Ultraviolet Radiation

Ultraviolet (UV) radiation has been used to maintain the postharvest quality and extend the shelf life of several fresh fruits and vegetables (Perkins-Veazie et al. 2008). The UV portion of the electromagnetic spectrum ranges approximately 10-400 nm (Shama 2007). UV radiation has been applied to produce in long- (UV-A: 315-400 nm), medium- (UV-B: 280-315 nm), and short-wave (UV-C: 100-280 nm) dosages. The shortest wavelengths of the UV spectrum are the most energetic ones and more effective biocide for surface sterilization of some food products (Shama 2007; Perkins-Veazie et al. 2008).

Low UV doses induce production of antifun-gal compounds, ripening delay, and reduction of chilling injury (Pombo et al. 2009). The exposure to UV-C delays fruit softening which is one of the main factors determining fruit postharvest life (Pombo et al. 2009). UV-C decreased the activity of enzymes involved in tomato cell wall degradation and delayed the fruit softening (Pombo et al. 2009; Liu et al. 2011). Treatment with UV-C increases ascorbic acid and total phenolic contents and improves nutritional qualities of tomato fruit (Liu et al. 2011).

UV radiation can affect physiological processes at the genetic level. In parsley, UV-B upregulates genes encoding the flavonoid biosynthetic pathway, such as chalcone synthase and phenylalanine ammonia lyases (PAL), which are key enzymes in anthocyanin formation (Perkins-Veazie et al. 2008) . In tomato, this exhibits ethylene production with ripening onset; UV-C treatment has disrupted ethylene production by decreasing the formation of ACC synthase (Perkins-Veazie et al. 2008). Peaches treated with UV-C showed increased activation of genes for b-1,3-glucanase and PAL (Perkins-Veazie et al. 2008).

Hormetic doses of UV-C radiation have been used as a physical treatment to extend posthar-vest life of several fruits and vegetables (Pombo et al. 2009). Hormesis has been defined as the use of potentially harmful agents at low doses in order to induce a beneficial stress response (Shama and Alderson 2005). Hormetic effects manifest themselves in treated plant tissue through the action of a variety of induced chemical species. They include phytoalexins, such as scoparone in oranges and resveratrol in grapes (Shama 2007) . Also induced are enzymes, such as chitinases and glucanases in peaches and PAL in tomatoes (Shama 2007) . The deleterious effects of UV light on plant tissues, such as decreased protein synthesis, impaired chloroplast function, and DNA damage, have been shown (Costa et al. 2006) . However, low doses of UV could inflict repairable damage to DNA, and this slight trauma would activate repair mechanisms for radiation-induced DNA damage. Sublethal radiation may stimulate vital processes inside the cells and create a positive change in the homeo-stasis of a plant (Shama and Alderson 2005).

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