Stress tolerance responses of two cotton cultivars exposed to ultraviolet-A (366 nm) radiation: photosynthetic performance and chemical constituents

Abstract

The 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 (366 nm) radiation were investigated. The 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, the seed germination of Giza 45 was progressively inhibited by UV-A exposure and ceased at 160 min, so doses of 40 and 80 min were selected for further studies. In contrast to seed germination, the seedling growth of Giza 86 was negatively affected at 40 min. UV-A stress induced a great reduction in the leaf carbohydrates as well as in the viability and dry mass production of the shoots of both cultivars, but the response was comparatively higher in Giza 45. It also decreased the chorophyll (Chl) and carotenoid contents, coupled with an increase in the Chla/bratio, diminished the Hill reaction activity, and quenched the Chlafluorescence both in the presence and absence 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 PSII (acceptor side). These changes reflect a disturbance in the structure, composition and function of the photosynthetic apparatus as well as the sensitivity of PSII to UV-A stress. Nucleic acids (DNA and RNA) were markedly damaged by exposure to UV-A for 80 min, while 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, thicker leaf blades. Since Giza 86 showed a comparatively higher level of adaptation, it tolerates UV-A stress better than Giza 45.Abbreviations:Car, carotenoids; Chl, chlorophyll; DCMU, 3-(3,4-dichlorophenyl)-1,1-dimethylurea; DCPIP, 2,6-dichlorophenol indophenol; DNA, deoxyribonucleic acid; d.m., dry mass; f.m., fresh mass; PSII, photosystem II; RNA, ribonucleic acid; TSP, total soluble proteins; UV-AR, ultraviolet-A (366 nm) radiation.