Differences in the photosynthetic plasticity of ferns and Ginkgo grown in experimentally controlled low [O2]:[CO2] atmospheres may explain their contrasting ecological fate across the Triassic-Jurassic mass extinction boundary.
Fluctuations in [CO 2 ] have been widely studied as a potential driver of plant evolution; however, the role of a fluctuating [O 2 ]:[CO 2 ] ratio is often overlooked. The present study aimed to investigate the inherent physiological plasticity of early diverging, extant species following acclimation to an atmosphere similar to that across the Triassic-Jurassic mass extinction interval (TJB, approx. 200 Mya), a time of major ecological change.



Mature plants from two angiosperm ( Drimys winteri and Chloranthus oldhamii ), two monilophyte ( Osmunda claytoniana and Cyathea australis ) and one gymnosperm ( Ginkgo biloba ) species were grown for 2 months in replicated walk-in Conviron BDW40 chambers running at TJB treatment conditions of 16 % [O 2 ]-1900 ppm [CO 2 ] and ambient conditions of 21 % [O 2 ]-400 ppm [CO 2 ], and their physiological plasticity was assessed using gas exchange and chlorophyll fluorescence methods.



TJB acclimation caused significant reductions in the maximum rate of carboxylation ( V Cmax ) and the maximum electron flow supporting ribulose-1,5-bisphosphate regeneration ( J max ) in all species, yet this downregulation had little effect on their light-saturated photosynthetic rate ( A sat ). Ginkgo was found to photorespire heavily under ambient conditions, while growth in low [O 2 ]:[CO 2 ] resulted in increased heat dissipation per reaction centre ( DI o / RC ), severe photodamage, as revealed by the species' decreased maximum efficiency of primary photochemistry ( F v / F m ) and decreased in situ photosynthetic electron flow ( Jsitu ).



It is argued that the observed photodamage reflects the inability of Ginkgo to divert excess photosynthetic electron flow to sinks other than the downregulated C 3 and the diminished C 2 cycles under low [O 2 ]:[CO 2 ]. This finding, coupled with the remarkable physiological plasticity of the ferns, provides insights into the underlying mechanism of Ginkgoales' near extinction and ferns' proliferation as atmospheric [CO 2 ] increased to maximum levels across the TJB.
Earth Institute, O'Brien Centre for Science, University College Dublin, Belfield, Ireland.|School of Biology and Environmental Science, University College Dublin, Belfield, Ireland.
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Yiotis
Earth Institute, O'Brien Centre for Science, University College Dublin, Belfield, Ireland.|School of Biology and Environmental Science, University College Dublin, Belfield, Ireland.
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Evans-Fitz Gerald
Earth Institute, O'Brien Centre for Science, University College Dublin, Belfield, Ireland.|School of Biology and Environmental Science, University College Dublin, Belfield, Ireland.
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McElwain
D001272:Atmosphere; D005075:Biological Evolution; D002245:Carbon Dioxide / Q000737:chemistry*; D053476:Extinction, Biological; D029624:Ferns / Q000502:physiology*; D020441:Ginkgo biloba / Q000502:physiology*; D010100:Oxygen / Q000737:chemistry*; D010788:Photosynthesis; D018515:Plant Leaves / Q000502:physiology
D002245:Carbon Dioxide; D010100:Oxygen
Ginkgo biloba; Triassic–Jurassic boundary; angiosperms; gymnosperms; high CO2; low O2; mesophyll conductance; monilophytes; photodamage; photorespiration; photosynthetic plasticity; stomatal conductance