Modification of flavonoid content and photosynthesis by ultraviolet-B radiation. Atrazine-tolerant and -sensitive ultivars of Brassica napus.

Levels of ultraviolet-B (UV-B, 280-320 nm) radiation reaching the Earth´s surface increase due to stratospheric ozone depletion. In this thesis the effect of enhanced UV-B radiation was studied on the atrazine-tolerant mutant Stallion, susceptible to photoinhibition, and the atrazine-sensitive cultivar Paroll of Brassica napus (oilseed rape). Photosynthetic parameters were studied with chlorophyll a fluorescence, photoacoustic measurements, CO2 fixation, immunoblotting and 35S-methionine labelling of the D1 protein. UV- screening was determined from internal radiation gradients in leaves by fibre optics and the content of UV-absorbing pigments by HPLC-diode array spectroscopy, -MS and NMR. Leaf anatomy was investigated by scanning electron microscopy. After two weeks of supplemental UV-BBE radiation (13 kJ m-2 day-1) and 800-900 µmol m-2 s-1 photosynthetically active radiation (PAR, 400-700 nm), the leaf area was reduced and leaf thickness increased, especially in the mutant. The penetration of UV-B radiation (310 nm) decreased in the leaves due to the accumulation of UV-screening phenolics. In the adaxial epidermis there was an increase especially in the flavonol quercetin, which is a better scavenger of free radicals than kaempferol, the latter of which dominates in control plants. Mostly kaempferol and hydroxycinnamic acid derivatives were found in the abaxial epidermis. The photosynthetic efficiency was unaltered or even higher after UV-B exposure in both cultivars. A concomitant increase in penetration of 430 nm radiation was noted. There was also an increase in carbon dioxide fixation after UV-B exposure. Short-term exposure (3-4 h) to UV-B and high PAR (1600-2000 µmol m-2 s-1) resulted in decreased photosynthetic efficiency and increased D1 degradation in both cultivars, although more severely in the mutant. Recovery under growth light was almost completed within four hours. However, it was less efficient after UV-B exposure, especially in the mutant. The D1 turnover was lower during recovery than under treatment and control conditions. UV-B radiation counteracted the reduction in photosynthetic capacity observed after cold stress (4C) under photoinhibitory conditions.

The results indicate that Brassica napus acclimatises to even high levels of UV-B radiation. However, the atrazine-tolerant mutant is more susceptible to short-term photoinhibition and UV-B radiation than the control cultivar.