Effect of rising CO2 levels on C3 and C4 plants

GREENHOUSE GASSED  In a long-running field experiment in Minnesota, scientists are studying the effects of rising atmospheric carbon dioxide levels on plots of grassland.


Unexpected reversal of C3 versus Cgrass response to elevated CO2 during a 20-year field experiment

by Reich P. B., Hobbie S. E.,  Lee T. D. Pastore M. A. (2018)

  1. Peter B. Reich1,2,*,
  2. Sarah E. Hobbie3,
  3. Tali D. Lee4,
  4. Melissa A. Pastore3

  1. 1Department of Forest Resources, University of Minnesota, St. Paul, MN 55108, USA.

  2. 2Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW 2753, Australia.

  3. 3Department of Ecology, Evolution, and Behavior, University of Minnesota, St. Paul, MN 55108, USA.

  4. 4Department of Biology, University of Wisconsin, Eau Claire, WI 54701, USA.



in Science 360(6386): 317-320 – DOI: 10.1126/science.aas9313 – 




Rising CO2 levels might not be as good for plants as we thought

Long-term experiment finds a surprising flip in the rules for plant photosynthesis

APRIL 19, 2018

Screen Shot 2018-04-25 at 17.55.13

A short-term trend reversed

Theory and empirical data both support the paradigm that C4 plant species (in which the first product of carbon fixation is a four-carbon molecule) benefit less from rising carbon dioxide (CO2) concentrations than C3 species (in which the first product is a three-carbon molecule). This is because their different photosynthetic physiologies respond differently to atmospheric CO2 concentrations. Reich et al. document a reversal of this pattern in a 20-year CO2 enrichment experiment using grassland plots with each type of plant (see the Perspective by Hovenden and Newton). Over the first 12 years, biomass increased with elevated CO2 in C3 plots but not C4 plots, as expected. But over the next 8 years, the pattern reversed: Biomass increased in C4 plots but not C3 plots. Thus, even the best-supported short-term drivers of plant response to global change might not predict long-term results.

Science, this issue p. 317; see also p. 263


Theory predicts and evidence shows that plant species that use the C4photosynthetic pathway (C4 species) are less responsive to elevated carbon dioxide (eCO2) than species that use only the C3 pathway (C3species). We document a reversal from this expected C3-C4 contrast. Over the first 12 years of a 20-year free-air CO2 enrichment experiment with 88 C3 or C4 grassland plots, we found that biomass was markedly enhanced at eCO2 relative to ambient CO2 in C3 but not C4 plots, as expected. During the subsequent 8 years, the pattern reversed: Biomass was markedly enhanced at eCO2 relative to ambient CO2 in C4 but not C3 plots. Soil net nitrogen mineralization rates, an index of soil nitrogen supply, exhibited a similar shift: eCO2 first enhanced but later depressed rates in C3 plots, with the opposite true in C4 plots, partially explaining the reversal of the eCO2 biomass response. These findings challenge the current C3-C4 eCO2 paradigm and show that even the best-supported short-term drivers of plant response to global change might not predict long-term results.



Oscillations in stomatal conductance in the light and dark.



Oscillations in stomatal conductance of hybrid poplar leaves in the light and dark.

by Reich P. B. (1984)

Peter B. Reich

in Plant Physiology 1984;61:541-548 –DOI: 10.1111/j.1399-3054.1984.tb05167.x –

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Cycling of stomatal conductance in three hybrid poplar (Populus sp.) cultivars was observed under a variety of conditions. Illumination of plants kept previously in the dark induced very large oscillations with a period of about 40 min and large oscillations with a shorter period (< 10 min) were superimposed on the longer cycles. During these oscillations, large changes in conductance could occur very rapidly (1.0 cm s−1 in 3 min).

Plants in constant light also displayed both long and short term cycles in conductance, but these were smaller in amplitude than those induced by sudden illumination.

Stomatal oscillations were also observed in darkness and after darkening of previously illuminated plants. These oscillations had shorter (< 30 min) and less regular periods than those observed in the light. Such cycling in the dark is rare.

Cycling of the two leaf surfaces was sometimes in synchrony in the light, and more so after a perturbation. Little synchrony between the two surfaces was observed in the dark.

Stomatal movements of different leaves on a plant were usually relatively independent. Transient stomatal opening occurred following leaf excision in the light or dark, and often after sudden darkening of intact leaves. Also, stomata of intact leaves sometimes transiently closed following illumination.

Stomatal density, diffusive conductance and stomatal responses



Leaf stomatal density and diffusive conductance in three amphistomatous hybrid poplar cultivars.

by Reich P. B. (1984)

Peter B. Reich

in New Phytologist 98 (2): 231-239 –DOI: 10.1111/j.1469-8137.1984.tb02733.x  –

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Stomatal density and diffusive conductance were characterized for both leaf surfaces in three amphistomatous hybrid cultivars of poplar. Consistent differences in stomatal density were observed between cultivars and between leaf surfaces within a cultivar.

Mean stomatal density in the three cultivars ranged from 150 to 330 stomata mm−2 for abaxial leaf surfaces and from 75 to 100 stomata mm−2 for adaxial surfaces. The density of stomata on the abaxial versus adaxial surface was related to the spatial orientation of leaves with respect to the horizon and this stomatal ratio ranged from 1–4 to 4–0 in the three clones. Also, stomatal density was greater in leaves at higher rather than lower nodal positions.

Differences in diffusive conductance between cultivars and leaf surfaces were observed on intact and detached leaves in the light and dark. Within each cultivar mean abaxial conductance (Kab) was greater than adaxial conductance (Kad).

Mean conductances in the light for the three cultivars ranged from 0–22 to 0–62 cm s−1 for abaxial, and from 0–15 to 0–17 cm s−1 for adaxial surfaces, and in the dark they were between 0.06 and 0.26 cm s−1 for abaxial, and from 0–04 to 0–06 for adaxial surfaces.

The differences in conductance between cultivars and between leaf surfaces were correlated with their respective stomatal densities.

Stomatal response to light and to leaf excision also varied between cultivars and between the two leaf surfaces.