44
O. Bjorkman and B. Demmig-Adams
consistent with the finding that, upon depoxidation, Z and A do indeed
replace V in the PS II light-harvesting complexes (Thayer and Bjorkman
1992). The finding that deep oxidation of V to Z also takes place in PS I may
indicate that a simultaneous increase in NRD occurs in this photosystem as
well.
2.5 Conclusions
In this chapter we have documented that plants have evolved a remarkable
array of mechanisms that enable them to regulate both the interception and
the dissipation of light energy, both in the short term (second to minutes)
and in the longer term (hours to days). Some of these mechanisms, such as
rapid leaf movements, are present in relatively few species, while other
processes, such as nonradiative dissipation, are most probably ubiquitous in
green plants. At present, our understanding of the molecular mechanism of
nonradiative energy dissipation in the pigment bed is only rudimentary;
current hypotheses are likely to be subject to change and yet other mechanisms of light regulation may also be discovered. It is certain, however, that
if one has seen one mechanism of light regulation, one has not seen them
all.
The fact that such a wide array of photoprotective processes have evolved
is fully consistent with the assumption that overexcitation of the photosynthetic system is very harmful and must be avoided. Moreover, laboratory
experiments have provided strong evidence that such overexcitation can
cause injury to the photosystems, especially to the PS II reaction center
complex. However, little is known about the actual occurrence of photoinhibitory damage in nature and even less about the consequences with
regard to primary productivity. Our present guess is that photoinhibitory
damage is uncommon in natural plant stands. Responses that in the past
were thought to be indicative of photoinhibitory damage (reduced photon
yield of CO2 uptake and O2 evolution and quenching of the maximum
fluorescence yield) in many cases now appear to be reflections of the
operation of photo-protective processes.
References
Adams WW III, Demmig-Adams B (1992) Operation of the xanthophyll cycle in higher
plants in response to diurnal changes in incident sunlight. Planta 186: 390-398
Adams WW III, Smith SD, Osmond CB (1987) Photoinhibition of the CAM succulent
Opuntia basi/aris growing in Death Valley: evidence from 77K fluorescence and quantum
yield. Oecologia 71: 221-228
O. Bjorkman and B. Demmig-Adams
consistent with the finding that, upon depoxidation, Z and A do indeed
replace V in the PS II light-harvesting complexes (Thayer and Bjorkman
1992). The finding that deep oxidation of V to Z also takes place in PS I may
indicate that a simultaneous increase in NRD occurs in this photosystem as
well.
2.5 Conclusions
In this chapter we have documented that plants have evolved a remarkable
array of mechanisms that enable them to regulate both the interception and
the dissipation of light energy, both in the short term (second to minutes)
and in the longer term (hours to days). Some of these mechanisms, such as
rapid leaf movements, are present in relatively few species, while other
processes, such as nonradiative dissipation, are most probably ubiquitous in
green plants. At present, our understanding of the molecular mechanism of
nonradiative energy dissipation in the pigment bed is only rudimentary;
current hypotheses are likely to be subject to change and yet other mechanisms of light regulation may also be discovered. It is certain, however, that
if one has seen one mechanism of light regulation, one has not seen them
all.
The fact that such a wide array of photoprotective processes have evolved
is fully consistent with the assumption that overexcitation of the photosynthetic system is very harmful and must be avoided. Moreover, laboratory
experiments have provided strong evidence that such overexcitation can
cause injury to the photosystems, especially to the PS II reaction center
complex. However, little is known about the actual occurrence of photoinhibitory damage in nature and even less about the consequences with
regard to primary productivity. Our present guess is that photoinhibitory
damage is uncommon in natural plant stands. Responses that in the past
were thought to be indicative of photoinhibitory damage (reduced photon
yield of CO2 uptake and O2 evolution and quenching of the maximum
fluorescence yield) in many cases now appear to be reflections of the
operation of photo-protective processes.
References
Adams WW III, Demmig-Adams B (1992) Operation of the xanthophyll cycle in higher
plants in response to diurnal changes in incident sunlight. Planta 186: 390-398
Adams WW III, Smith SD, Osmond CB (1987) Photoinhibition of the CAM succulent
Opuntia basi/aris growing in Death Valley: evidence from 77K fluorescence and quantum
yield. Oecologia 71: 221-228