A sufficiently high blue photon flux density can promote accumulation of phenolic compounds in hydroponic lettuce

Authors: MENG, QINGWU - University Of Delaware; Ranger, Christopher; Boldt, Jennifer; RUNKLE, ERIK - Michigan State University

Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/23/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Indoor vertical farming provides an alternative production system to grow edible crops with value-added traits. Adjusting how much and what type of light plants receive can change the color, aroma, flavor, and yield. Lettuce plants grown indoors with a higher fraction of light in the blue waveband had increased concentrations of some compounds that have health-promoting benefits in humans, without an increase in leaf bitterness. This information will help growers tailor light recipes for indoor-grown lettuce to produce a premium-product with improved color, taste, and health benefits.

Technical Abstract: The photon spectrum of sole-source lighting regulates whole-plant photosynthesis and secondary metabolism of indoor hydroponic crops. Phenolic compounds are major secondary metabolites that protect plants against abiotic and biotic stresses, impart color and organoleptic properties, and provide health-promoting benefits in the human diet. However, the influence of varying proportions of blue (B, 400 to 499 nm), green (G, 500 to 599 nm), and red (R, 600 to 699 nm) light on phenolic compounds is unclear. We grew red-leaf lettuce (Lactuca sativa) ‘Rouxai’ in deep-flow hydroponic trays in an indoor growth room under five sole-source photon spectra or in containers filled with a peat-based substrate in a greenhouse with supplemental lighting. Under a 20-h photoperiod, indoor plants received lighting treatments of equivalent intensity but varied spectra, consisting of R180, B20R160, B20G60R100, B100R80, or B100G60R20, where each subscript denotes the photon flux density (PFD) in µmol·m–2·s–1. After 17 or 20 d of treatments in two replications, we extracted, identified, and quantified key phenolic compounds in lyophilized lettuce tissue. The concentration of caffeoylmalic acid or dicaffeoyltartaric acid was similar across all six treatments. In contrast, the concentration of chlorogenic acid or quercetin 3-glucoside generally did not change as the B PFD increased from 0 to 20 µmol·m–2·s–1 but increased as the B PFD increased from 20 to 100 µmol·m–2·s–1. The inclusion of G photons did not influence the concentration of any of these phenolic compounds, irrespective of the B PFD. The concentration of chlorogenic acid or quercetin 3-glucoside was similar in lettuce grown indoors under a B PFD of 100 µmol·m–2·s–1 and in the greenhouse, despite the latter receiving a higher daily light integral. We conclude that increasing the B PFD can increase the concentrations of some, but not all, phenolic compounds in hydroponic lettuce.