Anethum graveolens L. (Dill), an annual herb belonging to the Apiaceae family and indigenous to southwest Asia or southeast Europe, is extensively used in food industry for its distinctive taste and aroma [1]. In addition to being used as flavouring agents for baking mixes, sauces, salads, soups, and seafood, dill greens enhance meals by adding complimentary ingredients including vitamins, mineral salts, and compounds that impact the sensory traits of food [2]. A synergistic relationship contributed by α-phellandrene and dill ether was found to be responsible for characteristic dill aroma [1]. Along with α-phellandrene, volatile phenylpropenes (dill apiole, apiole, and myristicin) also contribute to the distinctive aroma of dill as unravelled by preceding studies [3,4]. Apart from being primarily used as a taste enhancer, dill also possesses a plethora of therapeutic (antioxidant, antibacterial, antifungal, anti-inflammatory, organ-protective, etc.) properties because of the presence of carotenoids and phenolic acids [5].

Light is among the most important abiotic factors that are integral for plant growth and development. In addition to providing energy for photosynthesis, light also dictates specific signals which regulate the biosynthesis of phytochemicals [6]. The three light parameters viz. quantity (intensity and photoperiod), quality (spectral composition) and distribution affect physiological responses in plants among which, the spectral composition of light was shown to play a crucial role in plant specialized metabolism [6]. Implications of inadequate light quality on plants as a consequence of climate change due to seasons, locations, and unfavourable environmental conditions may lead to fluctuations in yield and phytonutrient concentrations of herbs. To combat this natural unpredictability, controlled environment agriculture (CEA) such as high tunnels, greenhouses, and indoor vertical farms are being used for the production of herbs [7]. Moreover, the attempt to increase the nutritional quality of plants via the use of different artificial light sources under controlled environmental conditions has been practiced for decades. The physiological requirements of plants for photosynthesis and photomorphogenic development must be met with compatible spectral properties under an indoor artificial lighting system [8]. The spectral distribution and difference of conventional light sources (such as, sodium vapour lamps, fluorescent lamps, and metal halides) are immutable, and might not be optimal for the light demands of different plant species. This spectral rigidity of conventional lights is now vanquished by light-emitting diodes (LEDs) allowing fine-tuning to obtain a desired spectral composition for diverse plant species. Apart from providing spectral adjustments, LED lights are frequently used in indoor lighting systems due to their effective energy usage, less heat output, and longer lifespans [9]. Spectral distributions that involve red (R) and blue (B) wavelengths in particular, regulate seed germination, photomorphogenesis, photosynthesis, stomatal opening, chloroplast movement, shade avoidance, circadian rhythms, and flower induction through photoreceptor signalling [6]. The use of an indoor LED light system of specific wavelengths or a combination of different wavelengths in the cultivation of aromatic and medicinal plants showed enhanced growth and improved phytochemical constituents [7]. Plants exposed to a range of spectral light quality through LED may differentially trigger the enzymatic pathways involved in the biosynthesis of specialized metabolites [10].

The use of LED to study the impact of a range of spectral lights on dill has been scarcely investigated. Only two studies have so far been conducted on the consequence of LED lights on the growth and metabolite content in dill. When exposed to different spectral combinations, no significant changes in phenol content were noticed in dill, though an increase in terpenoids was observed under a higher abundance of R light when compared to B light [11]. In another study, enhancements of chlorophyll b, internal volatile pool, and phenolic compounds under different spectral combinations of R and B lights were observed by Litvin et al. [12].

The aim of the present study was to determine the physiological behaviour of dill plants grown under different spectral lights. Plants were exposed to a range of LED light treatments including 100% R (100R:0B), 50% R:50% B (50R:50B), and 100% B (0R:100B) to see perceptible changes in the growth and morphology of dill. Further, physiological parameters such as photosynthetic efficiency, antioxidant defence mechanism, and variation in the internal pool of volatiles were also determined. Besides, the differences in the amounts of active enzymes (in terms of in vitro activities) pertaining to phenylalanine ammonia-lyase (PAL) and shikimate dehydrogenase (SKDH) involved in the biosynthesis of major phenolic compounds were assessed. In addition, differences in stem anatomy and histochemical localization of specialized metabolites at the cellular level were also observed. Moreover, the expression patterns of candidate genes involved in the biosynthesis of major antioxidant compounds (phenolic acids and carotenoids), terpenoids, major photosynthetic enzymes, and light dependant PsbDII thylakoid protein were compared among all spectral LED light treatments. The purpose of the research was to foster a clearer perception of how spectral manipulations may be associated with variations in plant biochemical profiles (particularly the status of pigments associated with photosynthetic apparatus, antioxidants, and volatiles). A comprehensive evaluation of these dimensions revealed the photo-regulation by different spectral LED lights that affect growth and nutritional development of dill, the outcome of which can be utilized for indoor cultivation of dill aiming for improved phytonutrient contents.