PHOTOSYSTEM-II EXCITATION PRESSURE AND PHOTOSYNTHETIC CARBON METABOLISM IN CHLORELLA-VULGARIS

Chlorella vulgaris grown at 5 degrees C/150 mu mol m(-2) s(-1) mimics cells grown under high irradiance (27 degrees C/2200 mu mol m(-2) s(-1 )). This has been rationalized through the suggestion that both popula tions of cells were exposed to comparable photosystem II (PSII) excita tion pressures measured as the chlorophyll a fluorescence quenching pa rameter, 1 - qP (D.P. Maxwell, S. Falk, N.P.A. Huner [1995] Plant Phys iol 107: 687-694). To assess the possible role(s) of feedback mechanis ms on PSII excitation pressure, stromal and cytosolic carbon metabolis m were examined. Sucrose phosphate synthase and fructose-1,6-bisphosph atase activities as well as the ratios of fructose-1,6-bisphosphate/fr uctose-6-phosphate and sucrose/starch indicated that cells grown at 27 degrees C/2200 mu mol m(-2) s(-1) appeared to exhibit a restriction i n starch metabolism. In contrast, cells grown at 5 degrees C/150 mu mo l m(-2) s(-1) appeared to exhibit a restriction in the sucrose metabol ism based on decreased cytosolic fructose-1,6-bisphosphatase and sucro se phosphate synthase activities as well as a low sucrose/starch ratio . These metabolic restrictions may feedback on photosynthetic electron transport and, thus, contribute to the observed PSII excitation press ure. We conclude that, although PSII excitation pressure may reflect r edox regulation of photosynthetic acclimation to light and temperature in C. vulgaris, it cannot be considered the primary redox signal. Alt ernative metabolic sensing/signaling mechanisms are discussed.