sylvestrisL.) branches subject to abscission was analyzed for a cost-benefit approach [92]. All but one of
the branches studied were found to be nonproductive (the budget was negative) over the growing season.
Following a decrease in photosynthetic capacity in July, the cumulative budget became negative and the
branches died, indicating that a negative carbon budget corresponds to the onset of abscission of the low-
est branches. It has been suggested that maintenance respiration per unit biomass is frequently not con-
stant. Rather, it decreases as biomass increases, so that the ratio of photosynthesis versus respiration is of-
ten approximately constant [93]. In these investigations, the photosynthesis/respiration ratio was
measured as a quantitative measure of the capability of frost hardening in wintering cereal plants. It was
found that the average level of the photosynthesis/respiration ratio in the hardened Triticum aestivum
plants at 0°C was two times higher than that in the control plants at 18°C [94].
Experiments suggested that a clear separation of respiration into growth and maintenance compo-
nents might not be a useful concept because they cannot be unambiguously measured or defined in terms
of biochemical processes [95]. Accordingly, growth yield calculations from biochemical pathway analy-
sis, from biomass molecular composition, from biomass heat of combustion, and from biomass elemen-
tal composition do not include all of the energy costs for biosynthesis; thus, they are not accurate mea-
sures of the carbon cost for plant growth. Improper definitions of growth respiration relations are
impeding the use of physiological properties for prediction of plant growth as a function of environmen-
tal variables. In accordance with the preceding argument, it was concluded that the failure to account for
tissue nitrogen effects on respiration rates results in serious errors when estimating annual maintenance
costs [90].
D. Chlororespiration
The phenomenon of chlororespiration is based on experiments in the 1960s in the laboratories of Bessel
Kok, who demonstrated a significant effect of light not only on photosynthesis but also on the respiratory
activity of algae that might be interpreted in the sense of a link between the photosynthetic and the respi-
ratory electron transport chains [96]. Later, this observation was substantiated by Pierre Bennoun, who
demonstrated respiratory activity in the chloroplast membranes in a green alga (Chlamydomonas). The
idea was that there should be an electron transfer from NAD(P)H via the plastoquinone pool to oxygen
so that PQH 2 acts as the component common to both the photosynthetic and the respiratory electron path-
ways [97]. Achim Trebst’s group partially purified the respiratory enzyme NAD(P)H-plastoquinone ox-
idoreductase (NDH complex) and demonstrated that NAD(P)H could feed electrons into the photosyn-
thetic transport chain [98,99]). For a long time the question was whether the phenomenon was restricted
to lower photosynthetic organisms or might also exist in higher plants. Some but not too much evidence
from physiological and biochemical data was given to support the latter possibility, e.g., from fluores-
cence studies in tobacco (Nicotiana tabacum), peas (Pisum sativum), and maize (Zea mays). Interestingly,
“chlororespiratory activity” was stronger under conditions of nitrogen deficiency [100–103].
Comparative analyses with cyanobacteria, green algae, and higher plants were carried out to investi-
gate the effect of cyanide on light-induced redox reactions of the Cyt b 6 /ƒ complex. The authors described
a clear enhancement of the reduction rate of oxidized cytochrome ƒ by appropriate concentration of
cyanide, and no significant differences were obtained for pro- and eukaryotes [103]. Moreover, it was ob-
served that coding genes in the liverwort chloroplast exactly corresponded to the genes of mitochondrial
NADH dehydrogenase components [104]. This type of activity might also explain the nonphotochemical
reduction of plastoquinone in pea leaves [105]. In investigations with sunflower (Helianthus annuusL.),
leaves exhibited an increased minimal fluorescence following a light-to-dark transition. The increase could
be reversed by far-red illumination and inhibited by rotenone or methyl viologen. It was interpreted in the
sense that the capacity of the plant for plastoquinone reduction might directly affect the chloroplast ATP
synthase [106]. Antimycin A [an inhibitor of the ferredoxin quinone reductase (FQR)] inhibited the cyclic
electron flow around photosystem I via NDH in tobacco measured as an enhanced dark fluorescence after
a light phase. Interestingly, these reactions were also inhibited by Amytal (an inhibitor of mitochondrial
NADH dehydrogenase [NDH]) and by nigericin. The inhibition of plastoquinone reduction showed a
biphasic behavior that was taken as evidence for two different mechanisms; the inhibition at low Antimycin
A concentrations might be correlated with FQR and the one at higher concentrations with NDH [107].
Mass spectrometric analyses in our laboratory using a mixed isotope composition of the reaction as-
says have been performed with chloroplasts from Nicotiana tabacumto investigate the oxygen gas ex-
PHOTOSYNTHETIC GAS EXCHANGE AND RESPIRATION 315