Heat stress reduces stomatal numbers in Ginkgo biloba: Implications for the stomatal method of palaeo-atmospheric [CO2] reconstruction during episodes of global warming

The stomata of fossil plants are commonly used as proxies to reconstruct palaeo-atmospheric carbon dioxide concentrations (palaeo-[CO₂]). Stomatal reconstruction of palaeo-[CO₂] during global greenhouse periods or episodes of global warming, are particularly important to our understanding of the role of CO₂ as a climate system driver. However, the efficacy of the 'stomatal method' for palaeo-[CO₂] reconstruction depends upon the strength of the inverse relationship between stomatal number and the [CO₂] in which the leaf developed. However, the impact of heat stress on stomatal initiation and development are largely unknown. Ginkgo biloba, a living fossil species, seedlings were grown in controlled environment chambers under 20/25 °C and 30/35 °C night/day temperature regimes. Heat stress in the 30/35 °C treatment impaired photosynthetic function, decreased stomatal conductance (G_s), and reduced stomatal index (SI), indicative of lower stomatal initiation. Modelled theoretical G_s did not correlate with observed measured G_s, undermining the utility of palaeo-[CO₂] reconstructions based on stomatal diffusion modelling. The lower stomatal initiation of G. biloba leaves from the higher temperature resulted in greater estimates of [CO₂] based on SI values using the nearest living equivalent and SI-[CO₂] transfer function approaches. Heat stress may diminish the effectiveness of the stomatal method in reconstructing palaeo-[CO₂] during intervals of global warming marked by floral turnover in Earth history.