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Effect of temperature on the CO2/O2 specificity of ribulose-1,5-bisphosphate carboxylase/oxygenase and the rate of respiration in the light
Authors:A. Brooks  G. D. Farquhar
Affiliation:(1) Department of Environmental Biology, Research School of Biological Sciences, Australian National University, P.O. Box 475, 2601 Canberra City, A.C.T., Australia
Abstract:
Responses of the rate of net CO2 assimilation (A) to the intercellular partial pressure of CO2 (pi) were measured on intact spinach (Spinacia oleracea L.) leaves at different irradiances. These responses were analysed to find the value of pi at which the rate of photosynthetic CO2 uptake equalled that of photorespiratory CO2 evolution. At this CO2 partial pressure (denoted Gcy), net rate of CO2 assimilation was negative, indicating that there was non-photorespiratory CO2 evolution in the light. Hence Gcy was lower than the CO2 compensation point, Gamma. Estimates of Gcy were obtained at leaf temperatures from 15 to 30°C, and the CO2/O2 specificity of ribulose 1,5-bisphosphate (RuBP) carboxylase/oxygenase (E.C. 4.1.1.39) was calculated from these data, taking into account changes in CO2 and O2 solubilities with temperature. The CO2/O2 specificity decreased with increasing temperature. Therefore we concluded that temperature effects on the ratio of photorespiration to photosynthesis were not solely the consequence of differential effects of temperature on the solubilities of CO2 and O2. Our estimates of the CO2/O2 specificity of RuBP carboxylase/oxygenase are compared with in-vitro measurements by other authors. The rate of nonphotorespiratory CO2 evolution in the light (Rd) was obtained from the value of A at Gcy. At this low CO2 partial pressure, Rd was always less than the rate of CO2 evolution in darkness and appeared to decrease with increasing irradiance. The decline was most marked up to about 100 mgrmol quanta m-2 s-1 and less marked at higher irradiances. At one particular irradiance, however, Rd as a proportion of the rate of CO2 evolution in darkness was similar in different leaves and this proportion was unaffected by leaf temperature or by [O2] (ambient and greater). After conditions of high [CO2] and high irradiance for several hours, the rate of CO2 evolution in darkness increased and Rd also increased.Abbreviations and symbols A rate of net CO2-assimilation - Gamma CO2 compensation point - Gcy CO2 compensation point in the absence of Rd - pi intercellular partial pressure of CO2 - Rd (ldquoday respirationrdquo) rate of non-photorespiratory CO2 evolution in the light - Rn (ldquonight respirationrdquo) rate of CO2 evolution in darkness - RuBP ribulose-1,5-bisphosphate - Rubisco RuBP carboxylase/oxygenase
Keywords:Carbon dioxide compensation point  Dark respiration  Ribulose-1,5-bisphosphate carboxylase/oxygenase  Specificity factor  Spinacia (CO2 assimilation)  Temperature and CO2 assimilation
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