**1. Introduction**

During the cultivation of plants in vertical farms, the cost of electrical energy can be quite high due to the many hours of operation of the irradiation system, which can be a limiting factor for the development of technologies and the profitability of the farm. One of the ways to reduce the cost of electricity for plant irradiation is the use of radiation sources with an impulse mode, which saves up to 12% of electricity compared to the constant mode without reducing plant productivity by varying the duration of working cycles and the ratio of light and dark (l/d) periods in each cycle [1,2].

Energy savings are also achieved due to a slight increase in the light output of LEDs with impulse power. For example, Cree LEDs of the X-Lamp XP-C series, when powered by an impulse current with a frequency of 1 kHz with a duty ratio (D) of 5%, showed a

**Citation:** Smirnov, A.A.; Semenova, N.A.; Dorokhov, A.S.; Proshkin, Y.A.; Godyaeva, M.M.; Vodeneev, V.; Sukhov, V.; Panchenko, V.; Chilingaryan, N.O. Influence of Pulsed, Scanning and Constant (16 and 24-h) Modes of LED Irradiation on the Physiological, Biochemical and Morphometric Parameters of Lettuce Plants (*Lactuca sativa* L.) while Cultivated in Vertical Farms. *Agriculture* **2022**, *12*, 1988. https:// doi.org/10.3390/agriculture12121988

Academic Editor: Athanasios Koukounaras

Received: 9 November 2022 Accepted: 19 November 2022 Published: 23 November 2022

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

10–15% higher light output compared to continuous mode [3]. Also, impulse irradiation has an influence on plant physiology, biochemistry, morphology and productivity. Thus, in basil plants, impulse irradiation caused a significant increase in biomass by 47% compared to continuous light [4], while the overall rate of photosynthesis (Pn) did not significantly decrease [5]. Interesting results were obtained while wheat was irradiated with impulse light. Instead of a photoprotective reaction, an increase in photosynthetic carbon assimilation occurred [6]. The use of intermittent illumination with a frequency of 1 kHz and D = 50%, 70% and 80% and with the same intensity on wheat plants showed that at D = 80% illumination, the plants showed the highest Pn, while the concentration of chlorophyll, antioxidant capacity, yield index and the weight of a thousand grains corresponded to the variant with constant illumination [7].

The etiochloroplasts defined from wheat leaves acquired green colour with repeated flashes of light at intervals of 15 min in the dark. These plastids exhibited light-induced decolourization of chlorophyll and carotenoids, and pigment stabilization occurred after additional repeated flashes with short intervals of darkness (5 s). However, when comparing plastids with similar photochemical activity, stabilization was much lower than under constant illumination [8]. Experiments carried out on pepper plants show that the addition of impulse radiation to the flux of fluorescent lamps affects the functional state of the photosynthetic apparatus—the efficiency of electron transport by open photosystem II (PSII) reaction centres and the quantum efficiency of PSII increased. Additional impulse irradiation contributed to an increase in the biomass and content of chlorophyll in the tissues of the pepper leaf blade [9].

A study of the response of tomato leaves to impulse irradiation showed that all light generated a photosynthetic photon flux (PPF) equivalent of 50 μmol m−<sup>2</sup> s−<sup>1</sup> from 5000 μmol m−<sup>2</sup> s−<sup>1</sup> pulses for 1.5 μs followed by periods of dark 148.5 μs (D = 1%, frequency 6.6 kHz), did not affect photosynthesis compared to continuous illumination of 50 μmol m−<sup>2</sup> s<sup>−</sup>1. When l/d pulses were extended to 2 ms of light and 198 ms of dark (D = 1%, frequency 5 kHz), net photosynthesis decreased by a factor of two [10]. In this work, the theory was also confirmed that the pigments of the xanthophyll cycle were not affected by any of the impulse light modes.

There are quite a lot of works on the study of the effect of impulse light sources on the physiology, biochemistry, and morphology of various varieties and species of plants [11–16], but an all-round study of the effect on plant growth and their productivity has not been previously conducted. The parameters of irradiation modes that allow to reduce the consumption of electrical energy without loss in the quality of the grown products were not shown.

Another way to reduce energy costs during the artificial irradiation of plants is the use of scanning phytoemitters (Light-Mover). This is a device that combines a phytoilluminator mounted on a rail attached to the ceiling of a greenhouse or growing room, along which the lamp is constantly moving electrically. The most popular and widespread are the scanning phytoirradiators moved linearly. There are also systems with a circular motion of the irradiators. To cover large areas, it is possible to use several parallel guides. The advantage of scanning phytoirradiators is a more uniform distribution of radiation over the growing area and a significant expansion of the irradiation zone of each phytoirradiator.

Manufacturers claim that such devices reduce lighting costs (for any type of lamp) or increase yields for equal energy costs. It has been established that shade-loving plants can quickly adapt to changing periods of diffuse and direct radiation under natural conditions [17]. Relatively few experiments have been carried out on photophilous plants that have shown the species-specific response of plants to scanning irradiation [18,19]. The results of studies on the use of impulse and scanning radiation systems in crop production are controversial and require further study. The effectiveness of such systems depends on many factors, such as plant species and variety, planting frequency, intensity and spectral composition of radiation. The purpose of this work is to study the time modes of LED irradiation (constant, impulse and scanning) of lettuce grown on racks.

The purpose of this work is to study the effect of impulse, scanning, and constant 16- and 24-h LED irradiation modes on the physiological, biochemical, and morphometric parameters of lettuce (*Lactuca sativa* L.) grown in vertical farms. In contrast to earlier studies, for the scanning mode of irradiation, we used LED irradiators, which have several advantages over gas discharge lamps. They can be placed closer to plants without causing leaf burns from excess radiation, have a longer lifespan, and are promising for vertical farms.
