Far-Red Light Supplementation in Tomato Cultivation: A Quantitative Review of Morphological and Physiological Responses.

Far-red (FR) light at wavelengths 700-800 nm is abundant in sunlight but largely absent from artificial light sources. Standard Light-Emitting Diodes emit little to no FR light. Recently, research has focused on the effects of FR light on the growth and development of crops, like tomatoes. However, findings on FR effects on tomatoes vary, with some effects even depending on the geographic location. To unravel the variability of the results, this review identified all relevant studies that quantified the FR-induced responses in tomato growth, morphology, fruit development and photosynthetic parameters. The studies were categorised into controlled-environment studies with artificial lighting (23 studies) and semi-controlled studies with mixed artificial and sunlight (20 studies). To allow comparison between the FR treatments, three categories were established representing the given FR proportions at shade, sunlight or artificial levels. While FR responses were often cultivar-dependent, known responses were confirmed for several parameters. Supplemental FR light increased plant height and often enhanced fresh weight, dry weight and leaf area, while stem diameter showed a decreasing response to FR. FR typically reduced leaf chlorophyll content, but its effects on photosynthetic efficiency varied. Importantly, variability in results appeared to arise from factors related to experimental setups rather than from confirmed FR quantities. To our knowledge, this is the first review providing a deep comparative analysis of FR treatments and experimental conditions in relation to tomato responses. This knowledge is crucial for planning controlled-environment lighting strategies and will greatly facilitate and increase the value of future studies on tomato performance.