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Optimizing light quality for growth, nutritional quality, and food safety of lettuce in vertical farming
With the rapid growth in population and urbanization, an increased supply of fresh, nutritious, and safe food in urban areas is required. Relying solely on conventional agriculture for food production can be risky due to climate change and decreasing natural resources (i.e., water, and arable land). Vertical farming (VF) involves growing food crops (primarily leafy greens and small-statured fruits) at multiple levels in controlled environments with less land and water inputs. However, high operational costs have resulted in low-profit margins in VF, which are challenging the economic sustainability of the VF industry. With the present VF technology, it may be difficult to significantly reduce the operational costs. Therefore, maximizing the wholesale value of produce, which is determined by the total yield and sale price per unit quantity (or crop value), is critical for increasing profits in VF. In this research, the overall goal is to increase lettuce growth, nutritional quality, and food safety by optimizing the light quality of light-emitting diodes (LED) in VF to increase the whole value of produce and thus increase the VF profits. The objectives of the research were to (i) study the independent and interactive effects of monochromatic wavebands of light from UV-A (365 nm) to far-red (750 nm) on lettuce growth and nutritional quality; (ii) identify the effects of substituting moderate proportions of UV-A and a high proportion of UV-A coupled with far-red light in growth lighting on lettuce growth and nutritional quality; (iii) study the effects of lettuce cultivars and UV LED light on the survival of E. coli O157:H7 on lettuce in VF production.
In the first study, we investigated the effects of different wavebands of light ranging from UV-A (370 nm) to far-red (733 nm), both independently and in combination with commercial growth lighting on lettuce growth, incident light-use efficiency (LUEinc), and levels of phytochemicals. Results showed that the monochromatic wavebands 389 and 733 nm had positive interactions with the growth lighting on lettuce. In addition, results also indicated that UV-A light at a peak wavelength of 389 nm could potentially increase phytochemical concentrations. In the second study, the effects of 40% UV-A (UV 389 nm) and 60% UV-A (UV 389 nm) plus 10% far-red (FR 733 nm) light for growth light during the plant stationary growth stage on lettuce biomass and biosynthesis of phytochemicals were examined. Results showed that substituting UV-A for 40% growth lighting during the plant stationary growth stage for seven days resulted in significantly increased levels of beta-carotene and phylloquinone in lettuce while slightly lowering lettuce growth. And the addition of far-red light to UV-A did not result in the expected increase in vegetative growth, while the levels of phytochemicals were not affected. In the third study, we first investigated the effects of four lettuce cultivars, including oakleaf, romaine, butterhead, and leaf lettuce on the survival of E. coli O157:H7 gfp+. Results showed that leaf lettuce had the lowest while oakleaf and romaine had the highest concentrations of E. coli O157:H7 gfp+ when sampled on days 2 and 7 after the inoculation, and on day 7 after harvest and storage at 4 °C. Then we examined the feasibility of supplementing UV-A, UV A+B, and UV A+C during plant growth stages to reduce E. coli O157:H7 gfp+ contamination on lettuce. Our results indicated that only the UV A+C light at an intensity of 54.4 μmol·m-2·s-1 for 15 minutes per day after inoculation reduced E. coli O157:H7 gfp+ contamination by 0.33 log CFU·g-1 without affecting plant growth and levels of phytochemicals.
The outcomes from our research suggested that the interactive effects of monochromatic wavebands should be considered in developing light recipes. In addition, VF growers who are interested in improving the nutritional phytochemical levels such as beta-carotene and phylloquinone in lettuce while maintaining growth should consider adding a moderate proportion (< 40%) of near-blue UV-A (i.e., 389 nm) radiation during the plant stationary growth phase to growth lighting. However, shorter wavelengths of UV-A radiation are not recommended due to their negative effects on plant growth and high economic cost. For growers who are at high risk of E. coli O157:H7 contamination, it is suggested that growing leaf lettuce and supplementation of UV A+C LED light during the plant-growth period should be considered to reduce the E. coli O157:H7 contamination levels.
- Doctor of Philosophy
- West Lafayette