spheric oxygen to the OEC with the subsequent formation of a peroxidic component might well represent
such a regulatory mechanism, and this peroxide may have played an essential role in evolution as a “tran-
sitory” electron donor [3,5]. In this context, it should be mentioned that hemoglobin was described to ex-
ist in various plants and crops such as barley [18] which might also be seen in context with the fine reg-
ulation of the oxygen content in plants. This hemoglobin has an oxygen dissociation constant of 3 nmol
L^1 and seems to act as a direct oxygenase and/or to regulate the energy status of the plant under condi-
tions of low oxygen [18].
In this chapter we have tried to summarize substantial features of the complex mechanisms of gas
exchange reactions that have to be (coarse- and fine-) regulated in plant physiology. Although we did not
restrict the presentation to oxygen exchange reactions, it appears clear that this gas represents the most
complicated task of regulation for a photosynthetic organism as evolution and uptake reactions take place
in the immediate vicinity of each other and, in some cases, even concomitantly. In some organisms such
as cyanobacteria a similar problem exists for hydrogen gas exchange reactions; e.g., hydrogen oxidation
(i.e., hydrogen uptake) and proton reduction (forming molecular hydrogen) occur virtually in parallel or
in highly regulated transitions [19,20].
B. Technical Aspects of Plant Physiological Gas Exchange
In photosynthesis, molecular water is oxidized and oxygen is produced as a waste product of the reaction.
However, physiological processes in photosynthetic organisms include (various) reactions that require or
include the oxidation or oxygenation of compounds so that technically an oxygen uptake from the sur-
rounding atmosphere takes place. On the basis of the overall oxygen gas exchange, the immanent prob-
lem for scientific investigation is obvious. Normally, oxygen gas analyses are carried out using oxygen
electrodes, which, however, cannot discriminate between evolution and concomitant uptake processes.
Consequently, such systems quantify the overall balance or difference following an “event” (e.g., illumi-
nation) by simply adding up positive and negative changes. Thus, an important goal of plant physiologi-
cal investigations has been to develop techniques allowing the simultaneous recording of gas evolution
and uptake reactions, e.g., in a liquid reaction assay and from an artificial gas atmosphere over the aque-
ous phase without interference. (For reasons of clarity, the interference of an evolution signal increasing
the atmospheric partial pressure and thus affecting the composition of the gas phase is neglected here.)
With respect to this and other requirements of plant physiological investigations, mass spectrometry has
been shown to be specifically well suited and the possible applications of this technique in biology in gen-
eral and in plant physiology in particular have been described [e.g., 21,22]. Early instruments, however,
suffered from both limited sensitivity and insufficient time resolution of the signals. Moreover, an im-
proved response of the mass spectrometer to dynamic changes of gas partial pressure was required, in par-
ticular following the first application of short (in the region of sec) flash illumination techniques. Now,
quite a few instruments with setups specifically adapted to the needs and requirements of plant physio-
logical investigations exist and important studies concerning gas exchange reactions in plants and algae
have been performed in photosynthesis research laboratories.
II. PHOTOSYNTHESIS
The terminus photosynthesis defines and summarizes the complex process(es) by which radiation energy
of light is used to form carbohydrates according to the simple formula
CO 2 D^2 /2H→CH 2 ODOwhere D(onor) means any reduced photo-oxidizable compound. Thus, in the case of anoxygenic photo-
synthesis D might be a sulfide or a ferric salt, whereas D^2 /2HH 2 O for oxygenic photosynthesis. In
the course of the light-induced electron transport from an ultimate donor through two consecutively op-
erating photosystems, energy is conserved in the form of ATP and reducing equivalents (NADPH 2 ) are
built. By now, essentially two gas exchange reactions can be investigated that reflect the photosynthetic
capacity of a plant, namely the evolution of molecular oxygen as a waste product of the water oxidation
and the decrease of the carbon dioxide partial pressure of the surrounding atmosphere due to carbon diox-
ide assimilation (CO 2 uptake).
Taking the trivial formula from before, one essential question still has to be clarified, namely the ori-
PHOTOSYNTHETIC GAS EXCHANGE AND RESPIRATION 301