Horticultural greenhouse production in circumpolar regions (>60° N latitude), but also at lower latitudes, is dependent on artificial assimilation lighting to improve plant performance and the profitability of ornamental crops, and to secure production of greenhouse vegetables and berries all year round. In order to reduce energy consumption and energy costs, alternative technologies for lighting have been introduced, including light-emitting diodes (LED). This technology is also well-established within urban farming, especially plant factories. Different light technologies influence biotic and abiotic conditions in the plant environment. This review focuses on the impact of light quality on plant–microbe interactions, especially non-phototrophic organisms. Bacterial and fungal pathogens, biocontrol agents, and the phyllobiome are considered. Relevant molecular mechanisms regulating light-quality-related processes in bacteria are described and knowledge gaps are discussed with reference to ecological theories.

Chemical plant growth regulators (PGRs) are used in the production of ornamental potted and bedding plants. Growth control is needed for maximizing production per unit area, reducing transportation costs and to obtain a desired visual quality. However, the use of PGRs is associated with toxicity risks to humans and the environment. In many countries the availability of PGRs is restricted as few substances are registered for use. A number of alternative methods have been suggested. The methods include genetic methods (breeding) and crop cultivation practices such as fertigation, temperature and light management. A lot of research into “alternative” growth regulation was performed during the 1980-1990s, revealing several possible ways of using different climatic factors to optimize plant growth with respect to plant height. In recent years, the interest in climatic growth regulation has been resurrected, not least due to the coming phase-out of the plant growth regulator chlormequat chloride (CCC). Today, authorities in many countries are aiming towards reducing the use of agrochemicals. At the same time, there is a strong demand from consumers for products produced without chemicals. This article provides a broad overview of available methods for non-chemical growth control. It is concluded that a combination of plant breeding and management of temperature, fertigation and light management has the potential of replacing chemical growth regulators in the commercial production of ornamental pot- and bedding plants.

Moving food production into the urban and peri-urban areas is one way of facilitating a closed-loop approach, integrating waste handling with food production in order to recirculate nutrients and at the same time reduce the use of mined and fossil resources in the production. Using anaerobic digestion as a way of converting urban wastes to an energy source (methane) and a nutrient-rich biodigestate with subsequent use as fertilizer for food production seems like a feasible approach. However, utilizing urban wastes in plant production systems implies some challenges, such as high salinity of the waste, imbalanced composition of nutrients, and abundance of less favorable forms of nitrogen. In a series of experiments, these problems were addressed. Vegetables (Pak Choi) were cultivated hydroponically in a controlled climate. Experiments included increased salinity, elevated levels of nitrite, and different concentrations of the biogas digestate-based nutrient solution, with mineral based solutions as controls. In general, the mineral controls yielded around 50% higher fresh biomass than the organic solutions. However, the quality of the produce with respect to content of secondary metabolites such as vitamins was enhanced when the plants were cultivated with organic nutrient solutions. Increasing the concentration of NaCl to 241 mg Cl L⁻¹ did not negatively affect plant performance. Increasing the concentration of nitrite negatively affected plant growth, with reductions in biomass production by up to 50%. Given this well-functioning nitrification process that did not result in high nitrite concentrations, the use of anaerobic digestates seems feasible for hydroponic production of vegetables.

In greenhouse organic horticulture there is a great challenge in supplying the crop with adequate amounts of nutrients at the right stage of crop development. This has been identified as one of the main factors compromising yields in organic systems as compared with conventional hydroponic systems based on the use of synthetic fertilisers. In organic systems, the supply of nutrients is reliant on microbial degradation of organic complexes, a process that is dependent on factors such as temperature, soil water content and pH. Different organic fertilisers will also have different characteristics with respect to mineralisation of nutrients. In order to evaluate different strategies for organic fertilisation in long-term greenhouse crops such as high-wire tomato crops, an experiment with three different treatments was performed. The different strategies evaluated were one based on blood meal, kalimagnesia and the commercial product Baralith Enslow (composed of clay and ground lucerne), one with poultry manure and kalimagnesia, and one with solid biogas digestate and kalimagnesia. A five-month tomato crop was grown. Lysimeter samples were taken from the growing media biweekly for monitoring of plant available nutrients. The results suggested that nitrogen was likely to have been the limiting factor for plant growth, however, the biogas digestate delivered mineralised nitrogen throughout the experiment.

Recovery and reuse of nutrients is a major challenge in agriculture. A new process contributing to a circular economy is the anaerobic digestion of food waste, which is a sustainable way of recycling nutrients as the digestate can be used as fertiliser in agriculture and horticulture. However, the digestate may be polluted with contaminants of emerging concern (CECs) that can be circulated back into the food chain, posing a risk to the environment and human health. In this work, the nutrient solution was spiked with 18 selected CECs frequently detected in food waste biogas facilities, and subsequent uptake and fate of these CECs were evaluated in pak choi grown in two different nutrient solutions (mineral and organic). All spiked compounds except two (propylparaben, fenbendazole) were taken up by pak choi plants, with perfluorobutanoic acid (PFBA) and sertraline displaying the highest concentrations (270 and 190 μg/kg fresh weight, respectively). There were no statistically significant differences in uptake between mineral and organic nutrient solutions. Uptake of per- and polyfluoroalkyl substances (PFAS) was negatively correlated with perfluorocarbon chain length and dependent on the functional group (r = -0.73). Sixteen transformation products (TPs) were tentatively identified using suspect screening, most of which were Phase II or even Phase III metabolites. Six of these TPs were identified for the first time in plant metabolism and their metabolic pathways were considered.