Tolerance Responses of Two Cotton Cultivars Exposed to Ultraviolet-A: Photosynthetic Performance and Some Chemical Constituents

Stress tolerance of two Egyptian cotton cultivars (Gossypium barbadense L.) (Giza 45 and Giza 86) exposed to various doses (40, 80, 160, and 320 min) of artificial ultraviolet-A (UV-A) radiation (366 nm) was investigated. Seed germination of cv. Giza 86 was promoted at 40 min, progressively inhibited at 80 and 160 min, and completely suppressed at 320 min irradiation. However, seed germination of cv. Giza 45 was not promoted but inhibited by UV-A light and stopped at the dose of 160 min. In contrast to seed germination, seedling growth of cv. Giza 86 was negatively stressed even at 40 min-dose. UV-A radiation reduced leaf carbohydrate content and shoot growth of both cultivars, but the response was comparatively higher in cv. Giza 45. UV-A radiation decreased chlorophyll (Chl) and carotenoid contents in parallel with an increase in the Chl a/b ratio, diminished Hill reaction activity, and quenched Chl a fluorescence independent of the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, suggesting an inhibitory effect on both the water-splitting system and electron transport from the primary to the secondary acceptors of photosystem II (PSII) (acceptor side). UV-A radiation also decreased the ratio of unsaturated to saturated fatty acids in thylakoid membranes, thus indicating that the inhibition of PSII activity was followed by lipid peroxidation and changes in thylakoid membrane fluidity. These changes reflect the disturbance of structure, composition, and functioning of the photosynthetic apparatus, as well as the sensitivity of PSII to UV-A stress. Both cultivars developed adaptive mechanisms for damage alleviation involving the accumulation of flavonoids, total lipids, and total soluble proteins, as well as development of smaller and thicker leaf blades. Since cv. Giza 86 showed comparatively higher level of adaptation, it tolerates UV-A stress more successful than cv. Giza 45.