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toquinol to P700+ .photosynthetic control) versus the
rate of photooxidation of P700. The higher P700+ levels
consistently observed for the mutant could be due to a
larger absorption cross-section for PSI. However, the
PSII absorption cross-section .a2, see above) and Chl a/b
level .Table 1) were unaected by the mutation so it is
unlikely that the PSI absorption cross-section was
altered. Maximum steady-state accumulation of P700+
occurred at CO2 levels £ 5 lM .Fig. 3). Asthe Ci in-
creased there was a progressive shift to a lower degree of
oxidation .Fig. 3) associated with an increase in alloca-
tion of electrons to ATP-consuming carbon metabolism
.Fig. 8). The ensuing decline in DpH would decrease the
xanthophyll cycle carotenoidsand chlorophyll ¯uorecsence in
light intensity-dependent chlorophyll-de®cient mutants of
wheat and barley. Photosynth Res 42:191±202
Foyer C, Furbank R, Harbinson J, Horton P .1990) The mecha-
nisms contributing to photosynthetic control of electron
transport by carbon assimilation in leaves. Photosynth Res
25:83±100
Funk C, Schroder WP, Napiwotzki A, TjusSE, Renger G,
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resistance to plastoquinol oxidation by Cyt b f allowing
6
an increased ¯ow of electrons to P700+ .Harbinson and
Hedley 1989; Foyer et al. 1990). The decline in levelsof
P700+ at very low Ci levelsisdue to blockage of P700
photooxidation by reduced acceptors.Laiks and Oja
1995). Nevertheless, the relative increase in oxidation of
the PSI donor in the mutant compared to the WT is
highest under these conditions, consistent with dier-
ential eects on inter-photosystem electron transport.
The PSII acceptor side was under all conditions more
reduced .lower qP) and the PSI donor side was more
oxidized .higher DA820/DA820max) in the mutant. This
indicates a greater resistance to electron ¯ow from
plastoquinol to P700+. Higher P700+ levelsin the mu-
tant may result from maintenance of a higher DpH,
altered sensitivity to the DpH, or a change in the inter-
action between the plastoquinone pool and the PSII
acceptor side involving a shift in the midpoint redox
potential of QA .Krieger and Weis1993). It isunclear at
thistime how the npq4-9 mutation altersboth the NPQ
mechanism and control of electron ¯ow between PSII
and PSI.
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leaves: a novel in situ probe of thylakoid functioning. Plant Cell
Environ 12:357±369
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and relationsto nonphotochemical quenching. Plant Phyisol
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the photosynthetic apparatus and its dependence on the leaf
developmental stage in the npq1 arabidopsis mutant de®cient in
the xanthophyll cycle enzyme violaxanthin de-epoxidase. Plant
Physiol 124:273±284
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Jansson S .1999) A guide to the Lhc genesand their relativesin
Arabidopsis. TrendsPlant Sci 4:236±240
Krieger A, WeisE .1993) The role of calcium in the pH-dependent
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912
Acknowledgements We thank Krishna Niyogi and Alba Phippard
for performing the DNA sequencing and providing the antibody
against PSII-S. We also extend thanks to Neil Schultes for helpful
discussions and to Carol Clark for skillful technical assistance.
Laisk A, Oja V .1995) Coregulation of electron transport through
PSI by Cyt b6f, excitation capture by P700 and acceptor side
reduction. Time kineticsand electron tranpsort requirement.
Photosynth Res 45:11±19
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