Relation between relative growth rate and tipburn occurrence of romaine lettuce under different light regulations in a plant factory with LED lighting

摘要

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 tight intensities: 85, 125, and 187 mu mol 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 mu mol 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 similar to 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 similar to 26 days after sowing would be critical for limiting tipburn occurrence.