Light Quality Regulates Source–Sink Dynamics and Mini-Tuber Formation in Aeroponic Potato

Light intensity and spectral composition regulate plant physiological processes and productivity, particularly under low-light greenhouse conditions. This study was designed to address two main objectives in aeroponically grown potato (Solanum tuberosum L. cv. Agria). First, we evaluated the effects of supplemental light quality, focusing on different red (R), blue (B), and white (W) combinations at a constant intensity of 100 μmol m−2 s−1. Second, we assessed the specific effects of far-red (FR) light on plant performance and biomass allocation patterns. Potato plants were grown under greenhouse conditions in a completely randomized design consisting of eight supplemental LED spectral treatments and a natural-light control. Supplemental lighting increased net photosynthesis, stomatal conductance, chlorophyll content, and biomass compared to the control, demonstrating that moderate increases in light intensity improved plant performance under low-light conditions. Among the spectral treatments, W light and balanced R–B combinations increased net photosynthetic rate by 93.7–198.7% and total biomass by 23.8–132.1% relative to the control, suggesting improved coordination of stomatal activity, electron transport, and chlorophyll biosynthesis under the experimental light environment. In contrast, FR inclusion reduced the net photosynthetic rate and mini-tuber biomass by 15.0–38.6% relative to the corresponding FR-free treatments, particularly under treatments with lower red proportions, suggesting that FR effects are more likely associated with phytochrome-mediated regulation of photosynthetic efficiency and assimilate partitioning under modified red to far-red spectral balance rather than classical shade-avoidance responses. Mini-tuber yield was strongly affected by light treatments. White light and balanced R:B spectra produced the highest tuber number and biomass, increasing mini-tuber number and biomass by 26.6–62.5% and 15.4–87.7%, respectively, compared with the control, whereas FR reduced yield. Although FR appeared to increase the relative allocation of biomass to tubers, overall photosynthetic performance and biomass accumulation remained lower, resulting in lower productivity. Overall, mini-tuber production appeared to be associated with source–sink relationships, where light intensity enhanced photosynthetic performance and biomass production, light quality optimized photosynthetic performance, and FR light appeared to modify biomass allocation patterns. These findings highlight the importance of optimizing spectral composition and FR management in aeroponic seed potato production under low-light greenhouse conditions.