tion of PQ by reduction equivalents from the stroma. High amounts of accumulated starch were observed
in transformants with deletions within the ndhC-K-J region, and this result was explained by suboptimal
oxidation of glucose in both glycolysis and the oxidative pentose phosphate pathway [110]. In largely
identical experiments with tobacco plants defective in NAD(P)H dehydrogenase, altered chlorophyll
emission behavior (Fv/Fm) was taken as evidence for enhanced sensitivity to photoinhibition in the case
of the transformants. Repetitive illumination of the ndh-defective plants at high light intensity even led to
severe responses with respect to the pigments; i.e., the plants showed strong chlorosis and were much less
able to recover from the treatment than the wild type, which also showed a smaller effect [107,112]. In
the case of the diatom Phaeodactylum tricornutum, it was shown that chlororespiration appeared to con-
tribute to the proton gradient needed for the formation of diatoxanthin and that this proton gradient was
as effective as is that in the case of a light-driven pH [113]. A type of control of photosynthesis by vary-
ing reaction rates of chlororespiration appeared to be much more significant (and important for the plants)
under heat stress, i.e., under conditions of elevated temperatures [103].
Interesting investigations of the distribution of chlororespiratory activities within a plant came from
molecular biological and immunological analyses. Fragments of NAD(P)H-plastoquinone oxidoreduc-
tase from barley were expressed in Escherichia coliand antisera against a protein of approximately 70
kDa were prepared. From these experiments, enhanced ndhF levels were calculated for etiolated tissue in
relation to greening leaves. The values were higher in roots than in leaves, and on a timely basis ndhF val-
ues decreased during senescence. Photo-oxidative treatments generally increased the levels [114].
Chlororespiration appears to control and regulate the activity of photosystem II as it “mediates” the over-
all rate of electron flow through the transport chain between the two photosystems and affects the redox
condition within the sequence. At least in green algae it was observed that, e.g., acetate enhances
chlororespiration rates. Under these conditions the photosystem II activity is down-regulated, probably in
order to avoid overreduction at specific sites (where, e.g., destructive reactive oxygen species might oth-
erwise be produced). In principle, this corresponds to any condition of heterotrophic growth where the
water-splitting capacity is decreased because of the presence of reduced carbon sources [115]. Generally,
it was suggested that chlororespiration from the onset of illumination serves to prevent any overreduction
of the transport carriers involved in the electron transport system of higher plants (maize) correlated with
lower Calvin cycle rates [102]. Structural evidence for the necessity for a chlororespiratory mechanism
in plants might as well be derived from the complete nucleotide sequencing of the genome of EpifagusPHOTOSYNTHETIC GAS EXCHANGE AND RESPIRATION 317Figure 13 Dependence of the quantum fluence rate of blue light (448 nm) and of red light (679 nm)
of^16 O 2 evolution and^18 O 2 consumption of isolated chloroplasts from Pisum sativum. (From Ref. 109.)