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A cycad's non‐saturating response to carbon dioxide enrichment indicates Cenozoic carbon limitation in pre‐historic plants
Author(s) -
Nackley Lloyd L.,
Midgley Guy F.,
Bösenberg Jacques de Wet,
Donaldson John S.
Publication year - 2018
Publication title -
austral ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.688
H-Index - 87
eISSN - 1442-9993
pISSN - 1442-9985
DOI - 10.1111/aec.12581
Subject(s) - cycad , photosynthesis , carbon dioxide , botany , biology , sugar , relative growth rate , biomass (ecology) , perennial plant , carbon sink , ecology , growth rate , ecosystem , biochemistry , geometry , mathematics
Cycads were a dominant plant functional type during the Mesozoic Era when atmospheric carbon dioxide [ CO 2 ] greatly exceeded current conditions. Cycads, now rare and endangered, are slow‐growing perennial gymnosperms that develop carbon‐rich structural biomass, such as sclerophyllous leaves, dense stems and massive reproductive cones. Is cycad carbon partitioning to specific organs a constraint of their high [ CO 2 ] evolutionary history ( CO 2 legacy hypothesis, CLH )? To explore changes in cycad growth, carbon partitioning and assimilation responses that could be expected during the CO 2 depletion of the Cenozoic Era, individuals of the cycad species Encephalartos villosus plants were grown at four CO 2 levels: 400, 550, 750 and 1000 μmol mol −1 . The CLH predicts that cycad biomass and growth rates would increase in elevated [ CO 2 ] due to increased net assimilation rates, and that carbon‐dense structures would provide sufficient carbohydrate sinks to prevent photosynthetic down‐regulation even under super‐ambient [ CO 2 ] of 1000 μmol mol −1 . Both hypotheses were confirmed, though the latter less strongly. Plant relative growth rates increased 23% and biomass accumulation increased 65% in 1000 μmol mol −1 relative to 400 μmol mol −1 treatment groups. Mean net assimilation rates increased 130% at 1000 μmol mol −1 relative to 400 μmol mol −1 CO 2 , though there was some down‐regulation of maximum rate of carboxylation (Vc max ). Assimilation rates, relative growth rates, biomass and mean leaf sugar content were linearly related to [ CO 2 ] over the entire experimental range. Photosynthesis appears to be regulated by stomata at low CO 2 levels and by non‐stomatal (i.e. biochemical limitations) at greater concentrations. In general, our results suggest that growth and physiological performance of cycads have been severely compromised by declining [ CO 2 ] during the Cenozoic Era, possibly contributing to the current rare and endangered status of this functional type.