heat energy than is produced by the cytochrome pathway, which generates more ATP. Generally, changes
in heat development are increased or decreased in close correspondence with the rate of respiration (i.e.,
the oxygen consumption rate). Thus, in soybean, dQ/dO 2 (heat production per oxygen consumed) ratios
were higher when the alternative pathway activity was higher [51]. Both the capacity and the activity of
the alternative pathway were found to be much higher in cotyledon-purified mitochondria (CPM) than in
hypocotyl-purified mitochondria (HPM) and, accordingly, dQ/dO 2 ratios were again higher in CPM. In
4-day-old roots, respiration of soybean proceeded almost entirely via cytochrome coxidase (COX). By
day 17, however, more than 50% of the flux occurred via alternative oxidase (AOX), which resulted in a
substantial decrease in the theoretical yield of ATP synthesis and concomitantly root relative growth rate
[52]. Decreases in whole-root respiration during growth of soybean seedlings can be largely explained by
decreases in maximal rates of electron transport via COX. In the case of increased AOX, the ubiquinone
pool can be maintained in a moderately reduced state.
In wheat (Triticum aestivum) the initial growth during the first 21–24 hr showed no sensitivity to
KCN. Salicylhydroxamic acid and disulfiram as inhibitors of the alternative path were, however, almost
completely inhibitory if added at any time until at least day 4 or for 3 days after inhibition, respectively
[53]. The alternative path was dominant and decreased with the concomitant development of the cy-
tochrome path, indicating that the initial growth of germinating wheat seedlings depends essentially on
the alternative path. In sunflower plants, the highest respiration rates were observed in young leaves fol-
lowed by old and mature leaves [54]. Cyanide had no effect on young leaves but it enhanced respiration
in mature and old leaves. SHAM reduced respiration in young leaves, indicating that the major portion of
respiration at this stage is based on alternative respiration, which coincides with the results obtained in
the case of wheat. In extracts from whole roots of different ages, the ubiquinone pool was maintained at
50 to 60% reduction, whereas the pyruvate content fluctuated without a consistent trend. The amount of
mitochondrial protein on a dry-mass basis, however, did not vary significantly with root age.
In rice plants, switching from the cytochrome pathway to the alternative cyanide-resistant respira-
tory pathway can be exogenously induced, e.g., by application of the rice blasticide SSF126. This chem-
ical (like others) catalyzes the transformation of the high-molecular-weight form of the oxidase to the
low-molecular-weight form in which the alternative pathway is preferentially operational. Thus, applica-
tion of this chemical is suited to artificially switching between the two pathways, affecting the recovery
from the rice blast symptoms [55].
B. Photorespiration
Photorespiration is still a somewhat enigmatic process whose significance for plant physiology is not re-
ally understood. It is based on the bifunctionality of the enzyme responsible for carbon dioxide assimila-
tion, namely ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). The reaction of the enzyme
with atmospheric oxygen did not appear to be of any obvious advantage for the plant as it did not result
in a net catabolic gain with respect to the carbohydrate content of the cells. On the contrary, it looked like
a mere waste of energy and a waste of carbon compounds as a consequence of the oxygenase reaction—
carbon dioxide is evolved in the light (instead of being taken up) and oxygen is taken up (instead of be-
ing evolved). However, detailed bioenergetic investigations of the process have shown that in fact pho-
torespiration is essential for plants and that inhibiting it or switching to nonphotorespiratory conditions is
detrimental for plants (compare Sec. V of this chapter). The counteracting partial reactions of photosyn-
thesis and photorespiration are illustrated schematically in Figure 11.
Following the oxygenation of the C5 compound ribulose bisphosphate, photorespiration comprises
a cyclic series of reactions with the participation of three different organelles—chloroplasts, peroxisomes,
and mitochondria—in a reaction sequence called the C2 cycle from the initial compound phosphoglyco-
late, the smaller product of the oxygenation reaction. (The splitting of the oxygenated five-carbon com-
pound produces a C2 structure, phosphoglycolate, together with a C3 compound, “normal” 3-phospho-
glycerate (3-PGA), the “same” as that is formed after carboxylation.) Specific conditions such as high
temperatures or low CO 2 /O 2 ratios steer the system toward higher rates of oxygenation and lower car-
boxylation. Details of the overall cycle can be found in every modern textbook on plant physiology. Prin-
cipally, the photorespiratory C2 cycle can be understood as recovery of three fourths of the carbon from
the formed phosphoglycolate because two glycine molecules are converted to one serine inside the mito-
chondria so that finally “only” one carbon of four is lost as carbon dioxide. The same reaction step yields
310 BADER AND ABDEL-BASSET