Dynamic Environmental Control of a Vertical Farm for Lettuce Production

Abstract

Dynamic environmental control systems in vertical farms have been posed as avenues towards improving agricultural productivity and crop quality while addressing sustainability concerns around energy consumed by lighting systems as well as overall resource use. Previous efforts in dynamic controls in vertical farming have built on an abundance of literature on yields, resource use efficiencies, crop indices, anthocyanin content, and tipburn mitigation reported under a variety of fixed light conditions to propose and evaluate dynamic light intensity and light quality treatments. However, many of these studies either neglect resource use efficiencies or fail to control the total amount of artificial lighting within the range of extended photosynthetically active radiation (ePAR, 400-750 nm) that is accumulated over the growing period. As accumulated ePAR light, consisting of blue, green, red, and far-red light is comprehensively responsible for supporting plant growth and these constitutive light colors can modify cellular responses from cell expansion to photoprotective investment, controlling these light attributes is a major opportunity for optimization. Furthermore, integration of airflow and CO2 enrichment, along with the light intensity and quality, has also been lacking in a more comprehensive dynamic environmental control in vertical farming system. This study proposed a treatment strategy of dynamic light quality with or without a dynamic light intensity in comparison to a treatment of fixed light intensity and quality as a control in a vertical farm that also incorporates existing practices established in the literature on intermittent airflow and CO2 enrichment. Fresh and dry shoot masses of red colored lettuce (Lactuca sativa L. cv. ‘Rouxaï’), along with light use efficiency and electrical use efficiency, were used to evaluate the proposed dynamic light control strategies for growth, while anthocyanin concentration, tipburn severity score, and crop quality indices (ARI, mARI, RGRI, and NDVI) of red colored lettuce (Lactuca sativa L. cv. ‘Rouxaï’) are used to evaluate the proposed dynamic light control strategies for quality. No differences in yields or resource efficiency were observed when replacing an initially elevated proportion of Fr light with B light towards the end of production –except with a slight impairment of fresh mass, which is supported by similar studies. In contrast, combining this strategy with a stepwise decreasing light intensity over time has a negative effect on yields and light use efficiency, but was not observed by electrical use efficiency by lighting. However, the consistency of resource efficiencies by both fresh and dry mass bases highlights the importance of controlling light accumulation in this research area. When replacing an initially elevated proportion of Fr light with B light towards the end of production, positive differences in ARI and mARI were observed. With or without adding a stepwise decreasing light intensity regime, tipburn was reduced, but without changing the overall marketable loss of the crop. However, some findings of this study appear contradictory within the context of the literature, such that the anthocyanin concentration was lower than expected by the high ARI and mARI indices, which is potentially explained by methodological inconsistencies in the literature between studies –to be addressed by future work. We further suggest that future work explores the space of light intensity and quality over time, while facilitating greater flexibility than pre-determined treatments by incorporating advanced algorithms and machine vision for real-time control of the growing environment.