and RGR value for growers who use plant factories with artificial lighting to limit tipburn occurrence.

E-mail: na.lu@chiba-u.jp  

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Received: 15 May 2019 | Accepted: 21 November 2019 | Published: 21 October 2020 | Available online: 21 October 2020