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photosynthesis 2

photosynthesis 2 - Physiologia Plantarum 131 19 2007...

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Physiologia Plantarum 131: 1–9. 2007 Copyright ª Physiologia Plantarum 2007, ISSN 0031-9317 TECHNICAL FOCUS Kinetics of leaf oxygen uptake represent in planta activities of respiratory electron transport and terminal oxidases Agu Laisk a, *, Vello Oja and Hillar Eichelmann Institute for Molecular and Cell Biology, Tartu University, Riia street 23, Tartu 51010, Estonia Correspondence *Corresponding author, e-mail: [email protected] Received 26 January 2007; revised 16 February 2007 doi: 10.1111/j.1399-3054.2007.00910.x We present, for the first time, the oxygen response kinetics of mitochondrial respiration measured in intact leaves (sunflower and aspen). Low O 2 concentrations in N 2 (9–1500 ppm) were preset in a flow-through gas exchange measurement system, and the decrease in O 2 concentration and the increase in CO 2 concentration as result of leaf respiration were measured by a zirconium cell O 2 analyser and infrared-absorption CO 2 analyser, respec- tively. The low O 2 concentrations little influenced the rate of CO 2 evolution during the 60-s exposure. The initial slope of the O 2 uptake curve on the dissolved O 2 concentration basis was relatively constant in leaves of a single species, 1.5 mm s 2 1 in sunflower and 1.8 mm s 2 1 in aspen. The apparent K 0.5 (O 2 ) values ranged from 0.33 to 0.67 m M in sunflower and from 0.33 to 1.1 m M in aspen, mainly because of the variation of the maximum rate, V max (leaf temperature 22 ° C). The initial slope of the O 2 response of respiration characterizes the catalytic efficiency of terminal oxidases, an important para- meter of the respiratory machinery in leaves. The plateau of the response characterizes the activity of the mitochondrial electron transport chain and is subject to regulations in accordance with the necessity for ATP production. The relatively low oxygen conductivity of terminal oxidases means that in leaves, less than 10% of the photosynthetic oxygen can be reassimilated by mitochondria. Introduction In plant leaves, CO 2 is taken up and O 2 is evolved from photosynthesis and simultaneously O 2 is taken up and CO 2 is evolved from respiration. The kinetics of leaf photosynthetic CO 2 exchange have been thoroughly investigated (Laisk et al. 2002) and mathematically ana- lysed, taking into account the complexity of the under- lying biochemical reaction network (Farquhar and von Caemmerer 1982, Laisk et al. 2006). This work has revealed that only under strictly limiting CO 2 concen- trations does the CO 2 acceptor RuBPaccumulate, and the RuBP carboxylation rate is then determined by the concentration of CO 2 , its diffusion rate and the kinetic properties of the CO 2 fixation enzyme ribulose- 1,5-bisphosphate carboxylase–oxygenase (Rubisco). At higher CO 2 concentrations, the CO 2 -binding reaction becomes limited by the regeneration rate of the second substrate, RuBP (Farquhar et al. 1980). As a result of this, the kinetic curve of photosynthesis with respect to CO 2 (the CO 2 response curve) follows the Rubisco-specific rectangular hyperbola only at relatively low CO 2 con- centrations, but at higher CO 2 concentrations the curve
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