The effect of continuous lighting (CL, 24 h) and light spectrum on growth and nutritional quality of arugula (Eruca sativa), broccoli (Brassica oleracea var. italic), mizuna (Brassica rapa. var. nipposinica), and radish (Raphanus sativus var. radicula) were investigated in growth chambers under light-emitting diode (LED) and fluorescent lighting. Microgreens were grown under four combinations of two photoperiods (16 h and 24 h) providing daily light integral (DLI) of 15.6 and 23.3 mol m⁻² day⁻¹, correspondingly) with two light spectra: LED lamps and fluorescent lamps (FLU). The results show that fresh and dry weights as well as leaf mass per area and robust index of harvested arugula, broccoli, mizuna, and radish seedlings were significantly higher under CL compared to 16 h photoperiod regardless of light quality. There were no visible signs of leaf photodamage. In all CL-treated plants higher chlorophyll a/b and carotenoid-to-chlorophyll ratios were observed in all plants except mizuna. CL treatment was beneficial for anthocyanin, flavonoid, and proline accumulation. Higher activities of antioxidant enzymes (catalase, superoxide dismutase, ascorbate peroxidase, and guaiacol peroxidase) were also observed in CL-treated plants. In most cases, the effects were more pronounced under LED lighting. These results indicate that plants under mild oxidative stress induced by CL accumulated more non-enzymatic antioxidants and increased the activities of antioxidant enzymes. This added nutritional value to microgreens that are used as functional foods providing health benefits. We suggest that for arugula, broccoli, mizuna, and radish, an LED CL production strategy is possible and can have economic and nutritional benefits.

The absence of an externally-imposed 24 h light/dark cycle in closed plant production systems allows setting the light environmental parameters in unconventional ways. Innovative lighting modes for energy-saving, high-quality, and yield production are widely discussed. This study aimed to evaluate the effects of the light/dark cycles of 16/8 h (control) and 24/12 h, 48/24 h, 96/48 h, 120/60 h (unconventional cycles) based on the same total light amount, and continuous lighting (360/0 h) on plant performance of some Solanaceae species. Responses of eggplant (Solanum melongena L.), sweet pepper (Capsicum annuum L.), tobacco (Nicotiana tabacum L.), and tomato (Solanum lycopersicum L.) plants to extended light/dark cycles and continuous lighting were studied under controlled climate conditions. Plants with two true leaves were exposed to different light/dark cycles for 15 days. Light intensity was 250 µmol m⁻² s⁻¹ PPFD, provided by light-emitting diodes (LEDs). After the experiment, tomato, sweet pepper, and eggplant transplants were planted in a greenhouse and grown under identical conditions of natural photoperiod for the estimation of the after-effect of light treatments on fruit yield. Extended light/dark cycles of 24/12 h, 48/24 h, 96/48 h, 120/60 h, and 360/0 h affected growth, development, photosynthetic pigment content, anthocyanin and flavonoid content, and redox state of plants. Effects varied with plant species and length of light/dark cycles. In some cases, measured parameters improved with increasing light/dark periods despite the same total sum of illumination received by plants. Treatments of tomato and pepper transplants with 48/24 h, 96/48 h, and 120/60 h resulted in higher fruit yield compared to conventional 16/8 h photoperiod. The conclusion was made that extended light/dark cycles can result in increased light use efficiency compared to conventional photoperiod and, therefore, reduced product cost, but for practical application, the effects need to be further explored for individual plant species or even cultivars.

This study addressed the following questions: How does continuous lighting (CL) impact plant physiology, and photosynthetic and stress responses? Does the impact of CL depend on the source of the light and other environmental factors (natural vs. artificial)? Do responses to CL differ for native and non-native plant species in the subarctic region and, if differences exist, what physiological reasons might they be associated with them? Experiments were conducted with three plants native to the subarctic region (Geranium sylvaticum L., Geum rivale L., Potentilla erecta (L.) Raeusch.) and three non-native plant species (Geranium himalayense Klotzsch, Geum coccineum Sibth. and Sm., Potentilla atrosanguinea Loddiges ex D. Don) introduced in the Polar-Alpine Botanic Garden (KPABG, 67°38′ N). The experimental groups included three species pairs exposed to (1) a natural 16 h photoperiod, (2) natural CL, (3) an artificial 16 h photoperiod and (4) artificial CL. In the natural environment, measurements of physiological and biochemical parameters were carried out at the peak of the polar day (at the end of June), when the plants were illuminated continuously, and in the second week of August, when the day length was about 16 h. Th experiments with artificial lighting were conducted in climate chambers where plants were exposed to 16 h or 24 h photoperiods for two weeks. Other parameters (light intensity, spectrum composition, temperature and air humidity) were held constant. The obtained results have shown that plants lack specific mechanisms of tolerance to CL. The protective responses are non-specific and induced by developing photo-oxidative stress. In climate chambers, under constant environmental conditions artificial CL causes leaf injuries due to oxidative stress, the main cause of which is circadian asynchrony. In nature, plants are not photodamaged during the polar day, as endogenous rhythms are maintained due to daily fluctuations of several environmental factors (light intensity, spectral distribution, temperature and air humidity). The obtained data show that among possible non-specific protective mechanisms, plants use flavonoids to neutralize the excess ROS generated under CL. In local subarctic plants, their photoprotective role is significantly higher than in non-native introduced plant species.

The study addressed the following questions: How does continuous lighting (CL) im-pact plant physiology, photosynthetic and stress response? Does the impact of CL de-pend on the source of light and other environmental factors (natural vs artificial)? Do responses to CL differ for native and non-native plant species in the Subarctic and, if differences exist, what physiological reasons might they be associated with? Experi-ments were conducted with three native to Subarctic (Geranium sylvaticum L., Geum rivale L., Potentilla erecta (L.) Raeusch.) and three non-native plant species (Geranium himalayense Klotzsch, Geum coccineum Sibth. & Sm., Potentilla atrosanguinea Loddiges ex D. Don) introduced in Polar-Alpine Botanic Garden (KPABG, 67°38′N). Obtained re-sults have shown that plants lack specific mechanisms of tolerance to CL. Protective responses are non-specific and induced by developing photo-oxidative stress. In cli-mate chambers under constant environmental conditions artificial CL causes leaf inju-ries due to oxidative stress, the main cause of which is circadian asynchrony. In nature plants are not photodamaged during the polar day as endogenous rhythms are main-tained due to daily fluctuations of several environmental factors (light intensity, spec-tral distribution, temperature and air humidity). The obtained data show that among possible non-specific protective mechanisms, plants use flavonoids to neutralize excess ROS generated under CL. In local subarctic plants, their photoprotective role is signif-icantly higher than in non-native introduced plant species.

The effects of shortened light/dark cycles (8/4 h and 4/2 h) on the growth, productivity, and nutritional value of baby leaf vegetables (borage, fenugreek, pea, and sunflower) and microgreens (arugula, broccoli, mizuna, and radish) were evaluated under controlled environment. Control plants were cultivated under 16/8 h photoperiod. The intermittent lighting influenced the quantitative and qualitative parameters of plant growth and yield (fresh weight, robustness index, dty matter content, nutritional value) , yet plant responses were species specific. It is suggested that plant response to shortened light-dark cycles depends on their tolerance/sensitivity to circadian asynchrony, which occurs when external L/D cycles do not coincide with the endogenous circadian rhythms. In areas with time-of-use electricity rates, scheduling lighting during off-peak hours can significantly reduce production costs. It was shown that shortened light-dark cycles can serve as the basis for cost-effective lighting regimes based on electricity tariffs with peak, standatd and off-peak time bands. Thus, intermittent irradiation regime with customized periods (7/4/9/4 h) has been shown to enhance crop yield and qualty or not affect them under equivalent energy consumption while reducing lighting energy costs by 25-45% compared to conventional 16/8 h photoperiod by maximizing the use of off-peak hours.

In controlled-environment agriculture systems, such as vertical farms, plants can be cultivated under artificial light-dark (L/D) cycles that deviate substantially from the natural 24 h photoperiod. While these regimes may enhance the growth and quality of certain crops, the physiological response is highly species-specific, and long-term implications for many crop species remain insufficiently characterized. This study aimed to evaluate the effects of a shortened L/D cycle (8/4 h, repeated twice daily) on the morphology, photosynthetic activity, and redox status of eggplant (Solanum melongena L., cv. ‘Almaz’) and tomato (Solanum lycopersicum L., cv. ‘Verlioka Plus’) seedlings, compared to a standard 16/8 h photoperiod. Plants were grown in chambers equipped with light-emitting diode (LED) and fluorescent (FLU) lamps under an equivalent daily light integral (DLI: 11.5 mol m⁻² day⁻¹). The shortened 8/4 h L/D cycle induced significant growth suppression, reduced leaf area, and decreased the maximum quantum yield of photosystem II (Fv/Fm), as well as chlorophyll and carotenoid content in both species. Under this regime, eggplants exhibited leaf chlorosis accompanied by necrotic lesions, whereas tomato plants displayed interveinal chlorosis. Notably, significant accumulation of oxidative stress markers—a 37% increase in malondialdehyde (MDA) and a 30% elevation in H₂O₂ levels—occurred exclusively in eggplant. These findings demonstrate that redistributing the same 16 h daily illumination into two 8/4 h L/D cycles induces stress in both eggplant and tomato plants, with eggplant exhibiting greater injury than tomato. The underlying circadian mechanisms and the apparent differences between LED and FLU lighting require further controlled validation.