PHOTOSYNTHETIC GAS EXCHANGE AND RESPIRATION 319Figure 14 Effect of inorganic salts (KCl, MnCl 2 , MnSO 4 , and CaCl 2 ) on photosynthetic oxygen evolution
amplitudes as a consequence of short (5 sec) saturating flashes in thylakoid preparation from the blue-green
algaOscillatoria chalybea. Flash frequency, 3.3 Hz; dark adaptation, 15 min. (From S Spiegel, KP Bader, sub-
mitted, 2000.)lowing transfer to a fresh medium without selenium but showed normal rates in the presence of the metal
(Figure 15F). Clear effects were also observed on the oxygen gas exchange, whereas the pigmentation
was not significantly affected. Selenium is a specific component of so-called selenoenzymes such as glu-
tathione peroxidase and is covalently bound to this enzyme in the form of selenocysteine. Functionally,
it appears to be relevant and even indispensable for the redox buffering of the respective organisms; the
effects are still under investigation (see Ref. 121 and references therein).
V. REFLECTIONS ON CROP YIELD
One of the most prominent goals of modern agriculture and applied plant physiology is to increase the
yield of agricultural crops. In principle, this goal can be approached by techniques of plant breeding
and/or exogenous applications of compounds such as fertilizers. One of the most obvious and seemingly
even trivial ideas was that there should be a direct link between an increase in crop yield and a decrease
in photorespiration. Surprisingly, this inverse correlation could not be substantiated. Even direct breed-
ing experiments selecting exclusively for high yields of crops did not decrease the rate of photorespira-
tion! This was one of the most convincing pieces of evidence that the process of photorespiration is by no
means the wasteful and useless phenomenon it was long taken for. Moreover, crop mutants that were de-
fective with respect to their photorespiratory activity grew well under nonphotorespiratory conditions
(e.g., artificial low oxygen partial pressure in a laboratory setup). As soon as the plants were transferred
to the ambient atmosphere, however, the mutation immediately proved lethal. Thus, the process of pho-
torespiration appears to have a beneficial effect for plants at least under appropriate conditions (justify-
ing the high energy demand of the reaction), and this has been discussed and investigated in recent years.
It must be kept in mind, however, that on an evolutionary level Rubisco developed its bifunctional-
ity at a time when there was no substantial oxygen in the atmosphere. Thus, although current investiga-
tions might suggest it, a protective function of the oxygenation of ribulose bisphosphate against detri-
mental concentrations of oxygen cannot have been the original “idea” behind this mechanism.
Surprisingly, the oxygenase function of Rubisco also requires preceding activation of the enzyme by a
carbon dioxide molecule that is bound to Lys201 of the large subunit of Rubisco, thus forming a carba-
mate together with the binding of magnesium ions [122,123]. In some cases, elevated productivity was
reported, e.g., for mutant genotypes of tobacco selected for under conditions of low carbon dioxide con-
centrations: The leaf area per plant was larger and photosynthetic rates were higher with similar rates of
respiration. However, none of the described positive modifications could be correlated with a decrease in