Roots: evolutionary origins and biogeochemical significance
J. A. Raven1,3 and D. Edwards2
1 Division of Environmental and Applied Biology, School of Biological Sciences, University of Dundee, Dundee DD1 4HN, UK
2 Department of Earth Sciences, Cardiff University, PO Box 914, Cardiff CF1 3YE, UK
Received 1 June 2000; Accepted 20 September 2000
Roots, as organs distinguishable developmentally and anatomically from shoots (other than by occurrence of stomata and sporangia on above-ground organs), evolved in the sporophytes of at least two distinct lineages of early vascular plants during their initial major radiation on land in Early Devonian times (c. 410–395 million years ago). This was some 15 million years after the appearance of tracheophytes and c. 50 million years after the earliest embryophytes of presumed bryophyte affinity. Both groups are known initially only from spores, but from comparative anatomy of extant bryophytes and later Lower Devonian fossils it is assumed that, during these times, below-ground structures (if any) other than true roots fulfilled the functions of anchorage and of water and nutrient acquisition, despite lacking an endodermis (as do the roots of extant Lycopodium spp.). By 375 million years ago root-like structures penetrated almost a metre into the substratum, greatly increasing the volume of mineral matter subject to weathering by the higher than atmospheric CO2 levels generated by plant and microbial respiration in material with restricted diffusive contact with the atmosphere. Chemical weathering consumes CO2 in converting silicates into bicarbonate and Si(OH)4. The CO2 consumed in weathering ultimately came from atmospheric CO2 via photosynthesis and respiration; this use of CO2 probably accounts for most of the postulated 10-fold decrease in atmospheric CO2 from 400–350 million years ago, with significant effects on shoot evolution. Subsequent evolution of roots has yielded much-branched axes down to 40 µm diameter, a lower limit set by long-distance transport constraints. Finer structures involved in the uptake of nutrients of low diffusivity in soil evolved at least 400 million years ago as arbuscular mycorrhizas or as evaginations of ‘roots’ (‘root hairs’).
Key words: Devonian, endodermis, phylogeny, Silurian, weathering.
Source: Journal of Experimental Botany, Vol. 52, No. 90001, pp. 381-401, March 2001. © 2001 Oxford University Press