Pre-dawn supplemental LED lighting enhances growth and yield of hydroponic Brassica juncea L. (mustard greens)

Optimizing the timing of supplemental lighting is a key strategy for improving crop performance in controlled-environment hydroponics. This study evaluated the effects of light-emitting diode lighting timing on mustard greens grown in a recirculating hydroponic system. Four treatments were compared: a control (no supplemental light) and three 4-hour lighting periods (18:00–22:00, 22:00–02:00, and 02:00–06:00). The 02:00–06:00 treatment significantly increased soil plant analysis development values, leaf number, plant height, and leaf width, resulting in 44% higher fresh weight (79.78 g) and 52% higher dry biomass (1.25 g) than the control, with the highest leaf-to-total ratio (90.88%) and lowest stem ratio (4.60%). Multiple regression showed that leaf number and width were strong positive predictors of productivity, while height was negatively correlated. Principal component analysis identified the 02:00–06:00 treatment as optimal based on the first principal component score (2.69), whereas the 22:00–02:00 treatment showed the strongest etiolation based on the second principal component score (−1.58); the first two principal components together explained 72.68% of the total variance. These results demonstrate that late-night or early-morning lighting best supports leaf development and yield. Future research should combine lighting-time optimization with spectral and energy-efficiency analyses to refine smart-lighting strategies for commercial hydroponics.