On the reconstruction of plant photosynthetic and stress physiology across the Triassic-Jurassic boundary

Authors: MATTHEW HAWORTH, ANGELA GALLAGHER, ELYSIA SUM, MARLENE HILL-DONNELLY, MARGRET STEINTHORSDOTTIR, JENNIFER MCELWAIN

Abstract: The Triassic-Jurassic boundary (TJB) coincides with major disruption to the carbon cycle and global warming as the Central Atlantic Magmatic Province developed. This resulted in both marine and terrestrial extinctions, with terrestrial plants thought to experience thermal stress as global temperatures and atmospheric CO_2 levels rose. As plant compression fossils typically only preserve external morphological features, it has not been possible to reconstruct plant paleophysiology in order to elucidate the mechanisms underlying plant stress and extinction. Here we present a new approach allowing us to infer the photosynthetic performance and stress physiology of fossil plants, applied to fossil Ginkgoales across the TJB. We use correlations between the adaxial epidermal cell density of extant Ginkgo biloba and photosynthetic and protective stress physiology to infer the paleophysiological condition of Late Triassic-Early Jurassic-aged plants from Astartekløft, East Greenland. The density of fossil leaf adaxial epidermal cells indicates that photosynthetic performance of Ginkgoales became increasingly impaired towards the latter stages of the Triassic, before improving into the Early Jurassic. This is consistent with \delta^{13}C isotope values, paleo-[CO_2] levels, and global mean temperatures, suggesting that photosynthetic performance was influenced by the prevailing environmental conditions during the TJB event. Dissipation of absorbed energy as heat would also have risen towards the boundary as plant stress increased, in order to protect the photosynthetic physiology. The increase in dissipation of energy as heat, associated with a reduction in convective heat loss due to reduced transpiration rates, would have exacerbated plant thermal stress at the TJB, thus contributing to sudden biodiversity loss and ecological change.

Keywords: Triassic-Jurassic boundary, ginkgo, plant stress, thermal stress, paleophysiology, nonphotochemical quenching

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