Effect of UV Radiation on Growth, Morphology and Phenology of Three Cornflower Cultivars (Centaurea cyanus)

In the last decades, human activities have had adverse effects on the atmosphere and the stratospheric ozone layer, resulting in an increase in the ultraviolet radiation on the ground, especially in highlands. Among living organisms, plants are the most exposed to ultraviolet rays due to their high and unavoidable need to light for photosynthesis, and are therefore more vulnerable to them. Plants show different responses to ambient UV radiation. The response of plants to ultraviolet light is manifested in two general ways, including tolerating the destructive effects of this radiation or/and avoiding it. The present study was conducted to evaluate the effect of ultraviolet light on growth, morphological and phenological characteristics of three cornflower cultivars under greenhouse conditions in 2018.

The experiment was performed as a split plot in a completely randomized design. Ultraviolet light was considered as the first factor in four levels (including: control, ultraviolet –A radiation, ultraviolet -B radiation and ultraviolet A + B radiations) and three cornflower cultivars (including: ‘Kornblume pink’, ‘Kornblume rot’ and ‘Kornblume blau’) as the second factor. The UV treatment was applied by lamps made by Q-Lab Co, USA. It should be noted that the 40-watt lamps used in this study were broadband and had the highest compliance with ultraviolet B (in the case of UV-B lamps) and ultraviolet A (in the case of UV-A lamps) received from the sun on the ground. So they provided the best possible simulation. During the growth period of plants, phenological traits were recorded and morphological traits and biomass traits were measured at the end of the experiment.

The results showed that UV-B radiation and simultaneous application of UV-A and UV-B radiations resulted in the reduction of the most morphological traits and yield traits including plant height, internode length, leaf width, leaf area, flowering stem length, plant fresh and dry weight, leaf dry weight, flower dry weight and number of flowers. However, flower yield was not affected by ultraviolet radiations. UV-A treatment reduced the flowering stem length and fresh and dry weight of plant in compared to the control treatment, but it had no significant effect on plant height, leaf width, leaf dry weight, flower dry weight and number of flowers. Plants response to ultraviolet radiation is very different. In many species, it has been observed that UV-A does not have a negative effect on plant growth, while, UV-B reduced the growth and yield of plants. It seems that the main reason for the reduction of plant growth and production is prevention of cell division caused by ultraviolet radiation. Degradation of plant pigments (chlorophyll) is also one of the main reason of photosynthesis decrease led to plant growth and yield reduction. The results of the present study showed that the application of UV-A reduced the number of days until the emersion of the first flower bud and the number of days until the opening of the first flower in cornflower cultivars compared to the control. UV-B treatment forced cornflowers to earlier flowering than UV-A. However, the fastest entry into the reproductive and flowering phase of cornflower cultivars was observed with the simultaneous application of UV-A and B radiations. Plants mechanisms against environmental stresses mainly depend on their origin and genetic factors. These mechanisms include three main strategies including “avoidance”, “tolerance” and “escaping”. One of the most important ways to reduce the life cycle is early flowering. It seems that the process of early flowering and completing the growth in cornflower species is a kind of stress escaping under ultraviolet radiation stress.

In this study, ultraviolet radiation reduced the growth and development of all three species of cornflowers. However, UV-A treatment showed the least negative effect on flower yield of plants. On the contrary, UV-B radiation and simultaneous application of UV-A and UV-B radiations reduced the growth and development of cornflowers, but did not have any significant negative effect on the flower yield (as the main useable organ of plant). Therefore, it seems that the ultraviolet radiation can be considered as a positive factor due to its positive effects on the production of secondary metabolites and early flowering and lack of significant negative effects on plant flower yield. Finally, ‘Kornblume pink’ cultivar, with higher flower yield and earlier flowering, is more suitable for cultivation than other cultivars.