change in the presence of (normal) H 216 O substrate water and^18 O 2 in an artificial gas atmosphere [108].
Upon illumination with 10 short (5 sec) saturating xenon flashes and under identical conditions with re-
spect to the background signals of both oxygen isotopes, the tobacco chloroplasts evolved oxygen (as
(^16) O 2 ) equivalent to 334 mol and (simultaneously) took up 233 mol oxygen (as (^18) O 2 ). The photosys-
tem II acceptor silicomolybdate quantitatively inhibited the oxygen uptake signal and so did DBMIB (2,5-
dibromomethyl-6-isopropyl-p-benzoquinone), a chemical that efficiently oxidizes the plastoquinone
pool. The results were interpreted in the sense that a chlororespiratory process in which the plastoquinone
pool appears to be involved must be the source for the light-induced oxygen uptake. Theses results were
extended by Kowallik’s group using chloroplasts from peas [109]. Figure 12 shows parallel recordings of
both oxygen evolution (at m/e32) and oxygen uptake (at m/e36) in continuous light. (The constant
decline in the^18 O 2 signal is technically explained as due to the unidirectional gas flow from the measur-
ing cell to the ion source of the mass spectrometer.) Both the photoevolution of oxygen and the light-in-
duced oxygen uptake are dependent on the intensity of light but independent of the light quality (Figure
13). Also, the light-dependent^18 O 2 uptake increased with increasing light intensity. The effect was iden-
tical in blue (679 nm) and in red (679 nm) light. This effect closely resembled the increase in
photosynthetic oxygen evolution in both spectral regions and no matter what the tested fluence rate was.
Opposing arguments that such light-induced oxygen uptake might be linked to photorespiration or
Mehler-type reactions rather than to chlororespiration may be ruled out by the observation that oxygen
consumption occurred even at very low light intensities (Figure 13).
- Physiological Significance
The function of chlororespiration might be related to the idea that under specific conditions algae and
plants require a type of valve or overflow mechanism to remove excess reduction equivalents within the
chloroplast [110] and supply oxidative pyridine nucleotides for chloroplastic metabolism in the dark
[111]. Biochemical and molecular biological studies have described details of the plastid-specific
NAD(P)H-plastoquinone oxidoreductase (ndhgenes) and the homology to the mitochondrial NADH-
ubiquinone oxidoreductase in higher plants. Mutagenesis experiments showed that uninjured ndhgenes
are essential for the viability of tobacco plants. Transformants with deleted genes lacked a rapid fluores-
cence rise in the dark following illumination—the signal that is supposed to indicate the transient reduc-
316 BADER AND ABDEL-BASSETFigure 12 Mass spectrometric recording of the oxygen gas exchange of isolated chloroplasts from peas
(Pisum sativum) in white light of 4 molm^2 sec^1. Oxygen evolution has been recorded as H 216 O ox-
idation at m/e32, whereas the concomitant oxygen consumption was measured at m/e36 from an artifi-
cial^18 O 2 gas atmosphere over the aqueous phase of the reaction assay. The inset shows an identical experiment
with heat -denatured chloroplasts (10 min, 100°C) as a technical control. (From Ref. 109.)