III. RESPIRATORY PROCESSES
A. Dark Respiration
In general terms, the expression “dark respiration” might mean any plant physiological reaction linked to
the consumption of molecular oxygen without(?) the participation of light. A correct and detailed defini-
tion cannot easily be given because the process as a whole comprises quite a few independent reactions
that (even separately) participate in the oxidation of carbohydrates: glycolysis, oxidative decarboxylation,
tricarboxylic acid (TCA) cycle, NADH oxidation. In this chapter we will not specifically describe and
elaborate on this process; rather, the reader is referred to one of the many chapters in modern physiolog-
ical textbooks and reviews [e.g., 35–37]. However, we want to stress some of the specific points that play
a role in modern plant physiology research, in particular with respect to environmental problems and to
interference with photosynthesis.
Because photosynthesis and respiration collaborate to fulfill energy needs of plant cells, it can be ex-
pected that these processes interfere—one affecting the other. In photosynthetic prokaryotes, the electron
transport systems of both photosynthesis and respiration even take place in the same membranes, which
might represent the highest challenge for coordinate regulation. Thus, photosystem II–generated electrons
together with those produced from substrate oxidation form a common electron pool for the cellular en-
ergy-consuming pathways. In higher plants and algae, despite the separation of photosynthesis and respi-
ration in different cell organelles (chloroplasts and mitochondria), they have been shown to interact quite
efficiently.
Apart from the interference of electron transport reactions, other photosynthetically relevant factors
such as CO 2 and light are reported to have an impact on respiration. It was found that CO 2 enrichment in-
creased the root respiration of wheat by 24% [38]. The dark respiration rate of gametophytes of the trop-
ical epiphytic fern Pyrrosia piloselloidesincreased substantially with increasing CO 2 concentrations dur-
ing growth [39]. The general stimulation of the rate of shoot respiration in plants by CO 2 enrichment was
clearly time dependent [40–42]. Unfortunately, stimulation of respiratory activity by an elevated carbon
dioxide partial pressure was not consistently observed and described in the literature. Thus, in the case of
strawberry (Fragaria xananassa) leaves, high CO 2 concentrations (up to 900 ppm) did not significantly
affect the dark respiration rate [43]. The rate of both shoot and root respiration in Plantago majorin-
creased with increasing internal nitrogen concentration but was not affected by CO 2 [44]. In accordance
with these finding, it was reported [45] that shoot and root respiration per unit dry weight was positively
correlated with the nitrogen content as a common phenomenon but was again not altered by the atmo-
spheric CO 2 concentration. Elevated CO 2 levels slightly but significantly increased dark respiration in
Abutilonbut had no significant effect on dark respiration in Ambrosia[46]. In shoot and root respiration,
it was found that the use of carbon compounds often decreased when the atmospheric CO 2 concentration
increased [41,47] and thus may have contributed to an increased relative growth rate at elevated CO 2. The
effect of carbohydrates on the expression of respiratory genes at the transcriptional level has been
described [48].
Light as an environmentally relevant factor affects respiration (in)directly via its impact on photo-
synthesis. (In photosynthetic systems, the mechanism might involve the export of energy out of the
chloroplasts under excess high light conditions.) Respiration (nonphotorespiratory mitochondrial CO 2 re-
lease) of tobacco leaves can be inhibited by light with a following stimulation in the dark. The inhibition
of respiration in the light took about 50 sec and was even evident at 3 mol photons m^2 sec^1 regard-
less of the light quality (red, blue or white) in tobacco leaves. Accordingly, two peaks of CO 2 release were
exhibited by tobacco leaves after switching off the light [49]. The initial CO 2 liberation was observed at
15–20 sec (the photorespiratory postillumination burst) and the second at 180–250 sec (light-enhanced
dark respiration, LEDR) following the offset of light. The increases of both LEDR and the light-induced
inhibition were positively correlated with each other and also positively correlated with the increasing ir-
radiance during the predark period, suggesting a dependence on the preceding photosynthesis. It has been
proposed that the cytochrome b 6 /ƒ complex should not be involved in respiratory electron transport be-
cause respiratory oxygen uptake was not suppressed by far-red illumination in SynechocystisPCC 6803
cells grown photoautotrophically [50].
Mitochondrial oxidation of respiratory substrates is usually catalyzed by the cytochrome oxidase or
by the so-called alternative oxidase. The alternative path, which is classically known to be cyanide resis-
tant but salicylhydroxamic acid (SHAM) sensitive, oxidizes respiratory substrates and produces more
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