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Additional file 2. Literature search protocol.

Additional file 9. Example of nutrient solution pH dynamics.

Additional file 5. Pigments, chlorophyll, and epidermal flavonol indices in response to different nutrient solutions.

Additional file 13. Chemical salts recipes used to prepare the full strenght nutrient solutions.

Additional file 10. Balloon plot of nutrient solution strenght and composition.

Additional file 7. Time course of leaf area in response to NaCl dose.

Additional file 3. Dry weight accumulation over time of best performing nutrient solutions.

Additional file 11. Photos of salt stress due to contact with nutrient solution.

Additional file 4. Dry weight accumulation over time for all solutions.

Additional file 8. Effect of nutrient solution concentration on number of leaves.

Additional file 1. Somerville and Ogren [21] solution concentration (EC) versus fresh weight [9].

Additional file 3. Dry weight accumulation over time of best performing nutrient solutions.

Additional file 1. Somerville and Ogren [21] solution concentration (EC) versus fresh weight [9].

Additional file 8. Effect of nutrient solution concentration on number of leaves.

Additional file 4. Dry weight accumulation over time for all solutions.

Additional file 12. Photos of germination and seedling system.

Additional file 6. Photos of plants on Murashige and Skoog (MS) solution.

Additional file 12. Photos of germination and seedling system.

Additional file 6. Photos of plants on Murashige and Skoog (MS) solution.

LED lighting in indoor farming systems allows to modulate the spectrum to fit plant needs. Red (R) and blue (B) lights are often used, being highly active for photosynthesis. The effect of R and B spectral components on lettuce plant physiology and biochemistry and resource use efficiency were studied. Five red:blue (RB) ratios (0.5-1-2-3-4) supplied by LED and a fluorescent control (RB = 1) were tested in six experiments in controlled conditions (PPFD = 215 μmol m−2 s−1, daylength 16 h). LED lighting increased yield (1.6 folds) and energy use efficiency (2.8 folds) as compared with fluorescent lamps. Adoption of RB = 3 maximised yield (by 2 folds as compared with RB = 0.5), also increasing leaf chlorophyll and flavonoids concentrations and the uptake of nitrogen, phosphorus, potassium and magnesium. As the red portion of the spectrum increased, photosystem II quantum efficiency decreased but transpiration decreased more rapidly, resulting in increased water use efficiency up to RB = 3 (75 g FW L−1 H2O). The transpiration decrease was accompanied by lower stomatal conductance, which was associated to lower stomatal density, despite an increased stomatal size. Both energy and land surface use efficiency were highest at RB ≥ 3. We hereby suggest a RB ratio of 3 for sustainable indoor lettuce cultivation.

Indoor plant cultivation can result in significantly improved resource use efficiency (surface, water, and nutrients) as compared to traditional growing systems, but illumination costs are still high. LEDs (light emitting diodes) are gaining attention for indoor cultivation because of their ability to provide light of different spectra. In the light spectrum, red and blue regions are often considered the major plants' energy sources for photosynthetic CO2 assimilation. This study aims at identifying the role played by red:blue (R:B) ratio on the resource use efficiency of indoor basil cultivation, linking the physiological response to light to changes in yield and nutritional properties. Basil plants were cultivated in growth chambers under five LED light regimens characterized by different R:B ratios ranging from 0.5 to 4 (respectively, RB0.5, Ra-1, RB2, RB3, and RB4), using fluorescent lamps as control (CK1). A photosynthetic photon flux density of 215 mu mol m(-2) s(-1) was provided for 16 h per day. The greatest biomass production was associated with LED lighting as compared with fluorescent lamp. Despite a reduction in both stomatal conductance and PSII quantum efficiency, adoption of RB3 resulted in higher yield and chlorophyll content, leading to improved use efficiency for water and energy. Antioxidant activity followed a spectral-response function, with optimum associated with RB3. A low RB ratio (0.5) reduced the relative content of several volatiles, as compared to CK1 and RB >= 2. Moreover, mineral leaf concentration (g g(-1) DW) and total content in plant (g plant(-1)) were influences by light quality, resulting in greater N, P, K, Ca, Mg, and Fe accumulation in plants cultivated with RB3. Contrarily, nutrient use efficiency was increased in RB