Interactions between Far-Red Light and Blue–Red Spectra on Growth, Pigment Content, and Mineral Composition of Hydroponic Lettuce Cultivated in a Vertical Farming System

The blue–red spectrum, widely used in vertical farming due to its alignment with chlorophyll absorption and its role in regulating secondary metabolism, can be supplemented with wavelengths such as far-red to improve cultivation in fully controlled environments. The aim of this research was to determine the impact of adding FR light to the blue (B), red (R), and blue–red (BR) spectra on the growth, pigment content, and mineral profile of green- and purple-leaf lettuce grown hydroponically in a vertical farm. Plants were cultivated under B, R, and BR spectra with or without FR addition (total PPFD ~150 µmol m⁻² s⁻¹; 16 h light). The addition of FR light, particularly to the red spectrum (R+FR), significantly increased fresh weight, between 36.6% and 141.5% in green lettuce and between 13.1% and 104.9% in purple lettuce compared to other treatments, due to the higher relative growth rate. Moreover, this treatment also caused a reduction in the percentage of dry matter, which ranged from 7.88% to 20.7% in green lettuce and from 8.43% to 24.8% in purple lettuce, compared to other treatments. In green lettuce, chlorophylls (Chl) a and b increased independently under BR and with FR addition. In purple lettuce, Chl a rose with B, while Chl b improved with the B+FR interaction. In green lettuce, carotenoid levels increased significantly with the BR-FR interaction, while in purple lettuce, they augmented significantly with B, and in the absence of FR, independently. Finally, anthocyanin content increased significantly with the B-FR interaction, followed by BR-FR. Although the intensity of these responses varied among cultivars, the results highlight the importance of adapting light spectra to maximize both yield and nutritional value in indoor growing systems. This knowledge lays the foundation for designing precise lighting protocols that respond to different production objectives in vertical farming with different lettuce cultivars.