PLANT PHYSIOLOGY , Vol 111, Issue 1 127-136, Copyright © 1996 by American Society of Plant Biologists
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CELL BIOLOGY AND SIGNAL TRANSDUCTION |
Photosystem II Excitation Pressure and Photosynthetic Carbon Metabolism in Chlorella vulgaris
L. V. Savitch, D. P. Maxwell and NPA. Huner
Department of Plant Sciences, University of Western Ontario, London, Canada N6A 5B7
Chlorella vulgaris grown at 5[deg]C/150 [mu]mol m-2 s-1 mimics cells grown
under high irradiance (27[deg]C/2200 [mu]mol m-2 s-1). This has been
rationalized through the suggestion that both populations of cells were
exposed to comparable photosystem II (PSII) excitation pressures measured
as the chlorophyll a fluorescence quenching parameter, 1 - qP (D.P.
Maxwell, S. Falk, N.P.A. Huner [1995] Plant Physiol 107: 687-694). To
assess the possible role(s) of feed-back mechanisms on PSII excitation
pressure, stromal and cytosolic carbon metabolism were examined. Sucrose
phosphate synthase and fructose-1,6-bisphosphatase activities as well as
the ratios of fructose-1,6-bisphosphate/fructose-6-phosphate and
sucrose/starch indicated that cells grown at 27[deg]C/2200 [mu]mol m-2 s-1
appeared to exhibit a restriction in starch metabolism. In contrast, cells
grown at 5[deg]C/150 [mu]mol m-2 s-1 appeared to exhibit a restriction in
the sucrose metabolism based on decreased cytosolic fructose-1,6-
bisphosphatase and sucrose phosphate synthase activities as well as a low
sucrose/starch ratio. These metabolic restrictions may feed-back on
photosynthetic electron transport and, thus, contribute to the observed
PSII excitation pressure. We conclude that, although PSII excitation
pressure may reflect redox regulation of photosynthetic acclimation to
light and temperature in C. vulgaris, it cannot be considered the primary
redox signal. Alternative metabolic sensing/signaling mechanisms are
discussed.
