1021-4437/05/5205- © 2005
Russian Journal of Plant Physiology, Vol. 52, No. 5, 2005, pp. 645–652. From Fiziologiya Rastenii, Vol. 52, No. 5, 2005, pp. 726–733.
Original English Text Copyright © 2005 by Ebrahim.
Since the discovery of ultraviolet wavelength band
at 200–390 nm, extensive studies have been made to
investigate its characteristics and possible effects in all
living organisms. These studies have subdivided it into
(1) UV-C (200–280 nm) extremely harmful to living
organisms, (2) UV-B (280–320 nm) is of particular
interest, because the depletion of ozone (O
mainly allows this radiation to reach the Earth surface,
and (3) UV-A (320–390 nm) represents the least haz-
ardous part of UV radiation . Solar electromagnetic
radiation does not reach the Earth due to the presence
of the O
layer in the stratosphere . Hence, UV radi-
ations at the Earth surface are UV-A and UV-B, which
can increase with the thinning and depletion of the O
layer due to the industrial emissions of chloroﬂuorcar-
bons . Over 90% of UV radiation reaching the Earth
surface is UV-A while the rest (10%) includes UV-B .
UV-B radiation exerts large negative effects on most
physiological processes in plants [1–3, 5, 6] due to its
absorption by proteins and nucleic acids .
UV can penetrate through plant leaves and be
absorbed by chromophores associated with the photo-
synthetic apparatus and/or by genes and gene products
[5, 7]. Cellular constituents that can directly absorb UV
include nucleic acids, proteins, lipids, and quinones .
Water-soluble phenolic pigments, such as ﬂavonoids,
can strongly absorb UV, whilst not absorbing photosyn-
thetically active radiation . Therefore, leaf penetra-
tion by UV may be negatively correlated with these
UV-absorbing compounds, as well as with the leaf
thickness. UV-stress potentially impairs photophospho-
ﬁxation, and/or a stomatal control of
supply ; hence, it may damage the photosynthesis
and consequently the plant growth and productivity.
Tolerance Responses of Two Cotton Cultivars
Exposed to Ultraviolet-A: Photosynthetic Performance
and Some Chemical Constituents
M. K. H. Ebrahim
Botany Department, Faculty of Science, Tanta University, Tanta, 31527 Egypt;
fax: +2-040-3350804; e-mail: email@example.com
Received February 26, 2004
—Stress tolerance of two Egyptian cotton cultivars (
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 pro-
moted 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
diminished Hill reaction activity, and quenched Chl
ﬂuorescence independent of the presence of 3-(3,4-
dichlorophenyl)-1,1-dimethylurea, suggesting an inhibitory effect on both the water-splitting system and elec-
tron transport from the primary to the secondary acceptors of photosystem II (PSII) (acceptor side). UV-A radi-
ation 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 ﬂu-
idity. 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 adap-
tation, it tolerates UV-A stress more successful than cv. Giza 45.
Key words: Gossypium barbadense - chlorophyll - ﬂuorescence - fatty acids - ﬂavonoids - germination - growth -
Hill reaction - photosynthetic apparatus - ultraviolet-A
: Car—carotenoids; Chl—chlorophyll; DCMU—3-
indophenol; FA—fatty acids; TSP—total soluble proteins; UV-A—
ultraviolet-A radiation (366 nm); PS—photosystem.
The text was submitted by the authors in English.