Cycads show no stomatal-density and index response to elevated carbon dioxide and subambient oxygen

The stomatal density (SD) and index (SI) of fossil plants are widely used in reconstructing palaeo-atmospheric CO2 concentration (palaeo-[CO2]). These stomatal reconstructions depend on the inverse relationship between atmospheric CO2 concentration ([CO2]) and SD and/or SI. Atmospheric oxygen concentration ([O-2]) has also varied throughout earth history, influencing photosynthesis via the atmosphericCO(2): O-2 ratio, and possibly affecting both SD and SI. Cycads formed a major component of Mesozoic floras, and may serve as suitable proxies of palaeo-[CO2]. However, little is known regarding SD and SI responses of modern cycads to [CO2] and [O-2]. SD, SI and pore length were measured in six cycad species (Cycas revoluta, Dioon merolae, Lepidozamia hopei, Lepidozamia peroffskyana, Macrozamia miquelii and Zamia integrifolia) grown under elevated [CO2] (1500 ppm) and subambient [O-2] (13.0%) in combination and separately, and compared with SD, SI and pore length under control atmospheric conditions of 380 ppm [CO2] and 20.9% [O-2]. The cycad species analysed showed no significant SD, SI or pore-length response to changes in [CO2] or [O-2].