UV-B Radiation Impacts on Growth, Bulb Yield and Antioxidants in Onion under Salt Stress

Ultraviolet radiation considered one of the environmental stresses that widely disturb most of the physio-biochemical processes in the plant. However, previous studies referred that irradiation of plant tissues with a low dose of UV-B radiation stimulates adaptive mechanisms and providing better protection to other stresses. In this approach, a pot experiment was laid out in the open field during the two growing seasons of 2014/2015 and 2015/2016. Transplants of red onion (Giza 20), six weeks old, were exposed to UV-B (280-320 nm) for 15 min, then grown plants were irrigated with three concentrations of NaCl (0, 50 and 100 mM) as a challenge. The effect of low dose of UV-B was evaluated by determination of vegetative growth characteristics, bulb yield/plant and some biochemical changes associated with UV-B induced tolerance. In general, plants irrigated with 50 and 100 mM of NaCl remarkably expressed a reduction in plant growth, yield and chlorophyll a and b. Flavonoids were significantly reduced under the highest level of salinity. Results showed that plants grown from UV-B treated transplants were less affected by salt stress which was obvious in increasing plant fresh weight, shoot height, number of leaves/plant, leaf length, leaf diameter and neck thickness. Moreover, UV treatment alleviated onion bulb yield in term of fresh bulb weight, bulb length, bulb diameter and harvest index. The enhanced tolerance was noticed by increasing organic osmolytes (free amino acids, reducing sugars and total sugars) and induced antioxidant system (total soluble phenols, carotenoids, flavonoids, peroxidase (POD), polyphenol oxidase (PPO) and ascorbate peroxidase (APX) in onion leaves and anthocyanin in bulbs. Regarding protein electrophoresis, 33% of polymorphism was produced; negative markers were detected by UV + 50 mM NaCl and UV + 100 mM NaCl. While one positive marker was detected by UV +100 mM NaCl at 87.545 kDa which indicated that the interaction between salinity stress and UV-B radiation resulted in plant salt tolerance.