Short-Term Measurements of Carbon Isotope Discrimination in Several C4 Species

Abstract

Carbon isotope discrimination (Δ) and leaf gas-exchange were measured simultaneously for a number of C4 species. Linear relationships were found between A and the ratio of intercellular to ambient partial pressures of CO2, pI/pa. These data were used to estimate the fraction of CO2 released by C4-acid decarboxylation in the bundle sheath, which subsequently leaks out to the mesophyll. We define this fraction as the leakiness of the system and it is also a measure of the extent to which phosphoenolpyruvate (PEP) carboxylations exceed ribulose 1,5-bisphosphate (RuBP) carboxylations. For Sorghum bicolor and Amaranthus edulis, leakiness was estimated at 0.2 and was constant over a wide range of irradiances (between 480 and 1600 μmol quanta m-2 s-1), intercellular CO2 pressures (between 30 and 350 μbar) and leaf temperatures (from 21�C to 34�C). At irradiances less than 240 μmol quanta m-2 s-1, leakiness appeared to increase. For a number of dicotyledonous and monocotyledonous species, of the various C4-decarboxylation types, leakiness was also estimated at 0.2. Contrary to expectation, amongst the 11 species examined, those with suberised lamellae did not show lower values of leakiness than those without suberised lamellae. For one NAD-ME and one PCK monocot, the estimates of leakiness were significantly higher at 0.30 and 0.25, respectively. Long-term discrimination (assessed from carbon isotope composition of leaf dry matter) did not correlate well with these short- term measures of discrimination. We suggest that this may be due to differences between species in fractionations occurring after photosynthesis.

Elements Required for an Efficient NADP-Malic Enzyme Type C4 Photosynthesis

“…The model further predicted that carbon isotope discrimination and CO 2 leakiness were reduced with increasing PPFD (Fig. 6), which is consistent with experimental observations (Farquhar et al, 1989;Henderson et al, 1992;Cousins et al, 2006;Kromdijk et al, 2008;Tazoe et al, 2008). These results demonstrate that the C4 model can realistically simulate the experimentally observed carbon isotope discrimination of C4 species.…”

The Operation of Two Decarboxylases, Transamination, and Partitioning of C4 Metabolic Processes between Mesophyll and Bundle Sheath Cells Allows Light Capture To Be Balanced for the Maize C4 Pathway

“…Previously, on the basis of theoretical considerations, it had been predicted that an unbalanced ATP supply would result in a disruption of the delicate equilibrium between BS and M functions with consequent loss in assimilatory efficiency (Henderson et al, 1992;Evans et al, 2007;Tazoe et al, 2008). This prediction arose because metabolic rigidity would be expected under some of the common simplifications used for C 4 biochemistry, whereby transamination is neglected, PGA is reduced at a fixed rate in BS, and NADPH delivery to BS is equimolar to CO 2 delivery Edwards, 2000, 2009).…”

“…Leakiness (F), the amount of CO 2 retrodiffusing relative to phosphoenolpyruvate (PEP) carboxylation rate, is therefore a proxy for the coordination between the CCM and assimilatory activity (Henderson et al, 1992;Tazoe et al, 2008;Kromdijk et al, 2010;Ubierna et al, 2011;Bellasio and Griffiths, 2013).…”

Fluxes of carbon, water and energy over Brazilian cerrado: an analysis using eddy covariance and stable isotopes

“…Respired CO2 from the soil would thus reflect recent litter inputs, rather than the carbon isotopic composition of the ecosystem as a whole. There are also uncertainties in the calculation due to differences between leaf carbon isotopic composition and other plant organs, and on the basis of what would be expected from on-line carbon isotope discriminatioti measurements (Henderson, Caemmerer & Farquhar 1992). Based on these considerations an estimate of somewhere between 30 and 35% of the annual sum of photosynthesis by C4 grasses seems reasonable for this ecosystem.…”