Phytochemicals in vegetables are important for human health, and their biosynthesis, metabolism and accumulation are affected by environmental factors. Light condition (light quality, light intensity and photoperiod) is one of the most important environmental variables in regulating vegetable growth, development and phytochemical accumulation, particularly for vegetables produced in controlled environments. With the development of light-emitting diode (LED) technology, the regulation of light environments has become increasingly feasible for the provision of ideal light quality, intensity and photoperiod for protected facilities. In this review, the effects of light quality regulation on phytochemical accumulation in vegetables produced in controlled environments are identified, highlighting the research progress and advantages of LED technology as a light environment regulation tool for modifying phytochemical accumulation in vegetables.

Vegetables, the indispensable staple produce providing humans with many beneficial substances, are readily contaminated by nitrate, heavy metals and pesticides during conventional cultivation. In particular, off-season vegetables grown in protected systems with low light intensity do tend to accumulate more nitrate in tissues due to excess N fertilization driven by farmers' desire for high yields. Over-the-limit accumulation of the harmful substances in vegetables constitutes a serious hazard to human health globally. Soilless cultivation, currently a fraction of vegetable cultivation in China, is a promising cultivation method to decrease the accumulation of harmful substances through nutrient solution regulation and environmental factor control. However, conventional inorganic nutrient solutions present few quality benefits besides plant nutrition for the widely acknowledged formulations. Currently, high-quality vegetables are urgently desired by humans globally, but they are difficult to grow for lack of an effective and practical cultivation method to lower the accumulation of harmful substances and to improve nutritional quality simultaneously. Although some attempts have been made, few commercial formulations have been applied in practice. Biogas manure (biogas slurry and biogas dregs) is a by-product of biogas production. It has been shown to be a good fertilizer with abundant nutrients, amino acids and bioactive substances. In China, as a product of the recycling process of agricultural wastes, biogas manure is an ever-growing resource due to the rapid development of biogas projects. Therefore, the need to utilize biogas manure is an urgent issue that relates both to environment protection and nutrient resources utilization. In this paper, the updated research results on yield and the quality effects of vegetables cultivated with biogas dregs and the solutions modified from biogas slurry in China are summarized, highlighting the feasibility and benefits of biogas manure in high-quality vegetable production. It is concluded that biogas manure is an effective nutrient source for high-quality vegetable production based on its synergistic effects and effectiveness in yield and quality improvement (particularly depression effects on nitrate accumulation), and stress resistance. However, deliberate component regulations need to be developed for better yield and quality of vegetables under soilless cultivation due to the large variability of components of biogas manure caused by various combinations of fermentative materials.

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