Utilising Far-Red Light: Photosynthetic and Physiological Adaptations in Shade-Tolerant Fittonia albivenis.

Shade plants have varying capacities to utilise far-red (FR) light, which substantially influences their growth and adaptability to shaded environments, but how plants acclimate to low-light conditions is unclear. This study investigates physiological and photosynthetic adaptations in shade-tolerant Fittonia albivenis during prolonged FR exposure. Plants displayed mixed shade-avoidance (internode and petiole elongation, upward leaf orientation) and shade-tolerance traits (delayed senescence) under FR treatment. Despite these morphological changes, chlorophyll content dropped by only 7.2% after 12 days of FR exposure, indicating robust photosynthetic acclimation. Notably, F. albivenis plants demonstrated a stable maximum quantum yield of PSII (Fv/Fm) and minimal changes in photosynthetic complex composition during FR treatment, emphasising their metabolic flexibility. PGR5/PGRL1-dependent cyclic electron flow emerged as the dominant pathway for ATP production under conditions of lower PSII activity, compensating for the limited electron flow. Additionally, long-term FR exposure induced non-photochemical quenching to dissipate excess energy, reflecting a regulatory photoinhibition mechanism. These findings highlight the ability of F. albivenis plants to efficiently use FR light as both an energy source and signal, providing insights into plant acclimation under low-light conditions. This study underscores the potential for leveraging FR-responsive traits to enhance crop productivity in shaded or controlled environments.