photosynthesis - Photosynthesis Research (2006) 88:357366...

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Regular paper Light- and CO 2 -saturated photosynthesis: enhancement by oxygen Juta Viil*, Hiie Ivanova & Tiit Pa ¨ rnik Division of Plant Physiology, Institute of Agriculture and Environment, Estonian Agricultural University, 76902 Harku, Estonia; *Author for correspondence (e-mail: [email protected]; fax: +372-6511142) Received 28 September 2005; accepted in revised form 14 February 2006/Published online: 9 June 2006 Key words: CO 2 -saturated photosynthesis, irradiance, oxygen concentration, pseudocyclic electron trans- port Abstract Oxygen may enhance CO 2 -saturated photosynthesis in intact leaves, which display the Warburg effect when illuminated at the current atmospheric level of CO 2 and O 2 , of about 350 l ll ) 1 and 21%, respectively. The magnitude of the stimulation depends on irradiance. The K M (O 2 ) of the stimulation is 128 l M (10.6% O 2 ). Maximum enhancement in wheat leaves is 6.1 and 5.3 l mol m ) 2 s ) 1 under 27.9 and 18.7 mW cm ) 2 , respectively, corresponding to a 25–30% increase in the ribulose 1,5-bisphosphate (RuBP) turnover rate if compared with O 2 -free ambient gas phase. The stimulation appears in 5–10 s after a sharp increase in O 2 .In response to a decrease in O 2 , the new stabilized rate is reached in 5–7 min. The stimulation does not involve any increase in the activity of Rubisco. The effect correlates with increased concentration of RuBP. Oxygen enhances CO 2 -saturated photosynthesis by acting as a terminal electron acceptor in the photosynthetic electron transport. The magnitude of the effect may be adopted as an index of the pseudocyclic photo- phosphorylation in vivo . Abbreviations: C a – ambient CO 2 concentration; C w – concentration of dissolved CO 2 ; E f – free reaction centers of Rubisco; E T – the total concentration of active carboxylation centers; ER – Rubisco-RuBP complex; R f – free RuBP; R s – RuBP synthesis rate; R T – total RuBP; Rubisco – ribulose-1,5-bisphosphate carboxylase/oxygenase; RuBP – ribulose 1,5-bisphosphate Introduction Under the normal atmospheric conditions oxygen reduces the efficiency of photosynthesis. O 2 com- petes with CO 2 for the acceptor, ribulose 1,5-bis- phosphate (RuBP), common to carboxylation and oxygenation. Oxygenation initiates also the gly- colate pathway that leads to the photorespiratory loss of CO 2 . When the concentration of CO 2 increases, oxygen loses its power as a competitor. The ratio of oxygenation to carboxylation decreases dramatically, and the photorespiratory loss of CO 2 is greatly reduced. Therefore at satu- rating CO 2 concentrations the influence of oxygen as of an inhibitor is not significant, and the steady- state carboxylation rate is independent of O 2 .A t high CO 2 concentrations (3000–4000 l ) 1 ) even an opposite effect has been found: oxygen can stimulate photosynthesis (Viil et al. 1972). The effect has been observed in the saturating light with fully expanded leaves of high photosynthetic capacity (Viil et al. 1977). During intensive leaf growth the effect has not been detected. Neither has it been observed with mature leaves which, by
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This note was uploaded on 01/16/2011 for the course BIO 1107 taught by Professor Devartanian during the Spring '08 term at UGA.

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photosynthesis - Photosynthesis Research (2006) 88:357366...

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