International Journal of Plant & Soil Science

Light occupies a singular position in plant biology because it supplies the energy for photosynthetic carbon gain while simultaneously providing information about time, season, canopy proximity, spatial orientation and environmental change. These energetic and informational functions are sensed across different scales and are integrated with endogenous developmental programmes, the circadian system, hormone signalling and temperature responses. This critical narrative review evaluates how light quantity, spectral quality, photoperiod, direction and temporal dynamics regulate plant growth, development and flowering, with particular emphasis on higher plants and on the translation of mechanistic knowledge from Arabidopsis thaliana to crops and controlled-environment horticulture. Literature was selected through live searches of accessible scholarly databases and indexes, with emphasis on peer-reviewed primary research, authoritative reviews and verifiable bibliographic records. The evidence is strongest for a conserved core in which phytochromes, cryptochromes, phototropins and UV RESISTANCE LOCUS 8 feed into interconnected regulators such as PHYTOCHROME-INTERACTING FACTOR proteins, CONSTITUTIVELY PHOTOMORPHOGENIC 1 and ELONGATED HYPOCOTYL 5. These networks coordinate de-etiolation, shade responses, organ growth and temporal development. Flowering provides the clearest example of light-time integration: circadian gating and photoreceptor control converge on florigenic signals, yet the upstream wiring differs substantially among long-day and short-day crops. Evidence from photosynthetic physiology also challenges simple wavelength rankings. Green and far-red photons can contribute substantially under high irradiance or within canopies, while spectral effects on growth often depend on photon dose, developmental stage and canopy architecture. Controlled-environment studies demonstrate practical opportunities for spectral and temporal manipulation, but generalisation is limited by cultivar specificity, short experimental duration, inconsistent spectral reporting and confounding between spectrum, daily light integral and temperature. Future work should integrate photoreceptor genetics, whole-canopy carbon balance, dynamic lighting and reproductive phenology under realistic fluctuating environments. Light is therefore best treated not as a single growth factor but as a multidimensional environmental signal whose effects emerge from context-dependent integration across molecular, organ and whole-plant scales.