Regulation of Photosynthetic Light Energy Capture
41
Thus, one may speculate that the events leading to an increased carotenoid
biosynthesis are triggered by a factor associated with excessive light energy
such as a low epoxidation state of the xanthophyll cycle. The finding that,
among the bulk pigments of the thylakoid membrane, the xanthophyll cycle
components are the only ones that exhibit large dynamic changes in response
to excessive light both in the short term (reversible deepoxidation) and in
the longer term (pool size adjustment) provides indirect evidence that this
cycle plays a role in the protection against excess light.
2.4.4 Mechanism of Nonradiative Dissipation
In Fig. 2.12 and Fig. 2.13 the de-epoxidation of V to Z was monitored in
intact leaves by measurements of changes in absorbance. These measurements are based on the fact that V, A, and Z differ in their absorption
spectra. The spectral changes caused by light-induced Z formation in a
Prunus leaf are shown in Fig. 2.17, curve a. The maximum change caused by
the deepoxidation occurs around 510 nm. Another absorbance change which
peaks around 540 nm (Fig. 2.17, curve b) is thought to reflect the conformational change of the thylakoid membrane considered to be involved in
4.5 r---~--.---.---.---~---.---,---.---.---.
4.0
3.5
3.0
:2i 2.5
x 2.0
o
o 1.5
1.0
0.5
0.0
Prunus ilicifolia
-0.5 L -__ .l..-__ --'----__ --'-__ ---L __ ---1 ____ L -__ .l..-__ --'----__ --'-_--l
440 460 480 500 520 540 560 580 600 620 640
Wavelength, nm
Fig. 2.17. Spectra of light-induced absorbance changes in a Prunus ilicifolia leaf caused
by de-epoxidation of violaxanthin to zeaxanthin (curve a) and by "light-scattering"
changes thought to reflect a conformational change in the thylakoid membranes (curve b).
The total spectral change caused by exposure of a pre darkened Prunus leaf to a PFD of
650llmol photons m- 2 s- 1 is shown by curve c. Curve b is the spectral change observed
during the relaxation upon return to darkness. Measurements were made with the instrumentation described by Brugnoli and Bjorkman (1992). (Original data by O. Bjorkman
and C. Shih)
41
Thus, one may speculate that the events leading to an increased carotenoid
biosynthesis are triggered by a factor associated with excessive light energy
such as a low epoxidation state of the xanthophyll cycle. The finding that,
among the bulk pigments of the thylakoid membrane, the xanthophyll cycle
components are the only ones that exhibit large dynamic changes in response
to excessive light both in the short term (reversible deepoxidation) and in
the longer term (pool size adjustment) provides indirect evidence that this
cycle plays a role in the protection against excess light.
2.4.4 Mechanism of Nonradiative Dissipation
In Fig. 2.12 and Fig. 2.13 the de-epoxidation of V to Z was monitored in
intact leaves by measurements of changes in absorbance. These measurements are based on the fact that V, A, and Z differ in their absorption
spectra. The spectral changes caused by light-induced Z formation in a
Prunus leaf are shown in Fig. 2.17, curve a. The maximum change caused by
the deepoxidation occurs around 510 nm. Another absorbance change which
peaks around 540 nm (Fig. 2.17, curve b) is thought to reflect the conformational change of the thylakoid membrane considered to be involved in
4.5 r---~--.---.---.---~---.---,---.---.---.
4.0
3.5
3.0
:2i 2.5
x 2.0
o
o 1.5
1.0
0.5
0.0
Prunus ilicifolia
-0.5 L -__ .l..-__ --'----__ --'-__ ---L __ ---1 ____ L -__ .l..-__ --'----__ --'-_--l
440 460 480 500 520 540 560 580 600 620 640
Wavelength, nm
Fig. 2.17. Spectra of light-induced absorbance changes in a Prunus ilicifolia leaf caused
by de-epoxidation of violaxanthin to zeaxanthin (curve a) and by "light-scattering"
changes thought to reflect a conformational change in the thylakoid membranes (curve b).
The total spectral change caused by exposure of a pre darkened Prunus leaf to a PFD of
650llmol photons m- 2 s- 1 is shown by curve c. Curve b is the spectral change observed
during the relaxation upon return to darkness. Measurements were made with the instrumentation described by Brugnoli and Bjorkman (1992). (Original data by O. Bjorkman
and C. Shih)