Optimizing Light Environment for Pakchoi in Plant Factories: Interactive Effects of Photoperiod and Light Intensity on Growth, Photosynthesis, and Energy-Use Efficiency

The light environment is a key factor in regulating crop growth and quality in plant factories, where both light intensity and photoperiod strongly influence photosynthetic productivity and energy consumption. This study aimed to elucidate the interactive effects of light intensity and photoperiod on the growth, photosynthetic performance, and energy-use efficiency of Pakchoi in a controlled environment, thereby optimizing lighting strategies. Here, three levels of light intensity (PPFD: 100, 175, and 250 μmol·m−2·s−1) and four photoperiods (8, 12, 16, and 20 h·d−1) were combined, resulting in twelve treatments. Plant growth parameters, chlorophyll content, gas exchange indices, CO2 response curves, and chlorophyll fluorescence characteristics were measured, along with analyses of light-use efficiency (LUE) and electrical energy-use efficiency (EUE). The highest biomass accumulation was observed under a 20 h·d−1–250 μmol·m−2·s−1 treatment. In contrast, the optimal LUE (9.69%) and EUE (4.98%) were observed under a 20 h·d−1–175 μmol·m−2·s−1 treatment. The best photosynthetic performance (Amax 32.61 μmol·m−2·s−1) occurred under a 16 h·d−1–250 μmol·m−2·s−1 treatment. This study integrates growth, photosynthetic physiology, and energy-use efficiency, revealing a trade-off between biomass production and energy utilization in Pakchoi cultivation. It clarifies that “moderate light intensity + long photoperiod” is the optimal strategy to balance yield and energy consumption in plant factories.