Groundsel (Senecio vulgaris L.) was grown in sand culture at a range of nutrient concentrations. Except when nutrient deficiency was severe, infection by the rust fungus Puccinia lagenophorae Cooke substantially reduced root dry weight but had little effect on root length. Thus, specific root length (SRL, cm root mg−1 d. wt) was significantly increased in rust-infected plants. The inhibition of root dry weight caused by rust infection was most pronounced late in development, especially after flowering when, in control plants, root elongation but not dry weight accumulation ceased. In rusted plants, and in all plants subjected to severe nutrient deficiency, dry weight accumulation in the roots ceased concurrently with root elongation. Late in development at high nutrient concentration adventitious roots with low SRLs were produced. However, infection did not modify the production of such roots and increases in SRL could not be attributed to changes in any single type of root.
There was an inverse relationship between SRL and root diameter. This relationship was unaffected by rust infection whilst nutrient deficiency changed only its intercept: at a given SRL roots of nutrient stressed plants were thinner than those of plants with adequate nutrient supply. Thus, the smaller diameter of roots of nutrient-stressed plants occurred independently of measured changes in SRL but, in the absence of nutrient stress, the decrease in root diameter caused by rust was closely related to increases in SRL.
Changes in the root: length relationships in rusted plants may have important implications for root activity in the field. In view of the reported changes in SRL, inhibition of root growth in terms of dry weight may be a poor indicator of potential changes in activity.