Effect of the low light intensity and ultraviolet radiation on the productivity of Karmesi lettuce

. Plants of the red - leaved form of Karmezi lettuce varietie was grown at three irradiation levels of 80.60 and 25 mmol m -2 • s -1 with subsequent transfer of part of the plants To a high level of illumination with additional ultraviolet light. The production process, gas exchange and accumulation of anthocyanins were studied. It was shown that ultraviolet and high irradiation (80 mmol m -2 • s -1 ) stimulate the formation of anthocyanins in the lettuce plants.


Introduction
Lettuce is one of the basic crop grown in greenhouses.At the same time, quite large expenses are spent on artificial lighting of greenhouses.Lettuce is grown at the light intensity of about 200 mmol m -2 • s -1 .Recently, the large amount of studies is devoted to the cultivation of this plant under LED irradiators.At the same time, it is important to consider not only the intensity of radiation, but also the presence of the red and blue parts of the specrum in the radiation sources and their ratio [1][2][3].
Plants containing anthocyanin are considered to have functional activity, gather especially valuable products.They are used in healthy nutrition for humans., Great lighting costs are required to grow these plants, especially compared with green-leaf forms.It is well known that ultraviolet radiation stimulates the formation of anthocyanins, but at the same time it inhibits growth processes and the accumulation of biomass.[4][5].
The aim of this work is to analyze the cultivation peculiarities of the red-leaved form of lettuce with stimulation of anthocyanin formation during transfer from low to high illumination and additional ultraviolet radiation.

Materials and methods
Data collection and processing were carried out on lettuce plants (Karmezi variety) grown on photobiological installations.Karmesi RZ variety is a dark red lettuce (Lactuca sativa L). of the Lollo rosso (triple red) type with rapid growth and development.It is used for cultivation in fields and greenhouses, including the tecknology of hydroponics.It is used sold by whole plants or individual leaves, as well as for slicing and adding to salad mixes.This variety has shiny, cherry-red leaves dyed to the base.It has a large open-type socket, a relatively high growth rate and is resistant to flowering.The variety is included in the State Register of Breeding Achievements approved for Use on the territory of the Russian Federation.
Lettuce plants were grown in soil culture in vegetative vessels with a volume of 2 liters.As the substrate the a low-decomposition sphagnum peat "Agrobalt C" was used.It has the humidity of no more than 65%.The peat is neutralized, with a full set of nutrients.In each vessel, 3 plants were grown to the final harvest.
After the emergence, the plants were placed on light modes in accordance with the scheme of the experiment (Table 1).Lettuce was grown at a low light level.The first variant received light content at the level of 80 mmol m 2 • s -1 .The second and third variants had 60 mmol m 2 • s -1 and 25 m 2 • s -1 .35 days after germination, half of the vessels were moved under lamps of photon flux density (230 mmol m -2 • s -1 ) with the addition of ultraviolet radiation (380 nm).The light installations maintained the constant temperature of 18-20 °C.Optimal soil humidity was provided for plants (70% of full soil moisture capacity).The irradiation source was narrow-band light-emitting diodes.Photoperiod was 18 h.The photon flux density and the ratio of the red and blue spectrum are presented in Table 2.During the entire growing season, phenological observations of the growth and development of lettuce plants were carried out.During the study, the dynamics of biomass accumulation was considered.The following indicators were measured: the total number of leaves from the entire plant, the length of the largest leaf, the leaf area.
The leaf area was determined using a LI-3100C photoplanometer (Li-Cor, Lincoln, Nebraska, USA).
The anthocyanin content in the leaves was determined by the standard spectrophotometric method during extraction with a 1% hydrochloric acid solution calculated for cyanidin [6].
The biological and analytical repetition was fourfold.Statistical data processing was carried out according to standard methods using statistical functions of the Microsoft Office Excel application.The tables and graphs show the average and standard deviations.

Results
Morphophysiological reactions of plants are largely determined by the intensity of light and it's spectral composition.At the initial stages of development, the difference between the options was not noticeable.But by the 25th day of vegetation, low illumination significantly affected growth processes (Table 3).The lower the light level, the less raw and dry biomass accumulates, the leaf area decreases.The lowest illumination stretched the leaves somewhat in length, but the difference between the variants was insignificant.The indicators of photosynthetic activity and transpiration are presented in Table 4.The decrease in illumination led to a sharp drop in the intensity of photosynthesis.The difference between 1 and 3 variant was fourfold.Transferring plants to high-intensity lighting with the addition of ultraviolet radiation led to the alignment of the options with each other.
Stomatal conductivity, which is determined primarily by the degree of openness of the stomata, under the influence of high light intensity and ultraviolet increased slightly in all variants.
The transpiration intensity in all low-light variants was approximately at the same level.When moving to intense light, this indicator increased by one and a half to two times.Table 5 shows the indicators of lettuce plants on the 55th day of the growing season.The high level of illumination slightly increased the yield of dry biomass from the plant.But if options 1 and 2 equalized, then option 3 could not reach their level.The raw biomass varied significantly less.Plants transferred from the lowest light intensity to ultraviolet radiation became significantly drier.The data on the leaf area were not unambiguous.If in variants 1 and 3 the transfer of plants to a high light intensity caused a decrease in the assimilation surface, then in variant 2 it even slightly increased.The length of the largest sheet and their number varied within the standart deviation One of the most important indicator of the quality of red-leaf lettuce is the content of anthocyanins.It was possible to determine thewm only in the first variant.Moreover, when transferring plants of these plants to high light intensity with the addition of ultraviolet radiation, the content of anthocyanins increased almost 2 times.

Discussion
The study of the light content influense on the lettuce is still of the great scientific interest [7].Low illumination significantly changes the morphology of lettuce plants [8].The introduction of ultraviolet rays of different spectra into the illumination changed the course of secondary metabolism and affected the content of flavone compounds and pigments [9].In red-leaved lettuce plants of the Skorokhod variety, grown under red-blue LEDs with the addition of ultraviolet rays with a wavelength of 355 nm, the anthocyanin content increased 1.5-2 times [10].In our case, this effect was observed only under illumination of 80 mmol m 2 • s -1 .At the same time, we used the soft ultraviolet.At lower light levels, the accumulation of anthocyanins did not occur, but the accumulation of dry matter increased.Plants were of the small size, but with well-colored leaves.They are convenient to implement in the form of slicing and salad mixes for a healthy diet.

Conclusion
Thus, it is possible to grow red-leaved forms of lettuce at a fairly low level of illumination -80 mmol m -2 • s -1 .It is very important to preserve lettuce useful properties.But during the pre-harvest period, these plants need to be transferred to the high level of illumination of 230 mmol m 2 • s -1 and illuminated by the ultra violet (380nm).

Table 1 .
Scheme of experiment.

Table 2 .
Spectral characteristics of LEDs.

Table 3 .
Biometric indicators of Kermezi lettuce plants grown in different light modes on the 25th day of vegetation.

Table 4 .
The effect of light intensity and ultraviolet radiation (UV) on the gas exchange rates of Karmesi lettuce plants on the 42nd day of the growing season.

Table 5 .
Biometric indicators of Carmezi lettuce plants variety 55 day of vegetation when grown in different light modes.