Introduction
Onion (Allium cepa L.) is a vegetable with great nutritional value. In the recent years, the cultivation of onion seedlings has expanded due to the rising production costs and limited water resources. The growth of high-quality seedlings is crucial for achieving optimal and high crop yield. Light quality is a significant factor influencing seedling quality, as it markedly affects the morphological and photosynthetic responses of plants. Currently, LED lights are employed in growth chambers to enhance seedling development. Another important factor influencing seedling growth is the cell volume of the seedling tray. Larger cells offer a more suitable environment for the optimal growth of seedlings; however, increasing the volume of the seedling tray may impose spatial constraints for producers. The aim of this study is to evaluate the effect various tray cell volumes in combination with different light conditions to improve the growth and quality characteristics of onion (Allium cepa L.) seedlings.
Material and methods
To this end, an experiment was conducted in a factorial design comprising two levels of cell volume (28 cc and 18 cc) and three levels of light composition (75% red: 25% blue, 50% red: 50% blue, and 75% blue: 25% red), with three replications, using the Azar Shahr onion cultivar (red and long-day onion). Seed were cultured in the growing substrates (2 coco peat: 4 perlite). After the emergence of the first true leaf (with a leaf length of approximately 5 centimeters), the plants were transferred to a growth chamber equipped with LED lamps and subjected to light treatments for a duration of 45 days. Ultimately, after the seedlings were removed from the growth chamber, various growth parameters were measured, including seedling height, length of the longest true leaf, root length, Pesodostem diameter and length, fresh and dry weight, photosynthetic pigments (chlorophyll a, chlorophyll b, total chlorophyll, carotenoid), fv/fm, photosynthesis rate, stomatal conductance, sub-stomatal CO2 concentration, transpiration rate, total carbohydrate content, and total phenolic content. At the end of the experiment, data analysis was performed using SAS software (version: 9.1).
Results and discussion
According to the results, in the 18 cc cell compared to the 28 cc cell and at all light treatment, seedling length, length of the longest leaf, and Pesodostem length increased, and root length, fresh and dry weight decreased. However, in the 18cc cell, using a 75% red: 25% blue light treatment resulted in the highest quality seedlings with a length of 25.5 cm, Pesodostem diameter (2.13 mm), Pesodostem length (3.4 mm), and dry weight (34 mg). Although at all light levels, cell volume reduction resulted in a decrease in photosynthetic pigments (amount of chlorophyll a, chlorophyll b, total chlorophyll, carotenoid), FV/FM, a decrease in substomatal carbon dioxide, photosynthetic rate, soluble carbohydrates, and phenol content. However, the highest levels of these parameters were observed in the small cell, in the 75% red: 25% blue light treatment. At all it seems that, using a smaller cell volume reduces the root's access to oxygen, water, and nutrients, leading to reduced root growth and some seedling quality parameters. Also, reducing the cell volume also increases the number of plants per tray. This increased density reduces the seedling's access to the appropriate light treatment. However, using an appropriate light treatment can improve growth and physiological parameters. Overall, in the smaller cell with high planting density, the use of the 75% red: 25% blue light combination can lead to improved growth and increased seedling quality. Under red light treatment, plants have smaller stomata and can more effectively control the process of stomatal opening and closing. In this light, in addition to the entry of carbon dioxide, a smaller amount of water is lost, which also increases water consumption efficiency. Therefore, combining red and blue light can offset the negative effects of red and blue light and increase the rate of photosynthesis. However, the response of different plant species to different ratios of red to blue light may vary.
Conclusion
In conclusion, our findings indicate that utilizing trays with smaller cell volumes (which allows for higher planting densities) can be economically beneficial and space-efficient. However, to ensure optimal onion seedling quality, it is crucial to implement a light composition of 75% red and 25% blue during the growth period. |
