Month encounters a considerably "thicker" ozone layer than at the equator, where it takes the shortest path through the atmosphere. The UV radiation is thus relatively high in the tropics and relatively low in polar regions. The higher intensity of UV radiation with increasing elevation above sea level is also known. This is caused by an attenuation of the tropospheric ozone layer as well as by a decrease in the intensity of haze.

UV radiation may damage cells and so is dangerous to organisms (Table 1.2.2); its intensity is therefore often not given as the flux of quanta, but in units expressing the damaging effects, e.g. as DNA thymine dimerisation or erythema formation (reddening of skin, sunburn). There is consensus that plants may suffer UV damage; thus the debate is how much UV radiation is tolerated by plants, or the extent to which damage can be repaired. Scientists vary considerably in their opinions. The phytoplankton of cooler seas appears to be particularly susceptible and significant reduction in their mass has been repeatedly shown, despite the shallow depth to which UV light penetrates into a body of water (Fig. 1.2.13). The effect of increased UV-B radiation on the growth and yield of food plants has been extensively studied. Significant differences in sensitivity of various cultivars, e.g. of soya bean or maize, have been shown. Decreased yields

■ Table 1.2.2. Physiological effects of increased UV-B radiation

DNA damage Dimerisation of thymine; breakage of strands

Biomembranes Lipid peroxidation Photosynthetic Inactivation of PS II; acceleration of apparatus the turnover of D1 (and D2?); damage to thylakoid membranes; bleaching of pigments; decrease in the activity of photosynthetic enzymes (particularly Rubisco); inactivation of photosynthetic genes Phytohormones Photooxidation of auxin UV avoidance:

Secondary meta- Activation of the UV-B/blue light re-boiism ceptors; activation of expression of the key genes of phenylpropane metabolism and accumulation of flavo-noids; accumulation of alkaloids, waxes and polyamines Radical scavengers Increased capacity of the antioxidative system (ascorbate peroxidase, SOD, glutathione reductase, and others)

have been statistically established occasionally, but the reductions were relatively small (usually <10%). For sensitive species, the onset of flowering was delayed. However, by harvest, this delay was usually balanced out. For a long-day plant (Hyoscyamus niger, henbane), UV even stopped the induction of flowering.

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