In the recent years, the plant factories have been developing very fast in the world due to the incomparable advantages, such as labor-saving, pesticide-free, clean, safe and annual planned leafy vegetables production with vertical multi-layer cultivation systems. However, because of the faster growth rate and the weaker transpiration rate of the newly developed leaves, the vegetables in plant factories are very susceptible to tipburn caused by calcium deficiency, and therefore the appearance and nutritional qualities of leafy vegetable are significantly reduced.

In this study, the effects of different relative humidity levels on the tipburn injury, growth and nutritional qualities of hydroponically grown lettuce were investigated. The objective of this study was to find out the optimum relative humidity levels for preventing or alleviating tipburn occurrence of vegetable in plant factories. Furthermore, this study also could provide the technical and theoretical support to prevent the tipburn occurrence, and improve vegetable quality and economic benefits of plant factories.

‘Tiberius’ lettuce, which was susceptible to calcium deficiency, was used in this experiment. Three different air relative humidity levels of 50% (RH50), 70% (RH70) and 90% (RH90) were set up during the light period (16 h), while the relative humidity was maintained at 70% during the dark period (8 h). The other environmental factors, such as light environment, air temperature and carbon dioxide concentration, were kept constant during this study. The tipburn occurrence, yield, photosynthetic parameters and nutritional quality of the lettuce plants were investigated.

Compared with RH70 and RH90, the calcium content in the newly developed lettuce leaves plants under RH50 was increased significantly, the percentage of lettuce plants with tipburn injury was decreased while harvesting, and the initial tipburn occurrence time was postponed significantly. The stomatal conductance and transpiration rate of lettuce plants increased significantly, which in turn enhanced transpiration and accelerated the absorption and transportation of Ca²⁺, under the lower relative humidity. The photosynthesis rate and fresh weight of lettuce plants were not significantly affected by different relative humidity levels. Moreover, the lower relative humidity also improved the nutritional quality of lettuce plants, including improving the contents of starch, ascorbic acid, and soluble protein.

The high humidity environment in a plant factory was also one of the reasons for the frequent tipburn occurrence of lettuce plants. Based on the above analysis, it could be concluded that the tipburn occurrence of lettuce plants could be alleviated, the nutritional and commercial qualities of lettuce plants could be improved without affecting its yield and photosynthetic capacity by appropriately controlling the air relative humidity in a plant factory.