Root porosities and radial oxygen losses of Rumex and Plantago species as influenced by soil pore diameter and soil aeration.

The effects of hypoxia in ballotini and quartz sand with respectively large and small soil pore diameters on root porosity was studied for Rumex palustris Sra., Rumex acetosa L. and Plantago major L. ssp. major. Under hypoxic conditions R. palustris produced large root pores when large soil pores were present. Absence of such large soil pores resulted in the collapse of the root structure and a reduced root growth. Increases in intercellular spaces in P. major roots seemed to result in a higher root porosity upon hypoxia, a small soil pore diameter or a combination of both but this was not significant. Only a limited number of roots with large root pores was produced. The morphological structure of P. major roots without large root pores remained intact also in soils with a small soil pore diameter. R. acetosa only slightly increased its root porosity upon hypoxia and its morphological structure also remained intact when soil pore diameter was small. Radial oxygen loss (ROL) was found in R. palustris roots when grown in waterlogged, uncompacted soils. P. major had some roots with ROL in all treatments except in waterlogged, compacted soil. R. acetosa did not produce any roots showing ROL. Under field conditions aerenchyma is of no use in water saturated, compacted soils. This explains why a species like R. palustris only grows on hypoxic soil when it is not compacted, in contrast to P. major which will grow on moist, compacted soils. R. acetosa cannot be found on either very moist or heavily compacted soil, since it does not produce a healthy root system under these conditions.