36
O. Bjorkman and B. Demmig-Adams
2.4.3.1 The Xanthophyll Cycle
In addition to chlorophyll, carotenoids constitute a major component of
the pigment-protein complexes of the thylakoid membranes and some of
these carotenoids, especially ~-carotene, have long been considered to protect the membrane against destructive events caused by overexcitation
of the chlorophyll (Krinsky 1979; Siefermann-Harms 1987; Young 1991). Of
particular interest with respect to NRD are the carotenoids violaxanthin
(diepoxy-zeaxanthin), antheraxanthin (monoepoxy-zeaxanthin) and zeaxanthin (dihydroxy-~-carotene), which are the carotenoid components of the
xanthophyll cycle. When there is an excess of excitation energy, violaxanthin (V) is de-epoxidized to zeaxanthin (Z) via antheraxanthin (A). This deepoxidation occurs when light-driven proton pumping causes a lowering of
the lumen pH because the enzyme catalyzing the V to Z de-epoxidation
(violaxanthin de-epoxidase) has its optimum activity around pH 5
(Yamamoto 1979). Conversely, when light becomes limiting to photosynthesis, the lumen pH rises and Z is reepoxidized to V by the action of the
enzyme zeaxanthin epoxidase (Hager 1980). A recent study confirms that V,
A, and Z are all localized within the light-harvesting complexes of PS II as
well as in PS I and indicates that deepoxidation of V to Z takes place in
these complexes (Thayer and Bjorkman 1992).
2.4.3.2 Dynamics of Changes in the Epoxidation State
and Pool Size of the Xanthophyll Cycle Pigments
Changes in the Epoxidation State. It is now well established that both the
time course and the extent of NPQ development in leaves are strongly
correlated with Z formation under a wide range of conditions, irrespective
of whether excess light was imposed by exposure to high PFDs or to water
stress, unfavorable leaf temperature, or subatmospheric CO2 pressures (see
reviews by Demmig-Adams 1990; Demmig-Adams and Adams 1992a). An
example of this close relationship is illustrated in Fig. 2.12, which compares
the steady state levels of NPQ and Z at different PFDs in a sun leaf
of Hedera canariensis and in Fig. 2.13 which shows that the temperature
dependence of the maximum rate of NPQ in a Malva leaf closely follows
that of Z formation. (Also see the relationship between the rate of development of NPQ and Z formation in Fig. 2.18, below).
The comparison between the diurnal changes in NPQ and in V, A, and Z
in leaves of cotton (Fig. 2.10) provides an example of the finding that a close
relationship between the Z content and NPQ also holds during the daily
course of natural variation in PFD in nature. That the epoxidation state of
the xanthophyll cycle pigments changes in response to diurnal changes in
PFD has been shown for many other C3 species (Demmig-Adams et al.
1989a; Thayer and Bjorkman 1990; Adams and Demmig-Adams 1992;
O. Bjorkman and B. Demmig-Adams
2.4.3.1 The Xanthophyll Cycle
In addition to chlorophyll, carotenoids constitute a major component of
the pigment-protein complexes of the thylakoid membranes and some of
these carotenoids, especially ~-carotene, have long been considered to protect the membrane against destructive events caused by overexcitation
of the chlorophyll (Krinsky 1979; Siefermann-Harms 1987; Young 1991). Of
particular interest with respect to NRD are the carotenoids violaxanthin
(diepoxy-zeaxanthin), antheraxanthin (monoepoxy-zeaxanthin) and zeaxanthin (dihydroxy-~-carotene), which are the carotenoid components of the
xanthophyll cycle. When there is an excess of excitation energy, violaxanthin (V) is de-epoxidized to zeaxanthin (Z) via antheraxanthin (A). This deepoxidation occurs when light-driven proton pumping causes a lowering of
the lumen pH because the enzyme catalyzing the V to Z de-epoxidation
(violaxanthin de-epoxidase) has its optimum activity around pH 5
(Yamamoto 1979). Conversely, when light becomes limiting to photosynthesis, the lumen pH rises and Z is reepoxidized to V by the action of the
enzyme zeaxanthin epoxidase (Hager 1980). A recent study confirms that V,
A, and Z are all localized within the light-harvesting complexes of PS II as
well as in PS I and indicates that deepoxidation of V to Z takes place in
these complexes (Thayer and Bjorkman 1992).
2.4.3.2 Dynamics of Changes in the Epoxidation State
and Pool Size of the Xanthophyll Cycle Pigments
Changes in the Epoxidation State. It is now well established that both the
time course and the extent of NPQ development in leaves are strongly
correlated with Z formation under a wide range of conditions, irrespective
of whether excess light was imposed by exposure to high PFDs or to water
stress, unfavorable leaf temperature, or subatmospheric CO2 pressures (see
reviews by Demmig-Adams 1990; Demmig-Adams and Adams 1992a). An
example of this close relationship is illustrated in Fig. 2.12, which compares
the steady state levels of NPQ and Z at different PFDs in a sun leaf
of Hedera canariensis and in Fig. 2.13 which shows that the temperature
dependence of the maximum rate of NPQ in a Malva leaf closely follows
that of Z formation. (Also see the relationship between the rate of development of NPQ and Z formation in Fig. 2.18, below).
The comparison between the diurnal changes in NPQ and in V, A, and Z
in leaves of cotton (Fig. 2.10) provides an example of the finding that a close
relationship between the Z content and NPQ also holds during the daily
course of natural variation in PFD in nature. That the epoxidation state of
the xanthophyll cycle pigments changes in response to diurnal changes in
PFD has been shown for many other C3 species (Demmig-Adams et al.
1989a; Thayer and Bjorkman 1990; Adams and Demmig-Adams 1992;