34
O. Bjorkman and B. Demmig-Adams
NPQ saturated already at moderate PFDs, indicating that the maximum
capacity is reached. Therefore, these shade leaves evidently were unable
to further counteract the accumulating excess excitation energy when the
incident PFD increased beyond 500/lmol photons m - 2 S -1. In Prunus, NPQ
did not reach saturation until about 1500/lmol photons m- 2 s- 1 and the
maximum level of NPQ was about twice as high as in Oxalis. In contrast to
Oxalis and Prunus, the Malva and cotton leaves showed only small increases
in NPQ up to 500/lmol photons m- 2 s- 1 and, beyond this point, NPQ
continued to rise more or less linearly with increasing PFD. Evidently, the
maximum capacity for NRD was not reached even in full sunlight in these
leaves, and there remained a "reserve" capacity for even higher NRD under
conditions where the photosynthetic capacity would be reduced by environmental stress.
In Fig. 2.10 are shown the diurnal courses of PFD, the efficiency of
energy conversion in PS II, the degree of reaction center closure, and NPQ
in fully exposed leaves of a cotton crop, growing in the field at Stanford,
California. (Also shown are the corresponding changes in the components of
the xanthophyll cycle; these will be referred to in Sect. 2.4.3.2) As expected,
NPQ closely followed the development of excessive light: it was very low in
the early morning, increased as the PFD increased to reach a peak around
solar noon, then gradually declined as the PFD decreased in the afternoon.
Conversely, the efficiency of energy conversion was highest in the morning
reached a minimum around noon and then rose again in the afternoon.
Although the fraction of closed centers, Qr/Ot' increased as the PFD increased, and then fell as the PFD decreased, it remained much lower than
one would expect in the absence of NRD, and at noon only 30% of the
centers were in the closed state.
Much larger diurnal changes in the photochemical efficiency of PS II and
in NPO than in this cotton crop can be seen in plants such as the cactus
Nopalea cochenillifera growing in the field (Fig. 2.11). The actual degree of
PS II center closure in a southeast facing cladode was maintained very low
even at peak PFD, presumably due to the very high NPQ level. The
hypothetical degree of reaction center closure that would have been reached
if there had been no NRD was 68%, compared with an actual closure of
only 20%.
2.4.3 Nonradiative Energy Dissipation and the Xanthophyll Cycle
There is much evidence that one prerequisite for the development of NPQ is
an acidification of the thylakoid lumen resulting from a buildup of a proton
gradient across the thylakoid membrane (Krause et al. 1982; Gilmore and
Yamamoto 1991, 1992). The extent of this gradient is determined by the
balance between the rate of electron transport driving the proton pumping
O. Bjorkman and B. Demmig-Adams
NPQ saturated already at moderate PFDs, indicating that the maximum
capacity is reached. Therefore, these shade leaves evidently were unable
to further counteract the accumulating excess excitation energy when the
incident PFD increased beyond 500/lmol photons m - 2 S -1. In Prunus, NPQ
did not reach saturation until about 1500/lmol photons m- 2 s- 1 and the
maximum level of NPQ was about twice as high as in Oxalis. In contrast to
Oxalis and Prunus, the Malva and cotton leaves showed only small increases
in NPQ up to 500/lmol photons m- 2 s- 1 and, beyond this point, NPQ
continued to rise more or less linearly with increasing PFD. Evidently, the
maximum capacity for NRD was not reached even in full sunlight in these
leaves, and there remained a "reserve" capacity for even higher NRD under
conditions where the photosynthetic capacity would be reduced by environmental stress.
In Fig. 2.10 are shown the diurnal courses of PFD, the efficiency of
energy conversion in PS II, the degree of reaction center closure, and NPQ
in fully exposed leaves of a cotton crop, growing in the field at Stanford,
California. (Also shown are the corresponding changes in the components of
the xanthophyll cycle; these will be referred to in Sect. 2.4.3.2) As expected,
NPQ closely followed the development of excessive light: it was very low in
the early morning, increased as the PFD increased to reach a peak around
solar noon, then gradually declined as the PFD decreased in the afternoon.
Conversely, the efficiency of energy conversion was highest in the morning
reached a minimum around noon and then rose again in the afternoon.
Although the fraction of closed centers, Qr/Ot' increased as the PFD increased, and then fell as the PFD decreased, it remained much lower than
one would expect in the absence of NRD, and at noon only 30% of the
centers were in the closed state.
Much larger diurnal changes in the photochemical efficiency of PS II and
in NPO than in this cotton crop can be seen in plants such as the cactus
Nopalea cochenillifera growing in the field (Fig. 2.11). The actual degree of
PS II center closure in a southeast facing cladode was maintained very low
even at peak PFD, presumably due to the very high NPQ level. The
hypothetical degree of reaction center closure that would have been reached
if there had been no NRD was 68%, compared with an actual closure of
only 20%.
2.4.3 Nonradiative Energy Dissipation and the Xanthophyll Cycle
There is much evidence that one prerequisite for the development of NPQ is
an acidification of the thylakoid lumen resulting from a buildup of a proton
gradient across the thylakoid membrane (Krause et al. 1982; Gilmore and
Yamamoto 1991, 1992). The extent of this gradient is determined by the
balance between the rate of electron transport driving the proton pumping