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 ecophysi ological interrelationships of these plants, a model dealing with their gas eous exchange, which is basic to transpiration and photosynthesis, is intro duced. 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. Necessa ry parameters consist of kinetical properties of the assimilation process a nd other physiological parameters (which have to be taken from extant plant s), as well as physical constants and anatomical parameters which can be ob tained from well-preserved fossil specimens. The model system is applied to an Early Devonian land plant, Aglaophyton major. The results demonstrate t hat, under an Early Devonian CO2 concentration, A. major shows an extremely low transpiration rate and a low, but probably sufficiently high assimilat ion rate. Variation of the atmospheric CO2 concentration shows that the ass imilation 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 pro bably able to exist under a wide range of atmospheric CO2 concentrations. F urther applications of this model system to ecophysiological studies of ear ly land plant evolution are discussed. (C) 2000 Academic Press.