Effect of ultraviolet irradiation and foliar application of some plant growth regulators on biomass and morphological characteristics of Fenugreek (Trigonella foenum-graecum L.)

Increase in ultraviolet radiation (UV (, especially UV B, as one of the consequences of climate change, can disrupt the growth of plants. UV light influences plant growth and metabolism. Various studies have shown that increasing in UV B can have detrimental effects on physiological processes and overall growth of some plants. The use of exogenous plant growth regulators has mostly been effective in reducing the effects of environmental stresses in plants. The present study was conducted to investigate the effect of UV-A and UV-B and foliar application of melatonin and ascorbic acid (ASC) on growth, biomass and morphological and stomatal characteristics of fenugreek in 2019.

The experiment was performed as a split plot in a completely randomized design in the research greenhouses of the Faculty of Agriculture and Natural Resources, Lorestan University. Ultraviolet radiation was considered in four levels as the first factor (including: control, UV-A, UV-B and UV-A+UV-B) and application of osmolytes was used in four levels (Control, melatonin, ASC and melatonin+ ASC) as the second factor. Finally, Plant height, plant width, number of leaves, leaflet length and width, petiole length, crown diameter, plant fresh weight, plant dry weight, leaf fresh weight, number of internodes, number of lateral branches, stomatal gurad cell length and width, stomatal index, stomatal density, chlorophyll and carotenoid contents were measured. The data obtained from the experiment was analyzed using Minitab software. Mean comparison was performed using LSD test method at 0.05.

The results showed that UV-A increased fresh leaf weight, crown diameter, leafstalk length, leaflet length and width, plant height and width, plant fresh weight (12.17%), plant dry weight, number of internodes and content of chlorophyll a (7.4%) and chlorophyll b (3.5%), but reduced the number of lateral branches (15.63%). In contrast, UV-B and UVA+UVB treatments reduced most of the studied traits including plant height (8.09%), leaflet length and width, leafstalk length, chlorophyll a, chlorophyll b, total chlorophyll, plant fresh and dry weight and leaf fresh weight. However, carotenoid content, the number of lateral branches (6.58%) and stomatal index (11.4%) were increased by UV-B and UV-A+UV-B. Application of ASC and melatonin reduced the negative effects of UV-B on fenugreek. In control and UV-A treatments, ASC foliar application had shown the best effects on the studied traits and the highest plant height, number of lateral branches, number of leaves, leaf length and width, chlorophyll, carotenoids, and stomatal index and density were observed in this treatment. While, simultantious application of melatonin and ASC led to the best results under UV-B and UVA+B treatments. Therefore, it can be concluded that the synergistic effect of these two compounds in modulating the effects of ultraviolet stress on fenugreek was remarkable.

In the present study, UV-B radiation and simultaneous radiation of UV-A and UV-B reduced the growth and development of fenugreek. In contrast, UV-A treatment improved yield and morphological characteristics of the plant. In addition, foliar application of melatonin and ascorbic acid modulated morphological destruction and growth indices caused by ultraviolet-B treatment.