Photosynthesis in Early Land Plants: Adapting to the Terrestrial Environment

Abstract

The embryophytic land plants evolved from charophycean green algae, one of the three clades of green algae which are important components of the microflora of present-day terrestrial habitats. The earliest embryophytes are recognised in the fossil record from their characteristic spores, with little evidence as to their vegetative structure. These earliest embryophytes presumably resemble the extant terrestrial green algae in being desiccation tolerant and poikilohydric. Only the embrophytes subsequently developed the homoiohydry which characterised the organism which today contribute most of the biomass and primary productivity on land, and allowed many of the organisms to become desiccation intolerant in the vegetative phase. Pre-Carboniferous land plant fossils have very few examples of bryophytes other than spores: exceptions are the Middle Devonian Metzgeriothallus and the Upper Devonian Pallaviciniites. Many of the other fossils are recognisable as polysporangiophytes, including vascular plants. Homoiohydry in some of these plants is shown by the occurrence of cuticle and stomata, although there is no fossil evidence bearing on desiccation tolerance/intolerance. In addition to the embryophytes there are many other fossils, e.g. Pachytheca, Parka, Protosalvinia, Prototaxites and Spongiophyton, which are probably photosynthetic organisms, but are not readily classified: algae, bryophytes and lichens have been suggested, in addition to the possibility that some represent terrestrial fungi. The high atmospheric CO2 concentrations in the early Phanerozoic would have permitted higher rates of photosynthesis than occurs today on the basis of the surface area of the plant exposed to the gas phase because large concentration gradients from the atmosphere to the carboxylase driving diffusive entry of CO2 are possible. Relatively complex morphologies (several layers of photosynthetic structures) and/or anatomy (ventilation within the organisms using gas spaces) are required if the light-harvesting capacity is to be matched by the CO2 assimilation capacity.