Senescence of plant roots is reviewed and related to laboratory and field observations on root turnover (death and replacement) of groundnut (<I>Arachis hypogaea</I> L.). Root death was assessed by browning, which correlated with failure of cells to plasmolyse after staining with neutral red. For five cultivars of groundnut grown in pasteurised soil in transparent tubes, death of root laterals commenced 3 - 4 weeks after sowing; root laterals died in proximal regions and new laterals formed in distal regions as the tap root extended. The half-life of individual roots was 3.7 - 4.4 weeks for all cultivars. Up to the time of plant maturity (14-20 weeks for different cultivars) 72.7% - 83.2% of cumulative (total) root length had died. Similar patterns were seen in field-grown plants in Malawi. Two pathogens of groundnut (<I>Aspergillus niger, Fusarium oxysporum</I>) and two saprophytes (<I>Idriella bolleyi</I> from cereals and <I>Mucor hiemalis</I> from groundnut) were induced or selected for tolerance of benomyl or pimaricin. The mutants had reduced hyphal extension rates on agar but sporulation similar to the wild-type parents in liquid culture. Spore suspensions of these mutants and of <I>Aspergillus flavus</I> were inoculated onto groundnut root laterals of different ages in tubes of soil. After 3 weeks, <I>A. niger</I> and <I>F. oxysporum</I> were not recovered; in other studies <I>A. niger</I> was found to germinate poorly in rhizosphere soil. <I>A. flavus, I. bolleyi</I> and <I>M. hiemalis</I> were recovered in higher numbers on young roots (lower in soil profile) than on older roots. Both growth at the expense of nutrients released early during root senescence <I>(A. flavus, I. bolleyi</I>) and spread in percolating water (<I>M. hiemalis</I>) are suggested to explain these findings. Spores of <I>M. hiemalis</I> (pimaricin-tolerant) were applied to groundnut hypocotyls and to the surface on non-planted soil in an experimental field plot in Malawi. The fungus spread rapidly and progressively down the rhizosphere, reaching at least 58 cm during 51 days, but it did not move far (7 cm) in non-planted soil.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:653562 |
| Date | January 1993 |
| Creators | Krauss, Ulrike |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/12111 |
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