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Assimilation of xylem-transported 13C-labelled CO2 in leaves and branches of sycamore (Platanus occidentalis L.).

McGuire MA, Marshall JD, Teskey RO - J. Exp. Bot. (2009)

Bottom Line: The majority was fixed in the woody tissue of the branches, with smaller amounts fixed in the leaves and petioles.Overall, the fixation of internally transported (13)CO(2) label by woody tissues averaged 6% of the assimilation of CO(2) from the atmosphere by the leaves.Woody tissue assimilation rates calculated from measurements of (13)C differed from rates calculated from measurements of CO(2) efflux in the lower branch but not in the upper branch.

View Article: PubMed Central - PubMed

Affiliation: School of Forestry and Natural Resources, University of Georgia, Athens, Georgia 30602, USA. mmcguire@uga.edu

ABSTRACT
Previous reports have shown that CO(2) dissolved in xylem sap in tree stems can move upward in the transpiration stream. To determine the fate of this dissolved CO(2), the internal transport of respired CO(2) at high concentration from the bole of the tree was simulated by allowing detached young branches of sycamore (Platanus occidentalis L.) to transpire water enriched with a known quantity of (13)CO(2) in sunlight. Simultaneously, leaf net photosynthesis and CO(2) efflux from woody tissue were measured. Branch and leaf tissues were subsequently analysed for (13)C content to determine the quantity of transported (13)CO(2) label that was fixed. Treatment branches assimilated an average of 35% (SE=2.4) of the (13)CO(2) label taken up in the treatment water. The majority was fixed in the woody tissue of the branches, with smaller amounts fixed in the leaves and petioles. Overall, the fixation of internally transported (13)CO(2) label by woody tissues averaged 6% of the assimilation of CO(2) from the atmosphere by the leaves. Woody tissue assimilation rates calculated from measurements of (13)C differed from rates calculated from measurements of CO(2) efflux in the lower branch but not in the upper branch. The results of this study showed unequivocally that CO(2) transported in xylem sap can be fixed in photosynthetic cells in the leaves and branches of sycamore trees and provided evidence that recycling of xylem-transported CO(2) may be an important means by which trees reduce the carbon cost of respiration.

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Woody tissue CO2 efflux of detached sycamore branches calculated from measurements made with an infrared gas analyser. Each branch was measured at two locations (lower, ∼25 cm from the cut end, and upper, ∼60 cm from the cut end). Measurements were made in sunlight after branches took up CO2-enriched water for at least 2 h, and repeated after the branches equilibrated in the dark for at least 2 h. Treatment branches (n=9) took up water enriched with 99 atom% 13CO2 at a concentration of 0.0116 mol l−1. Control branches (n=3) took up water enriched with 0.0114 mol l−1 CO2 at atmospheric isotope composition (∼1 atom% 13C). Light efflux was significantly different from dark efflux (t=3.731, P=<0.001, n=24), and light efflux was weakly correlated with dark efflux (light efflux=0.000621+0.430 (dark efflux), R2=0.23, P=0.011).
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fig3: Woody tissue CO2 efflux of detached sycamore branches calculated from measurements made with an infrared gas analyser. Each branch was measured at two locations (lower, ∼25 cm from the cut end, and upper, ∼60 cm from the cut end). Measurements were made in sunlight after branches took up CO2-enriched water for at least 2 h, and repeated after the branches equilibrated in the dark for at least 2 h. Treatment branches (n=9) took up water enriched with 99 atom% 13CO2 at a concentration of 0.0116 mol l−1. Control branches (n=3) took up water enriched with 0.0114 mol l−1 CO2 at atmospheric isotope composition (∼1 atom% 13C). Light efflux was significantly different from dark efflux (t=3.731, P=<0.001, n=24), and light efflux was weakly correlated with dark efflux (light efflux=0.000621+0.430 (dark efflux), R2=0.23, P=0.011).

Mentions: The difference between CO2 efflux measurements made in the dark and light was used to estimate the rate at which carbon was assimilated photosynthetically in the woody tissue of the branches (Fig. 3). Rates of dark efflux ranged from 0.003 to 0.023 μmol CO2 g−1 s−1. There was a weak positive correlation between efflux in the light and in the dark (light efflux=0.000621+0.430 (dark efflux), R2=0.23, P=0.011). On average, woody tissue photosynthesis in the light reduced CO2 efflux from the branch by about 52% (SE=0.07). Net assimilation based on CO2 efflux measurements was not significantly different between treatment and control branches (P=0.505). Assimilation of transported 13CO2 label by woody tissues calculated by 13C analysis was 130% greater than assimilation of CO2 calculated from efflux measurements in the lower part of the branches (Fig. 4). The difference between the upper and lower woody tissue assimilation rates seen in the 13C-based data was not observed in the efflux-based measurements.


Assimilation of xylem-transported 13C-labelled CO2 in leaves and branches of sycamore (Platanus occidentalis L.).

McGuire MA, Marshall JD, Teskey RO - J. Exp. Bot. (2009)

Woody tissue CO2 efflux of detached sycamore branches calculated from measurements made with an infrared gas analyser. Each branch was measured at two locations (lower, ∼25 cm from the cut end, and upper, ∼60 cm from the cut end). Measurements were made in sunlight after branches took up CO2-enriched water for at least 2 h, and repeated after the branches equilibrated in the dark for at least 2 h. Treatment branches (n=9) took up water enriched with 99 atom% 13CO2 at a concentration of 0.0116 mol l−1. Control branches (n=3) took up water enriched with 0.0114 mol l−1 CO2 at atmospheric isotope composition (∼1 atom% 13C). Light efflux was significantly different from dark efflux (t=3.731, P=<0.001, n=24), and light efflux was weakly correlated with dark efflux (light efflux=0.000621+0.430 (dark efflux), R2=0.23, P=0.011).
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC2736895&req=5

fig3: Woody tissue CO2 efflux of detached sycamore branches calculated from measurements made with an infrared gas analyser. Each branch was measured at two locations (lower, ∼25 cm from the cut end, and upper, ∼60 cm from the cut end). Measurements were made in sunlight after branches took up CO2-enriched water for at least 2 h, and repeated after the branches equilibrated in the dark for at least 2 h. Treatment branches (n=9) took up water enriched with 99 atom% 13CO2 at a concentration of 0.0116 mol l−1. Control branches (n=3) took up water enriched with 0.0114 mol l−1 CO2 at atmospheric isotope composition (∼1 atom% 13C). Light efflux was significantly different from dark efflux (t=3.731, P=<0.001, n=24), and light efflux was weakly correlated with dark efflux (light efflux=0.000621+0.430 (dark efflux), R2=0.23, P=0.011).
Mentions: The difference between CO2 efflux measurements made in the dark and light was used to estimate the rate at which carbon was assimilated photosynthetically in the woody tissue of the branches (Fig. 3). Rates of dark efflux ranged from 0.003 to 0.023 μmol CO2 g−1 s−1. There was a weak positive correlation between efflux in the light and in the dark (light efflux=0.000621+0.430 (dark efflux), R2=0.23, P=0.011). On average, woody tissue photosynthesis in the light reduced CO2 efflux from the branch by about 52% (SE=0.07). Net assimilation based on CO2 efflux measurements was not significantly different between treatment and control branches (P=0.505). Assimilation of transported 13CO2 label by woody tissues calculated by 13C analysis was 130% greater than assimilation of CO2 calculated from efflux measurements in the lower part of the branches (Fig. 4). The difference between the upper and lower woody tissue assimilation rates seen in the 13C-based data was not observed in the efflux-based measurements.

Bottom Line: The majority was fixed in the woody tissue of the branches, with smaller amounts fixed in the leaves and petioles.Overall, the fixation of internally transported (13)CO(2) label by woody tissues averaged 6% of the assimilation of CO(2) from the atmosphere by the leaves.Woody tissue assimilation rates calculated from measurements of (13)C differed from rates calculated from measurements of CO(2) efflux in the lower branch but not in the upper branch.

View Article: PubMed Central - PubMed

Affiliation: School of Forestry and Natural Resources, University of Georgia, Athens, Georgia 30602, USA. mmcguire@uga.edu

ABSTRACT
Previous reports have shown that CO(2) dissolved in xylem sap in tree stems can move upward in the transpiration stream. To determine the fate of this dissolved CO(2), the internal transport of respired CO(2) at high concentration from the bole of the tree was simulated by allowing detached young branches of sycamore (Platanus occidentalis L.) to transpire water enriched with a known quantity of (13)CO(2) in sunlight. Simultaneously, leaf net photosynthesis and CO(2) efflux from woody tissue were measured. Branch and leaf tissues were subsequently analysed for (13)C content to determine the quantity of transported (13)CO(2) label that was fixed. Treatment branches assimilated an average of 35% (SE=2.4) of the (13)CO(2) label taken up in the treatment water. The majority was fixed in the woody tissue of the branches, with smaller amounts fixed in the leaves and petioles. Overall, the fixation of internally transported (13)CO(2) label by woody tissues averaged 6% of the assimilation of CO(2) from the atmosphere by the leaves. Woody tissue assimilation rates calculated from measurements of (13)C differed from rates calculated from measurements of CO(2) efflux in the lower branch but not in the upper branch. The results of this study showed unequivocally that CO(2) transported in xylem sap can be fixed in photosynthetic cells in the leaves and branches of sycamore trees and provided evidence that recycling of xylem-transported CO(2) may be an important means by which trees reduce the carbon cost of respiration.

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