Regulation of Ribulose-1,5-Bisphosphate Carboxylase Activity in Response to Light Intensity and CO(2) in the C(3) Annuals Chenopodium album L. and Phaseolus vulgaris L |
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| Authors: | Sage R F Sharkey T D Seemann J R |
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| Affiliation: | Department of Botany, University of Georgia, Athens, Georgia 30602. |
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| Abstract: | The light and CO2 response of (a) photosynthesis, (b) the activation state and total catalytic efficiency (kcat) of ribulose-1,5-bisphosphate carboxylase (rubisco), and (c) the pool sizes of ribulose 1,5-bisphosphate, (RuBP), ATP, and ADP were studied in the C3 annuals Chenopodium album and Phaseolus vulgaris at 25°C. The initial slope of the photosynthetic CO2 response curve was dependent on light intensity at reduced light levels only (less than 450 micromoles per square meter per second in C. album and below 200 micromoles per square meter per second in P. vulgaris). Modeled simulations indicated that the initial slope of the CO2 response of photosynthesis exhibited light dependency when the rate of RuBP regeneration limited photosynthesis, but not when rubisco capacity limited photosynthesis. Measured observations closely matched modeled simulations. The activation state of rubisco was measured at three light intensities in C. album (1750, 550, and 150 micromoles per square meter per second) and at intercellular CO2 partial pressures (C1) between the CO2 compensation point and 500 microbars. Above a C1 of 120 microbars, the activation state of rubisco was light dependent. At light intensities of 550 and 1750 micromoles per square meter per second, it was also dependent on C1, decreasing as the C1 was elevated above 120 microbars at 550 micromoles per square meter per second and above 300 microbars at 1750 micromoles per square meter per second. The pool size of RuBP was independent of C1 only under conditions when the activation state of rubisco was dependent on C1. Otherwise, RuBP pool sizes increased as C1 was reduced. ATP pools in C. album tended to increase as C1 was reduced. In P. vulgaris, decreasing C1 at a subsaturating light intensity of 190 micromoles per square meter per second increased the activation state of rubisco but had little effect on the kcat. These results support modelled simulations of the rubisco response to light and CO2, where rubisco is assumed to be down-regulated when photosynthesis is limited by the rate of RuBP regeneration. |
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