The Effect of Different Qualities of LED Light on the Morphophysiological Indicators of Cucumis sativus L. var. Officer

 In addition to providing the necessary energy for photosynthesis, light controls many plant metabolic processes. Nowadays, the use of supplemental light significantly improves the quality of the food product in the conditions of lack of natural light in the autumn and winter seasons. Light-emitting diodes have been proposed as alternative light sources in controlled agricultural environments. These lamps are the first light sources with the ability to control the light spectrum. Therefore, by controlling the light spectrum and matching the wavelength of LED lamps with the photoreceptors of the plant, the performance and quality of the plant is improved. Cucumber is one of the most important greenhouse plants and its seedlings are generally produced during the autumn and winter seasons, when natural light is relatively low; therefore, the use of supplementary light is required. The use of artificial light sources in fully controlled conditions can change and improve the growth conditions of the plant and also improve its quantitative and qualitative traits. In this research, the aim was to investigate the effect of different qualities of light on the morphophysiological indicators of cucumber seedlings.

 This research was performed at the Horticultural Plants Biotechnology Department, Industrial Biotechnology Research Institute of Khorasan Razavi. First, the seeds of the cucumber plant (Cucumis sativus L. var. Officer) were planted in a planting tray, and placed under the light panel with different light treatments. The experimental treatments included four light qualities including white light (6000-6500K) blue light (460-470 nm), red light (625 nm) and combined light (blue + red + white). The amount of photosynthetic photon flux was considered the same in all light treatments, which was equal to 2.75 µol m-2 s-1 CO2 assimilation. Thirty days after planting of seeds, the growth of plants were measured in three stages every 10 days. In each stage, 4 plant samples were selected and then plant height, leaf area, fresh and dry weight of shoots and roots, plant dry matter index and leaf chlorophyll content were measured. Data preparation was done in Excel software, data analysis was done using JMP-8 software and treatment averages were compared using LSD test at 5% probability level.

The results of the experiments showed that the quality of light was significantly effective on the growth indicators of Cucumber plants. Plant height in red light and blue light treatments compared to the white light and combined light (red+blue+white with the same intensity) treatments, increased by more than 130% and 60%, respectively. The lowest height was observed in plants grown under combined light, 9 and 14.25 cm after 40 and 50 days of cultivation respectively. The exposure of cucumber seedlings to the red and combined lights recorded the highest and the lowest leaf area in the plants, 618.65 and 377.26 cm2 respectively. Also white light significantly improved the dry weight of the roots. It is worth to mention that the highest plant fresh weight was observed in blue and white lights, 20.9 and 19.5 g respectively. For parameters such as dry weight, dry matter index, and pigment content, the light treatments did not exert a significant effect. However, the utilization of red and blue light, both individually and in combination, positively impacted plant growth. Notably, exposure to red light alone led to a significant increase in leaf surface area, root dry weight, and plant height compared to other light conditions. Previous studies have indicated that red light enhances leaf area, stem length, and fresh weight of plants. Leaves play a critical role in plant photosynthesis and overall growth. Therefore, increasing the leaf area in the plant increases the amount of photosynthesis, growth and development of the plant. In this study, with the increases of the leaf area in the plant and the subsequent increases in the amount of photosynthesis, allocation of dry mater to root increased. The results of the research has been shown that the combination of blue and red wavelengths in pepper, tomato and cucumber seedlings is effective in stimulating plant growth and improving the morphological characteristics under controlled conditions. Blue and red lights can increase the proton flow rate of epidermal cells through the separation mechanism and thus affect leaf development. Blue light directly through the interaction with proton pumps and indirectly through receptors, affects proton pumps by modulating passive ion conduction of potassium and calcium channels.

 According to the results of this research, it was found that exposing the plant to different light quality had different responses in the cucumber plants. Although variables such as dry weight and the amount of photosynthetic pigments were not significantly affected by light quality, however, traits such as plant height, leaf area, root dry weight, and plant fresh weight were affected by light quality. The affected parameters are among the traits that are influenced by the gibberellin hormone and according to the reports related to the effect of light quality on the gibberellin biosynthesis and response to this hormone. It seems that plant action to the quality of light can be attributed to the regulation of this hormone. So it is possible to choose the appropriate light quality in fully controlled conditions according to the production goal and results. In this research, according to the plant leaf rea, root dry weight and plant height, it was determined that white light can be used in the seedling production stage.