PLANT PHYSIOLOGY , Vol 101, Issue 2 507-512, Copyright © 1993 by American Society of Plant Biologists
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ENVIRONMENTAL AND STRESS PHYSIOLOGY |
Temperature Dependence of the Linkage of Quantum Yield of Photosystem II to CO2 Fixation in C4 and C3 Plants
W. Oberhuber and G. E. Edwards
Department of Botany, Washington State University, Pullman Washington 99164-4238
The temperature dependence of quantum yields of electron transport from
photosystem II (PSII) ([phi]II, determined from chlorophyll a fluorescence)
and CO2 assimilation ([phi]CO2, apparent quantum yield for CO2
assimilation) were determined simultaneously in vivo. With C4 species
representing NADP-malic enzyme, NAD-malic enzyme, and phosphoenolpyruvate
carboxykinase subgroups, the ratio of [phi]II/[phi]CO2 was constant over
the temperature range from 15 to 40[deg]C at high light intensity (1100
[mu]mol quanta m-2 s-1). A similar response was obtained at low light
intensity (300 [mu]mol quanta m-2 s-1), except the ratio of
[phi]II/[phi]CO2 increased at high temperature. When the true quantum yield
for CO2 fixation ([phi]CO2*) was calculated by correcting for respiration
in the light (estimated from temperature dependence of dark respiration),
the ratio of [phi]II/[phi]C02* remained constant with varying temperature
and under both light intensities in all C4 species examined. Because the
[phi]II/[phi]CO2* ratio was the same in C4 monocots representing the three
subgroups, the ratio was not affected by differences in the bio-chemical
mechanism of concentrating CO2 in the bundle sheath cells. The results
suggest that PSII activity is closely linked to the true rate of CO2
fixation in C4 plants. The close relationship between [phi]II and [phi]CO2*
in C4 species under varying temperature and light intensity conditions is
apparently due to a common low level of photorespiration and a primary
requirement for reductive power in the C3 pathway. In contrast, in a C3
plant the [phi] II/[phi]CO2* ratio is higher under normal atmospheric
conditions than under nonphotorespiratory conditions and it increases with
rising temperature. This decrease in efficiency in utilizing energy derived
from PSII for CO2 fixation is due to an increase in photorespiration. In
both the C3 and C4 species, photochemistry is limited under low
temperature, and thus excess energy must be dissipated by nonphotochemical
means.
