Monochromatic Red Light Effects on Plants Under Optimal and Saline Environments: A Comprehensive Review of Photobiological Mechanisms and Adaptive Responses

Light-emitting diode (LED) technology allows for precise spectral tailoring in controlled-environment agriculture, with red light (R; 600–700 nm) acting as a central regulator of plant photophysiology through phytochrome (PHY)-mediated control of photosynthesis, morphology, and metabolic adjustment. This review synthesizes the current knowledge of the benefits and limitations of monochromatic and multichromatic R-containing LED systems under both optimal and saline conditions. Monochromatic R light enhances chlorophyll biosynthesis, carbon assimilation, and biomass accumulation; however, its exclusive application can restrict stomatal regulation, photoprotection, and secondary metabolism due to the absence of blue (B)- and green (G)-light-dependent signaling pathways. In contrast, multichromatic spectra incorporating R—particularly R-B, R-far-red (R-FR), and R-centered multi-spectral combinations with white (W) or G wavelengths—provide broader physiological advantages. These include improved photosystem II efficiency, pigment stability, ion homeostasis, antioxidant defense, and metabolic quality, while also optimizing canopy light distribution and energy use efficiency. Under salinity stress, R-containing spectral combinations consistently outperform monochromatic R by enhancing osmotic adjustment, reducing oxidative damage, and maintaining photosynthetic integrity. Nevertheless, species-specific sensitivity, ratio-dependent responses, and potential risks such as excessive elongation under FR enrichment highlight the need for careful spectral optimization. Despite substantial progress, the mechanisms underlying the integration of PHY signaling with salinity-responsive networks remain insufficiently resolved. Advances in multi-omics approaches and dynamic spectral management will be critical for the development of R-based LED strategies that sustainably enhance crop performance and stress resilience in controlled environments.