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 (Gs), and reduced stomatal index (SI), indicative of lower stomatal initiation. Modelled theoretical Gs did not correlate with observed measured Gs, 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.