Tipburn is a severe problem in producing butterhead lettuce under artificial lighting and develops as a consequence of decreased calcium concentrations in leaves. Here, we investigated the effects of light intensity on tipburn development and calcium concentration in leaves by comparing their growth rates. Butterhead lettuce was grown in a plant factory under artificial light at photosynthetic photon flux (PPF) densities of 150, 200, 250, and 300 μmol·m−2·s−1. Fresh and dry weights of shoots, relative growth rate, the number of leaves, and the number of tipburned leaves significantly increased with light intensity. Associations existed between growth and tipburn occurrence. Calcium absorption rate per plant also increased with light intensity in association with increased water absorption rate. Consequently, calcium concentrations in the entire plant and outer leaves increased with light intensity. In contrast, calcium concentration in the inner enclosed leaves did not increase with light intensity. This pattern can be attributed to the higher mass flow of calcium to outer leaves than to inner leaves, driven by transpiration, under high light intensities. Thus, a lack of calcium in the inner leaves resulting from rapid growth may contribute to the frequent tipburn development.
Yuki Sago and Airi Shigemura
To reduce nitrate concentrations in vegetables grown under artificial lighting, we determined nitrate accumulation under various conditions of nitrate availability. Butterhead lettuce plants were grown with nutrient solutions of various concentrations, which were maintained according to electrical conductivity (EC) throughout the cultivation period. Under these conditions, growth in nutrient solutions with lower EC led to slight decreases in nitrate concentrations in leaves, but also decreased fresh and dry weights of shoots, leading to the risk of a yield loss under EC control management (ECM). By contrast, when total nitrate requirement was supplied only at the start of cultivation, nitrate concentrations in leaves were decreased significantly with only slightly reduced plant growth. Hence, marketable-sized butterhead lettuces with lower nitrate concentrations can be produced by supplying total nitrate requirements in the nutrient solution at the start of cultivation.