The portable chlorophyll (Chl) meter SPAD-502 Plus (Konica Minolta Optics, Japan) is one of the most commonly used diagnostic tools for rapid and nondestructive estimation of Chl content in leaves of crops and tree species. The aim of this work was to evaluate the performance of a SPAD-502 Plus to estimate the content of Chl a, Chl b, and Chl (a + b) in Solanum lycopersicum L. leaves with mottled, interveinal chlorosis induced by continuous lighting. The results have shown positive correlation between the Chl content index (CCI) and contents per unit leaf area (mg/cm²) of Chl a (R² = 0.89), Chl b (R² = 0.79), Chl (a + b) (R² = 0.90) and between CCI and content per unit leaf mass (mg/g) of Chl a (R² = 0.88), Chl b (R² = 0.78), Chl (a + b) (R² = 0.89), suggesting that the portable Chl meter SPAD-502 Plus can be an effective tool for nondestructive estimation of Chl content in leaves with mottled, interveinal chlorosis. The accuracy of SPAD-502 Plus Chl meter tends to be higher for estimating Chl content per unit leaf area. Non-linear regressions were successfully used for estimating Chl content from SPAD measurements.

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.

Under continuous lighting (CL, 24 h photoperiod), both the energy and signaling components of light are continuously supplied to plants. In response to CL sensitive plant species exhibit leaf chlorosis and necrosis, down-regulation of photosynthesis, accumulation of carbohydrates and accelerated leaf senescence. Current research was undertaken to test if the CL-induced injury truly results from CL per se or from the excessive light received by the plant due to high daily light integral (DLI). Experimental design included combinations of light intensity and photoperiods providing either different (Experiment 1) or equal (Experiment 2) DLI for tomato and cucumber plants grown under CL or 14 h photoperiod. In experiments where equal DLI was provided CL-treated plants developed injury symptoms similar to those in CL treatments with higher DLI. Plants exhibited an entire spectrum of responses to increasing day length—epinastic movements of leaves and petioles, chlorosis, the significant decline in values of potential quantum yield of photochemical activity of PSII (F_v/F_m) and effective quantum yield of PSII (ΦPSII), elevation of the ratio of Chl a/b, decrease of Chl/Car ratio, reduction of LHCII, increase of lipid peroxidation degree measured in terms of the malondialdehyde (MDA) levels and enhanced activities of antioxidant enzymes (catalase, superoxide dismutase, ascorbate peroxidase, guaiacol peroxidase). Data obtained have shown that a higher DLI is not needed to trigger CL-induced injury. The continuity of light signaling, photosynthesis and photooxidative pressure may induce injury even if DLI values are not high. Extended photoperiod itself may also be a reason of the excess of absorbed light even if DLI is not higher than normally required by plants under shorter photoperiods.

Unlike the natural photoperiod that includes the alternation of day and night in the diurnal cycle, continuous (24 h a day) lighting provides uninterrupted supply of light energy required for photosynthesis, permanently promotes photooxidative processes, implies continuous signaling to the photoreceptors, and desynchronizes the internal circadian biorhythms from the external light/dark cycle (circadian asynchrony). The leaves of many plant species grown under constinuous lighting are prone to characteristic and potentially lethal interveinal chlorosis and necrosis. The photodamage of plant leaves exposed to long photoperiods, including daily 24-h illumination was described more than 90 years ago, but the causes of this phenomenon are still not entirely clear. Biological bases underlying this phenomenon are theoretically and practically important, because growing plants under a 24-h photoperiod at a relatively low photon flux density is seemingly an effective way to save resources and increase plant productivity in greenhouses and plant factories with artificial lighting. This review of available literature compiles and evaluates the arguments both supporting and confronting the hypothesis that carbohydrate accumulation, specifically the hyperaccumulation of starch in leaves, is the main cause of photodamage to plants grown under continuous lighting or long photoperiods. The analysis of a large number of studies indicates that the accumulation of carbohydrates is neither the main nor the only cause of leaf injuries in plants grown under a 24-h photoperiod, although the role of this factor in photodamage cannot be ruled out. The appearance and development of photodamage under a 24-h photoperiod is presumably due to several simultaneously acting factors, such as photooxidation, stress-induced senescence, and circadian asynchrony. The contribution of individual factors to photodamage may vary substantially depending on environmental conditions and biological properties of the object (plant species and variety, plant age, and the stage of development).

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.

Microgreens of four species of the family Brassicaceae (broccoli, mizuna, radish, and arugula) were grown under 16- and/or 24-h photoperiod conditions. In the first series of experiments, the daily light integral (DLI) was different (15.6 and 23.3 mol m^–2 day^–1 at PAR 270 µmol m^–2 s^–1), while it was the same (15.6 mol m^–2 day^–1 at PAR 270 µmol m^–2 s^–1 and 180 µmol m^–2 s^–1) in the second. In the third series of experiments, continuous lighting was used only in the last three days before harvesting. The results obtained showed that broccoli, mizuna, radish, and arugula plants in the early phases of growth are resistant to continuous lighting and do not show typical signs of leaf photodamage. In all three series of experiments, microgreens of all four species grown under 24-h photoperiod had a higher yield and nutritional value (higher content of substances with antioxidant properties—anthocyanins, flavonoids, carotenoids, and proline—as well as increased activity of antioxidant enzymes) and a lower content of nitrates compared to plants grown under 16-h photoperiod. It was concluded that it is possible through the use of continuous lighting without increasing energy costs (while maintaining the DLI) to increase the yield and nutritional value of the studied species of microgreens and reduce their nitrate content compared to the standard 16-h photoperiod. In addition, an increase in nutritional value and a decrease in nitrate content is also possible with the use of continuous lighting (as an agricultural practice) for several days immediately before harvesting.