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PLANT PHYSIOLOGY , Vol 103, Issue 1 83-90, Copyright © 1993 by American Society of Plant Biologists
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ENVIRONMENTAL AND STRESS PHYSIOLOGY |
C4 Photosynthesis (The CO2-Concentrating Mechanism and Photorespiration)
Z. Dai, MSB. Ku and G. E. Edwards
Department of Botany, Washington State University, Pullman, Washington 99164-4238
Despite previous reports of no apparent photorespiration in C4 plants based
on measurements of gas exchange under 2 versus 21% O2 at varying [CO2],
photosynthesis in maize (Zea mays) shows a dual response to varying [O2].
The maximum rate of photosynthesis in maize is dependent on O2
(approximately 10%). This O2 dependence is not related to stomatal
conductance, because measurements were made at constant intercellular CO2
concentration (Ci); it may be linked to respiration or pseudocyclic
electron flow. At a given Ci, increasing [O2] above 10% inhibits both the
rate of photosynthesis, measured under high light, and the maximum quantum
yield, measured under limiting light ([phi]CO2). The dual effect of O2 is
masked if measurements are made under only 2 versus 21% O2. The inhibition
of both photosynthesis and [phi]CO2 by O2 (measured above 10% O2) with
decreasing Ci increases in a very similar manner, characteristically of O2
inhibition due to photorespiration. There is a sharp increase in O2
inhibition when the Ci decreases below 50 [mu]bar of CO2. Also, increasing
temperature, which favors photorespiration, causes a decrease in [phi]CO2
under limiting CO2 and 40% O2. By comparing the degree of inhibition of
photosynthesis in maize with that in the C3 species wheat (Triticum
aestivum) at varying Ci, the effectiveness of C4 photosynthesis in
concentrating CO2 in the leaf was evaluated. Under high light, 30[deg]C,
and atmospheric levels of CO2 (340 [mu]bar), where there is little
inhibition of photosynthesis in maize by O2, the estimated level of CO2
around ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the
bundle sheath compartment was 900 [mu]bar, which is about 3 times higher
than the value around Rubisco in mesophyll cells of wheat. A high [CO2] is
maintained in the bundle sheath compartment in maize until Ci decreases
below approximately 100 [mu]bar. The results from these gas exchange
measurements indicate that photorespiration occurs in maize but that the
rate is low unless the intercellular [CO2] is severely limited by stress.
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