Carbon Isotope Fractionation during Photorespiration and Carboxylation in Senecio    

Abstract

The magnitude of fractionation during photorespiration and the effect on net photosynthetic 13 C discrimination (D) were investigated for three Senecio species, S. squalidus, S. cineraria, and S. greyii. We determined the contributions of different processes during photosynthesis to D by comparing observations (D obs ) with discrimination predicted from gas-exchange measurements (D pred ). Photorespiration rates were manipulated by altering the O 2 partial pressure (pO 2 ) in the air surrounding the leaves. Contributions from 13 C-depleted photorespiratory CO 2 were largest at high pO 2 . The parameters for photorespiratory fractionation (f ), net fractionation during carboxylation by Rubisco and phosphoenolpyruvate carboxylase (b), and mesophyll conductance (g i ) were determined simultaneously for all measurements. Instead of using D obs data to obtain g i and f successively, which requires that b is known, we treated b, f, and g i as unknowns. We propose this as an alternative approach to analyze measurements under field conditions when b and g i are not known or cannot be determined in separate experiments. Good agreement between modeled and observed D was achieved with f 5 11.6& 6 1.5&, b 5 26.0& 6 0.3&, and g i of 0.27 6 0.01, 0.25 6 0.01, and 0.22 6 0.01 mol m 22 s 21 for S. squalidus, S. cineraria, and S. greyii, respectively. We estimate that photorespiratory fractionation decreases D by about 1.2& on average under field conditions. In addition, diurnal changes in D are likely to reflect variations in photorespiration even at the canopy level. Our results emphasize that the effects of photorespiration must be taken into account when partitioning net CO 2 exchange of ecosystems into gross fluxes of photosynthesis and respiration.

Diel variations in the carbon isotope composition of respired CO<sub>2</sub> and associated carbon sources: a review of dynamics and mechanisms

“…While we are able to precisely predict changes in carbon discrimination and variations in δ 13 C of fresh assimilates in response to changes in VPD, light and temperature, much less is known on the isotopic effects of mesophyll CO 2 conductance (g m ), photorespiration and dark respiration throughout the day (e.g. Warren and Adams, 2006;Wingate et al, 2007;Lanigan et al, 2008;Tcherkez et al, 2010). There have been recent insights that there is active regulation of internal CO 2 conductance through aquaporins, which are transporting CO 2 across plasma membranes (Hanba et al, 2004;Flexas et al, 2008).…”

Bundle Sheath Leakiness and Light Limitation during C4 Leaf and Canopy CO2 Uptake

“…Phosphoenolpyruvate carboxylation and photorespiration rates were set at 1.5 and 0.03 times V c , respectively. A value of 11.5& was used for the fractionation factor associated with photorespiration, based on recent estimates by Lanigan et al (2008). …”

“…Furthermore, there might be additional fractionations associated with photorespiration (Gillon and Griffiths, 1997;Lanigan et al, 2008), variable mitochondrial respiration between old and young leaves (Villar et al, 1995), or different carbon sources for respiration (Ghashghaie et al, 2003). A sensitivity analysis (Table II) was undertaken, using the extended model by Farquhar (1983), with various combinations of values for mitochondrial respiration, the fractionation factor assigned to respiration (e), and carboxylation rate in BS cells (V c ).…”

Alocação de fotoassimilados marcados e relação fonte-dreno em figueiras cv. Roxo de Valinhos. 2. Tempo de alocação

“…A ocorrência do 13 C na massa vegetal é baixa, porque durante a carboxilação, o 12 C é favorecido pelas enzimas. A Rubisco favorece mais fortemente o 12 C, discriminando mais o 13 C, quando comparada com a PEP-carboxilase (-28‰ e -9‰, respectivamente), comparando enzimas de plantas C 3 e C 4 (Farquhar et al, 1989;Lanigan et al, 2008).…”