Red and White Supplemental LED Lighting Improves Lower-Tier Productivity and Fruit Quality of Strawberry in a Multi-Tier Hydroponic System

Multi-tier hydroponic systems improve land use efficiency in protected strawberry production; however, shading in the lower cultivation tier can substantially limit plant growth, fruit yield, and fruit quality. This study evaluated whether supplemental LED lighting with different spectral compositions could compensate for lower-tier light deficiency in the strawberry ‘Seolhyang’. Plants were grown under an upper-tier control (UT-C), a lower-tier control (LT-C), and four lower-tier supplemental lighting treatments: blue (LT-B), red (LT-R), red + white (LT-RW), and red + white + far-red (LT-RWF). The lower tier received 6–8 mol·m−2·d−1 less daily light integral (DLI) than the upper tier, whereas supplemental lighting increased DLI by approximately 6.5 mol·m−2·d−1. Among the tested spectra, LT-RW consistently produced the greatest improvements in vegetative growth, reproductive performance, fruit quality, and total yield. LT-RW increased total yield from 102.4 g·plant−1 in LT-C to 359.1 g·plant−1, corresponding to 65.6% of the UT-C yield, and achieved an estimated gross margin equivalent to 48.0% of the UT-C level, compared with 18.5% for LT-C. LT-RW also maintained a higher soluble solids content, SSC/TA ratio, fruit weight, and firmness than the other lower-tier treatments. LT-B showed the highest total sugar concentration on a dry-weight basis and the highest ellagic acid content, but these biochemical responses were not accompanied by comparable improvements in overall productivity or fresh fruit quality. LT-RWF showed intermediate performance, whereas LT-R provided only partial improvement. Principal component analysis further suggested that LT-RW produced the overall plant response most similar to that of UT-C. These findings demonstrate that red + white supplemental lighting is an effective strategy for improving lower-tier productivity, fruit quality, and estimated economic performance in multi-tier hydroponic strawberry production.