Results (
Thai) 2:
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Only data for those compounds which levels varied in more
than double as compared with the values found in control (without
UV-B) tissues are presented.
2.1. UV-B induces synthesis of membrane-related sterols
As shown in Fig. 1, the amount of sterols (1–3) measured in
leaves of grape plants significantly increased after both UV-B treatments
as compared with the control in which UV-B was filtered,
and these amounts were also affected by leaf ontogeny. It has been
reported that the most abundant plant sterols in nature are sitosterol
(1) and stigmasterol (2) (Croteau et al., 2000), which regulate
membrane fluidity and play a role in the adaptation of membranes
to different stresses (Piironen et al., 2000; Schaller, 2003). Consistent
with this, sitosterol (1) was the most abundant sterol and increased
16.4-fold in young leaves and 8.0-fold in mature leaves
under low UV-B irradiance, and 4.8-fold in young leaves and 1.8-
fold in mature leaves by high UV-B irradiance, as compared with
the control (Fig. 1a). Thus, sitosterol (1) levels were higher in younger
leaves than in mature leaves under both, low and high UV-B
irradiance treatments. The concentration of stigmasterol (2) was
augmented 3.2-fold under low UV-B in young leaves as compared
to control, and 2.3-fold with respect to high UV-B, without differences
in the rest of the treatments (Fig. 1b). Levels of another
triterpenoid related with antioxidant defense system, lupeol (3)
(Bracco et al., 1981), increased by low UV-B, 4.2-fold in young
leaves and 3.2-fold in mature leaves as compared with control
plants, without any effect of high UV-B (Fig. 1c). Under the three light treatments, lupeol (3) levels were always higher in mature
leaves than in young leaves.
2.2. High UV-B irradiance increases antioxidant diterpenes in mature
leaf tissues
Tocopherols (4–6) are antioxidant diterpenes with a well-recognized
ability to protect cells from oxidative stress (Fryer, 1992).
Levels of c-tocopherol (4), a-tocopherol (5) and phytol (6) increased
in grape plants exposed to high UV-B irradiation (Fig. 2).
c-Tocopherol (4) was augmented 1.9-fold in young leaves and 3.3-fold in mature leaves under high UV-B respect to low UV-B
irradiation, and 3.6- and 6.1-fold as compared to their respective
controls (Fig. 2a). a-Tocopherol (5) levels were 2-fold increased
in mature leaves under high fluence UV-B conditions with regard
to the rest of the treatment. The highest concentration was measured
in mature leaves exposed to high UV-B (Fig. 2b). Among
diterpenes measured in grapevine leaf tissue, phytol (6) was the
most abundant and its level was augmented 2.8-fold under high
UV-B as compared with controls and low UV-B irradiation, irrespective
of leaf ontogeny (young or mature, Fig. 2c).
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