Abstract
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.
Highlights
In the coming decades, agricultural production may be constrained by limited availability of water[1] and mineral nutrients[2] as well as reduced availability of labour and accessibility to land and fertile soil[3]
As compared to plants illuminated with a similar RB ratio provided by LED lamps, those grown under fluorescent lamps (FL) presented significantly lower dry matter content (Fig. 2C) due to lower plant dry weight (Fig. 2B) despite a statistically similar fresh weight (Fig. 2A)
The larger leaf area and higher specific leaf area (SLA) of lettuce grown under fluorescent lights instead of red + blue light was previously associated with puffiness and a loose shoot structure of the former[12], a response that was observed during our experiments
Summary
Agricultural production may be constrained by limited availability of water[1] and mineral nutrients[2] as well as reduced availability of labour and accessibility to land and fertile soil[3]. A simulation study with lettuce[6] showed that vertical farms can increase land, water and nutrients use efficiency as compared to greenhouses located in Sweden, The Netherlands or the United Arab Emirates, at the cost of elevated energy needs mainly associated with electric lighting. These achievements are related with the higher number of plants per unit land area thanks to the use of multiple growing layers[7], and to the possibility to recover water lost by transpiration and excess minerals and water draining from the growth media[5]. The present study aimed at the identification of how variations in the RB ratio of the incident light (see spectra in Fig. 1) affect sustainability and resource use efficiency in indoor grown lettuce, by dissecting growth performance and related physiological and biochemical adaptations of the plant
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