Evolutionary differences in Δ13C detected between spore and seed bearing plants following exposure to a range of atmospheric O2:CO2 ratios; implications for paleoatmosphere reconstruction

The stable carbon isotopes of fossil plants are a reflection of the atmosphere and environment in which they grew. Fossil plant remains have thus stored information about the isotopic composition and concentration of atmospheric carbon dioxide (pCO(2)) and possibly pO(2) through time. Studies to date, utilizing extant plants, have linked changes in plant stable carbon isotopes (delta C-13(p)) or carbon isotope discrimination (Delta C-13) to changes in pCO(2) and/or pO(2). These studies have relied heavily on angiosperm representatives, a phylogenetic group only present in the fossil record post-Early Cretaceous (similar to 140 million years ago (mya)), whereas gymnosperms, monilophytes and lycophytes dominated terrestrial ecosystems prior to this time. The aim of this study was to expand our understanding of carbon isotope discrimination in all vascular plant groups of C3 plants including lycophytes, monilophytes, gymnosperms and angiosperms, under elevated CO2 and sub-ambient O-2 to explore their utility as paleo-atmospheric proxies. To achieve this goal, plants were grown in controlled environment chambers under a range of O-2:CO2 ratio treatments. Results reveal a strong phylogenetic dependency on Delta C-13, where spore-bearing (lycophytes and monilophytes) have significantly higher C-13 discrimination than seed plants (gymnosperms and angiosperms) by similar to 5%. We attribute this strong phylogenetic signal to differences in C-i/C-a likely mediated by fundamental differences in how spore and seed bearing plants control stomatal aperture. Decreasing O-2:CO2 ratio in general resulted in increased carbon isotope discrimination in all plant groups. Notably, while all plant groups respond unidirectionally to elevated atmospheric CO2 (1900 ppm and ambient O-2), they do not respond equally to sub-ambient O-2 (16%). We conclude that (1) Delta C-13 has a strong phylogenetic or 'reproductive grade' bias, whereby Delta C-13 of spore reproducing plants is significantly different to seed reproducing taxa. (2) Delta C-13 increases with decreasing O-2:CO2 ratios (where significant) but is more likely a result of mechanistically uncoupled responses to elevated pCO(2) and sub-ambient O-2; and (3) due to this response we find delta C-13(a) cannot be calculated from delta C-13(p) unless environmental influences such as O-2:CO2 ratio can be independently constrained. Therefore, interpretations of trends in fossil plant delta C-13(p) to reconstruct paleoatmospheric CO2 concentration should include cross calibration based on a nearest living relative, appropriate nearest living equivalent, or utilizing the phylogenetic corrections produced from this study. (C) 2017 The Author(s). Published by Elsevier Ltd.