Abstract
Stress/tolerance responses of two Egyptian cotton cultivars (Giza 45 and 86) exposed to various doses (40, 80, 160 and 320 min) of artificial ultraviolet-A radiation (366 nm) were investigated. Seed germination of Giza 86 was promoted at 40 min, but substantially inhibited at 80 and 160 min, and completely suppressed at 320 min. However, seed germination of Giza 45 was progressively inhibited by UV-A exposure and stopped at 160 min, hence, the doses 40 and 80 min were selected for further studies. In contrast, to seed germination, seedling growth of Giza 86 was negatively affected at 40 min. UV-A stress induced great reduction in the leaf carbohydrates as well as in the viability and dry mass production by shoots of both cultivars, but the response was comparatively higher in Giza 45. It decreased chorophyll (Chl) and carotenoid contents coupled with an increase in Chl a/b ratio, diminshed Hill reaction. activity, and quenched Chl a fluorescence whether in the absence or presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, suggesting an inhibitory effect on the water-splitting system (donor side) as well as on the electron transport from the primary to the secondary acceptors of PS2 (acceptor side). It also decreased the ratio of unsaturated to saturated fatty acids, in thylakoid membranes, indicated that the inhibition of PS2 activity was tollowed by lipid peroxidation and changes in the thylakoid membrane fluidity. These changes reflect the disturbance of structure, composition and function of the photosynthetic apparatus as well as the sensitivity of PS2 to UV-A stress. Nucleic acids (DNA and RNA) were markedly damaged by exposing to UV-A for 80 min, in turn, both cultivars developed adaptive mechanisms for damage moderation. These mechanisms involved increasing the levels of flavonoids, total lipids and total soluble proteins as well as having smaller and thicker leaf blades. They also confirm a decisive role in stress-tolerance response. Since Giza 86 comparatively showed higher magnitude of adaptation, it tolerates UV-A stress more than Giza 45.