Abstract
A major challenge for growing vegetables in an indoor vertical farming system will be supplying not only sufficient quantity but also quality of light. It has been reported that yield of crops is enhanced under appropriate combination of red and blue light compared with red light alone. This project aims to investigate the effects of different combinations of red and blue. Plants were cultured for a 12-h photoperiod at 210 μmol·m–2·s–1 photosynthetic photon flux density (PPFD) under different combinations of red (R) and blue (B) light-emitting diodes (LED). The R:B-LED ratios are: 1) 100:0 (0B); 2) 92:8 (8B); 3) 84:16 (16B) and; 4) 76:24 (24B). All combined RB-LEDs significantly increased light-saturated photosynthetic CO2 assimilation rate (Asat), stomatal conductance (gs sat) and productivity compared with those under 0B. Results suggested that 16B was the most suitable combination of LEDs to achieve the highest productivity for B. alboglabra. To further substantiate these results, comparative studies were conducted under equal photoperiod and PPFD among 16B (RB-LED), white LED (RBW-LED) and high-pressure sodium (HPS) lamps. Shoot, root biomass, leaf number, leaf mass per area and Asat were higher in plants under HPS lamps and RB-LED, than under RBW-LED. However, gs sat was lower under RB-LED and RBW-LED, than under HPS lamps. Plants under RB-LED had higher electron transport rate and photochemical quenching but lower non-photochemical quenching than those under RBW-LED and HPS lamps. Thus, these results more conclusively affirmed that 16B was the most suitable light source to achieve the highest photosynthetic capacities. The findings of this study could also be used in vertical farming to achieve the highest productivity of vegetable crops such as B. alboglabra within the shortest growth cycle with reduced energy consumption.
Highlights
The maintenance of food security is increasingly challenging for modern cities, such as Singapore, where arable land is limited [1]
Among the different combined RB-light-emitting diodes (LEDs), 16B treatment resulted in the highest values of these parameters followed by those plants exposed to 24B and 8B conditions while plants grown under 0B conditions had the lowest values
Under the same amount of PPFD, our present study found that B. alboglabra plants grown under combined RB-LED enhanced both Asat and gs sat compared with those under red-LED alone (Table 1)
Summary
The maintenance of food security is increasingly challenging for modern cities, such as Singapore, where arable land is limited [1]. Urban indoor vertical farming system can offer a solution for the production of vegetable crops. For an indoor farming system, light is critical for vegetable production as the energy source for photosynthesis and as a signal for morphogenesis [2]-[4]. Poor light uniformity, high thermal radiation and high energy consumption limit more extensive usage of HPS lamps in large scale of vegetable production. In early 1990s, after the testing of light-emitting diodes (LEDs) for plant growth in space [7], much effort has been made to enhance the productivity and quality of crops using LED lighting as artificial light sources. LEDs have numerous advantages such as fast switching, lower energy cost, higher durability, longer lifetime, lower thermal radiation, and narrower variation in specific wavelengths [8] for targeted crops. LEDs are more costly in comparison with conventional HPS lamps, the variation in spectrum and quantity of light could control costs more economically
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