Transpiration and assimilation of early Devonian land plants with axially symmetric telomes - Simulations on the tissue level

Early terrestrial ancestors of the land flora are characterized by a simple, axially symmetric habit and evolved in an atmosphere with much higher CO2 concentrations than today. In order to gain information about the ecophysiological interrelationships of these plants, a model dealing with their gaseous exchange, which is basic to transpiration and photosynthesis, is introduced. The model is based on gas diffusion inside a porous medium and on a well-established photosynthesis model and allows for the simulation of the local gas fluxes through the various tissue layers of a plant axis. Necessary parameters consist of kinetical properties of the assimilation process and other physiological parameters (which have to be taken from extant plants), as well as physical constants and anatomical parameters which can be obtained from well-preserved fossil specimens. The model system is applied to an Early Devonian land plant, Aglaophyton major. The results demonstrate that, under an Early Devonian CO2 concentration, A. major shows an extremely low transpiration rate and a low, but probably sufficiently high assimilation rate. Variation of the atmospheric CO2 concentration shows that the assimilation is fully saturated even if the CO2 content is decreased to about one-third of the initial value. This result indicates that A. major was probably able to exist under a wide range of atmospheric CO2 concentrations. Further applications of this model system to ecophysiological studies of early land plant evolution are discussed. (C) 2000 Academic Press.