UV-B photons

Figure 12. Elevational variation of midsummer UV-B photon energy reaching the earth surface in a subtropical latitude (20° N, Hawaii) calculated according to Caldwell, 1977. Data from Sullivan et al.,1992, redrawn by author. Bars show the general subdivision of montane plant structures in low latitude humid mountain ranges.

However, not all related species or conspecific populations respond similarly under the same UV influence. For example, Betula pendula and B. resinifera, two closely related birch species collected from populations in Europe and Alaska, generally produce a number of flavonoids, hydroxycinnamic acids, and condensed tannins in larger quantity in exposed leaves to artificial UV-B photon fluxes (4.9 kJ m d for 3 hr/day of biologically active radiation) than in non radiated plants (Lavola, 1997). In B. pendula populations the major increase was in flavonoid synthesis but in B. resinifera the bias was towards the hydroxycinnamic acids and condensed tannins. Besides, allopatric (geographically distant) populations of birch reacted differently in the synthesis of specific compounds. For example, chlorogenic acid increased by 22 and 92% in plants from Germany and Alaska, respectively, but in Finnish plants there was no increment at all. 

Some microorganisms, finally, have been shown to be able to perceive and transduce short-wavelength ultraviolet radiation (UV-B = 280 - 315 nm). As in aquatic ecosystems and, in particular, in microorganisms, UV-B has been shown to impair photosynthetic activity, curb growth and metabolic rates, damage DNA, spoil photo- and gravi-orientation in the water column, and hurt cell viability and motility (see Chapter 116). This ability of cells to elaborate UV-B photons as environmental sensory stimuli can directly lead the cells into sheltered areas, thus avoiding harmful UV irradiation. ... 

UV-b radiation has wavelengths in the range 290-320 nm. It is much more dangerous to the skin than UV-a because each photon possesses more energy. In consequence, the photolytic processes caused by UV-b are more extreme than those caused by UV-a. For example, UV-b causes thermal degradation of the skin (we call it sunburn ) but, additionally, it inhibits DNA and RNA replication, which is why over-exposure to UV-b will ultimately lead to skin cancer. 

UV radiation has been the most widely used radiation method in initiating oxidation processes. The UV wavelengths of importance are UV-A (315 00 nm), UV-B (280-315 nm), and UV-C (200-280 nm) as illustrated in Chapter 10. The radiation energy associated with a photon (E) depends on the wavelength of light (k) according to the equation ... 

Increments in phenolics content after UV-B exposure have been recorded for a number of plant species (e.g. Caldwell et al., 1983 Chaves et al., 1997 Turunen et al., 1999). For example, a 10-fold increase in flavonoid concentration occurred in grape vine plants Vitis viniferd) exposed to natural sunlight relative to plants in which UV-A and UV-B radiation had been filtered out (Kolb et al., 2001). Similar responses include UV-B tolerant Arabidopsis mutants (Bieza and Lois, 2001) and lichens in which a clear attenuation of high frequency photons within the thallus was observed due to chemical filtration (Hall et al., 2002). Birch Betula... 

UV-A, -B and -C reveal differences in their ability to penetrate the human skin While UV-C barely reaches the upper epidermis, UV-B penetrates the epidermis and part of it reaches the dermis. UV-A, the radiation with the least photon energy and longest wavelengths within the UV band reaches the dermis and small amounts of it even reach the subcutaneous tissue [29,30] (Fig. 4). 

Fig. 1. (a) The two isomers of NsbF. (b) Photoionization efficiency curve of NasF. Ab initio vertical ionization energies of the isomers are indicated by vertical lines. The UV probe photon energy used in the photodepletion experiment, as well as the femtosecond pump+probe total energy, is indicated by down arrows, (c) Photoabsorption spectrum of NasF determined by photodepletion spectroscopy. The calculated oscillator strengths for vertical excitations are indicated for both the Cjy and Csv structures by straight and dotted lines respectiveIy.[S] The spectra of the femtosecond pulses are also indicated. 

---