Led lighting for indoor cultivation of leafy vegetables and herbs
Background: Due to the growth of urban population and to the reduction of resources availability (e.g., arable land, water and nutrients), new forms of agriculture that can be developed also in urban environment are gaining increasing popularity. This is the case of the so called vertical farms with artificial lighting (VFALs), where artificial light substitutes solar radiation and allows plants' growth. In VFALs, environmental factors (e.g., temperature, humidity, light and CO2) can be adjusted to the plant needs. By using multi-layer systems, it is possible to grow a greater number of plants per unit of cultivation area. Furthermore, by controlling environmental factors, a year-round production can be achieved. Thanks to the adoption of hydroponics systems and to the possibility to recover water lost from plant transpiration, these cultivation systems are also considered to be highly efficient in water use. Moreover, hydroponic solutions avoid run-off and leaching of mineral nutrients in soil, therefore reducing the environmental impact associated with agriculture. On the other hand, VFALs are characterized by high initial investment costs and elevated energy needs mainly associated with artificial lighting, thus the overall efficiency of the system is highly dependent on the lighting strategy. The main features that affect light management in VFALs are light composition (the light spectrum supplied by the lamps), light intensity (the light quantity expressed as ?mol m-2 s-1), and photoperiod (the number of hours of light per day). Light emitting diodes (LEDs) enables to improve light management as compared to other artificial lighting sources (e.g., high pressure or fluorescent lamps), thanks to the possibility to emit light in narrow wavebands of the spectrum, the lower surface temperature and the longer lifetime. In recent years, the main crops tested within VFAL environments were leafy vegetables and herbs, due to their small size that enables elevate plant densities and multi-layer cultivation and the short cycle that allows for multiple harvests across the year. Moreover, since their edible product is represented by vegetative organs, induction or management of the reproductive stages is avoided, their light management is simpler as compared to fruit crops and the harvest index is increased. Objectives: The general objective of the hereby presented research project was to identify the optimal light management for leafy vegetables and herbs cultivation in indoor environment by using artificial lighting. Accordingly, one aromatic herb (sweet basil, Ocimum basilicum) and three leafy vegetable crops (lettuce, Lactuca sativa; chicory, Chicorium intybus; and rocket, Eruca sativa) were tested in light insulated compartments in a controlled environment climate chamber (T=24°C, 450 ppm CO2) for their adaptability to different light managements, including spectral properties, light intensity and photoperiod. Accordingly, the targeted research questions (RQ) were the following: - RQ1: Which is the optimal red and blue spectrum for growing sweet basil? - RQ2: Which is the optimal red and blue spectrum for growing lettuce? - RQ3: Which is the optimal red and blue spectrum for reducing environmental impact of sweet basil, lettuce, chicory and rocket under artificial lighting? - RQ4: Which is the optimal light intensity of an improved red and blue spectrum for growing indoor sweet basil and lettuce? - RQ5: Which is the optimal photoperiod of an optimized red and blue spectrum and light intensity for growing indoor sweet basil, lettuce, chicory and rocket under artificial lighting? Results: The first step (RQ1 and RQ2) was to identify the role played by red:blue (RB) ratio on the resource use efficiency of indoor basil and lettuce cultivation, linking the physiological response to light to changes in yield and nutritional properties. Basil and lettuce plants were grown under five LED light regimens characterized by different RB ratios ranging from 0.5 to 4, using fluorescent lamps as control. A photosynthetic photon flux density (PPFD) of 215 ?mol m-2 s-1 was provided for 16 h per day (photoperiod of 16/8, day/night). LED lighting increased biomass production and resources use efficiency in both basil and lettuce crops as compared with fluorescent lamps. More specifically, the adoption of RB=3 resulted in higher yield, chlorophyll content, and antioxidant activity in both lettuce and basil crops, fostering improved performances in terms of growth, physiological and metabolic functions. This also resulted in increased efficiency in the use of resources, namely water use efficiency (WUE, g FW L-1 H2O), energy use efficiency (EUE, g FW kW-1 h-1) and land surface use efficiency (SUE, g FW m-2 day-1). The second step (RQ3) was to quantify the effect of varying the red and blue LED spectral components (RB ratios of 0.5, 1, 2, 3 and 4) on the eco-efficiency of indoor production of lettuce, chicory, rocket and sweet basil from a life cycle perspective. The functional unit of the assessment was 1 kg of harvested fresh plant edible product, and the International Reference Life Cycle Data System (ILCD) method was employed for impact assessment. Results highlighted that electricity consumption was the largest contributor to the environmental impacts (with the LED lamps being the main electricity consumers, approximately 70%), apart from the resources use indicator, where the materials of the lamps and the mineral nutrients were also relevant. RB=0.5 was the most energy-efficient light treatment, but had the lowest eco-efficiency scores due to the lower crop yields. The third step (RQ4) was to define the optimal light intensity for indoor cultivation of basil and lettuce, by linking resource use efficiency to physiological responses and biomass production under different light intensities. Basil and lettuce plants were cultivated under red and blue light (with RB ratio of 3) and a 16/8 photoperiod in growth chambers using five light intensities (respectively supplying PPFD of 100, 150, 200, 250 and 300 ?mol m-2 s-1). A progressive increase of biomass production for both lettuce and basil species up to a light intensity of 250 ?mol m-2 s-1 was observed. Despite the highest stomatal conductance associated to a light intensity of 250 ?mol m-2 s-1 in both lettuce and basil, WUE was also maximized under this light regime, as well as were EUE and light use efficiency (LUE, g DW mol-1). Furthermore, in lettuce grown under 250 ?mol m-2 s-1, the activity of secondary metabolism (e.g., antioxidant activity) resulted enhanced. Accordingly, a light intensity of 250 ?mol m-2 s-1 seems suitable for optimizing yield and resource use efficiency in red and blue LED lighting for indoor cultivation of lettuce and basil under the prevailing conditions of the used indoor farming set-up. The fourth and last step (RQ5) was to identify the optimal photoperiod for indoor cultivation of lettuce, basil, chicory and rocket, analysing their morphological and physiological responses to different long day regimes. Plants were cultivated under red and blue LED light (RB=3, PPFD=250 ?mol m-2 s-1) and underwent three treatments of photoperiod (16/8, 20/4, and 24/0 hours of light/dark). Response to different photoperiods differed among plants? species. In basil and rocket plants, changes in photoperiod did not affect biomass production. On the other hand, lettuce and chicory presented greater fresh weight for plants grown at 16/8. WUE did not vary among the different photoperiods for the plants? species considered. EUE resulted the greatest in lettuce, basil and chicory crops grown at 16/8, but was not affected by photoperiod in rocket plants. LUE did not change at variations of photoperiod (with the exclusion of chicory plants where it was greater at 16/8). Conclusions: From the results of this PhD research project, it can be argued that a RB ratio of 3 with a light intensity of 250 ?mol m-2 s-1 and a photoperiod of 16/8 hours light/dark are the optimal light conditions for leafy vegetables and herbs cultivation, in order to enhance resources use efficiency of the indoor system. However, it should be evidenced that these light conditions are strictly linked to the environmental conditions (e.g., temperature, relative humidity, CO2) of the system. Therefore, in order to improve the performances of the cultivation system, a multifactorial approach is needed, which shall consider the environmental conditions, the lighting management and the specific features and requirement of the grown plants.
Hortalizas, Técnicas de cultivo hortícola, Gestión de la producción vegetal, Life Cycle Assessment (LCA), Vegetables, Lettuce (Lactuca sativa L.), Aromatics herb, Chicory (Cichorium intybus L.), Basil (Ocimum basilicum L.)
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