Leaf area model based on thermal effectiveness and photosynthetically active radiation in lettuce grown in mini-plant factories under different light cycles

Abstract

To establish an effective leaf area model in a mini plant factory with artificial lighting (mini-PFAL) environment, we conducted three experiments exposing lettuce plants to different light cycles. Three treatments were set up in the first experiment: 12-h/12-h (light/dark), 6-h/6-h (light/dark), and 3-h/3-h (light/dark). We analyzed the dynamic changes in the lettuce leaf area under different light cycles in a mini-PFAL and established a leaf area model based on the accumulated product of thermal effectiveness and photosynthetically active radiation (TEP) method. Data from two subsequent experiments were used to validate the lettuce leaf area model and uncover the difference in plant growth caused by different light/dark cycle patterns from a physiological perspective. Results show that the TEP and light cycle influenced lettuce leaf area significantly. Under 12-h/12-h (light/dark) treatment, lettuce growth was better than in 6-h/6-h (light/dark) and 3-h/3-h (light/dark) conditions within the same TEP. The light cycle appeared to significantly affect the leaf area as well as morphogenesis. The shape of lettuce leaves was slenderer and the leaf angle was steeper under the longer light cycle. We believed the different illumination durations in a single light cycle altered the functional relationships between phytochromes and led to different plant shapes. At the same time, the plant shape caused different light interceptions that affected the growth and leaf areas. A relative longer light period (6-h light/6-h dark) increased leaf stomatal conductance, net photosynthetic rate, and plant growth, compared to those of a shorter light period (3-h light/3-h dark). Based on these results, this study provides useful insights on regulation strategies related to light cycles and their effects on lettuce growth.