Photosynthesis and Photorespiration in Whole ... - Plant Physiology

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GERBAUD AND ANDRE Plant Physiol. Vol. 64, 1979
-c 75
A a E 1/ A A 4
z
0
P 50
_
IC
A
.-I
0
w
0
w 25
a-
/~~~~
/~~~~~~~~~
n
0 200 400 600 800 12000
CO2 (p.l-1)
FIG. 3. Gas exchange of a plant of wheat as a function of CO2 concentration. Data were comparable by correcting them for the size of the plant,
supposing a uniform growth throughout the experiment. Abbreviations as in Figures I and 2.
(Fig. 2). 02 concentration was always normal. Diurnal transpiration
rates were nearly constant, indicating that stomata can be
considered open throughout the light periods.
As expected, photosynthetic rates follow the concentration of
CO2 and respiration varies as Pc but to a smaller extent: it
diminishes only 20%o when P is halved.
Photorespiration varies oppositely to photosynthesis, so that
their sum P, + U depends only on the age and size of the plant.
At high concentrations of CO2, where it is admitted that photorespiration
is inhibited, there remains some 02 uptake, about 10%
of Pc; we suppose this is due to the continuation of night respiration
in the light, which appears also in maize (1).
Pc + U corresponds to the utilization of reducing power in the
leaves of the plant; its production is measured by the 02 evolution
E. We found E always slightly superior to Pc + U showing that
some reducing power is also consumed in the roots (nitrate reduction).
If we abstract the influence of plant growth, E appears to be
independent of CO2 (Fig. 3) and P, and U follow symmetrical
curves. The mirror effect between Pc and U was observed by
Radmer and Kok (19) with algae during the induction phase of
photosynthesis at constant CO2 level and, with a lag time, after
CO2 exhaustion by photosynthesis (20). The main result of the
present work is the constancy of the gross evolution E under
various conditions of CO2, in spite of wide variations of apparent
photosynthesis and in long steady-state experiments, which shows
that it is not a transient effect but a permanent mechanism. The
permanency of this balance suggests that the electron transport
chain turns over at a constant rate, controlled by the light intensity
and independent of the nature of the final electron acceptor.
When CO2 concentrations are low the excess of reducing power
must be trapped by 02 reduction. If the mechanism is blocked for
example by the simultaneous absence of 02 and CO2, inhibition
or damage occurs (9) because unwanted compounds have to be
reduced instead of 02 or CO2.
Acknowledgments-We are grateful to Mr. A. Daguenet and Mrs. J. Massimino for their very
valuable contribution to the experiments.
LITERATURE CITED
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1 I I I I I I I AI
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