Annals of Botany

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A root in a hard place

It was apparent in one of the symposia sponsored by the Annals of Botany at the recent International Botanical Congress in Vienna that laboratory studies of root development may tell only part of the story. This is because, in contrast to laboratory conditions, agricultural and natural habitats are often far from ideal for root growth. One common shortcoming is soil compaction, studied here by Hanbury and Atwell (South Perth and Sydney, Australia, pp. 913–924). These authors imposed an air pressure of 15 kPa on the coarse sand in which roots of Lupinus angustifolius seedlings were growing, thereby mimicking the effects of impedance in a hard soil. Within 1 h, the extension rate of roots declined, and by 20 h the rate was less than 25 % of that in control roots. There was no effect on the root meristem; the effect on root extension being due to a dramatic reduction in cell elongation. Two other morphological effects were also very clear. Firstly, the zone of cell elongation was pushed towards the meristem. Secondly, the impeded roots were nearly twice as thick as control roots. This did not involve the generation of new cell files but was based entirely on increased lateral expansion of the cells. The authors then determined O₂ requirements for elongation. Control roots achieved their maximum elongation rate at approx. 10 % soil atmosphere O₂ whereas the corresponding figure for impeded roots was approx. 22 %. Overall, impeded roots consumed 80 % more O₂ per unit of elongation growth than control roots. In both situations O₂ requirement was much greater in the apical 5 mm, the zone containing the root meristem, but because, in impeded roots, cell elongation was initiated so close to the meristem, cell elongation in this zone was very vulnerable to lower O₂ tension. Thus, morphological responses to impedance made the roots more vulnerable to hypoxia, a situation likely to be exacerbated in hard soils where gaseous diffusion may be hindered.