Influence of Red–Blue LED Spectra on Growth Dynamics and Phytochemical Enrichment of Chinese Kale (Brassica oleracea var. alboglabra) in Hydroponic Vertical Farming System

The rapid pace of global population growth and urbanization continues to deplete arable land, emphasizing the urgent need for sustainable food production systems. This study examines the effectiveness of hydroponic vertical farming systems (HVFSs) equipped with light-emitting diode (LED) technology in cultivating Chinese kale (Brassica oleracea var. alboglabra). Specifically, it evaluates the impact of three distinct LED treatments—white LEDs (WL), 20% red + 80% blue (20%RL:80%BL), and 80% red + 20% blue (80%RL:20%BL)—on morphological development and phytochemical accumulation.

Plants were grown under controlled conditions (pH 5.8–6.5, EC 2.0–3.0 dSm⁻¹) and assessed at weeks 2, 4, and 6 post-transplanting. The 80%RL:20%BL treatment yielded superior fresh weight, leaf area, root length, and dry biomass, indicating that higher red light proportions significantly enhance morphological growth (P<0.05). In contrast, the 20%RL:80%BL treatment resulted in the highest chlorophyll (53.60 mg/g), anthocyanin (11.30 units), and total phenolic content (1.46 mg GAE/g), suggesting blue-light dominance improves phytochemical profiles.

Interestingly, leaf number and maximum quantum yield (QY) remained statistically similar across treatments, though all QY values fell below the ideal benchmark of ~0.83, indicating light-induced stress across the board. Notably, Chinese kale grown under WL exhibited the greatest stem elongation, likely triggered by a low red to far-red ratio that induced shade avoidance responses.

Overall, an 80%-RL:20%-BL ratio emerges as the optimal treatment for promoting vegetative growth, whereas a 20%-RL:80%-BL ratio proves more effective for enhancing phytochemical enrichment. These insights offer practical guidance for optimizing LED configurations in HVFS, balancing yield and nutritional quality for future urban agricultural systems.