Senecio vulgaris (groundsel), healthy or inoculated with rust (Puccinia lagenophorae), was grown in a nutrient-poor sand-soil mixture amended with a compound fertiliser to provide a range of nutrient regimes. The greatest number of pustules per plant occurred on unamended soil but fertiliser addition increased the sporulating area of each pustule: the leaf area occupied by pustules was unaffected by nutrient supply. Since total leaf area increased with increasing nutrient supply, the percentage of total area that was infected declined. Plant and root dry weights increased with nutrient supply and decreased with rust infection. Infection also reduced root fresh weight and length, which were little changed by nutrient supply, but reduced leaf area only under the most nutrient-rich conditions.
The ratios of root length: leaf area (R:L) and, especially, root:shoot dry weight (R:S) were reduced by infection. Root length per unit root weight was increased by infection when expressed on the basis of dry weight but not on the basis of fresh weight.
Conversely, increasing nutrient supply decreased root length:weight only on the basis of fresh weight. The concentrations of P and K per unit dry weight were increased in rusted leaves but that of N was unchanged. Because infection increased the dry weight: fresh weight ratio (d. wt:f. wt) of leaves, concentrations of P and K on a fresh weight basis increased by up to 3-fold over those in controls and that of N was also substantially increased. On a dry weight basis, the concentration of N and K, but not P, increased in the roots of rusted plants. However, since d. wt:f. wt of roots was decreased in infected plants, the concentrations of N and K were unchanged on a fresh weight basis and that of P was significantly reduced.
Rates of uptake of N, P and K were significantly increased in rusted plants both on the basis of root dry weight and root length. This was attributed to increased ‘shoot demand’ on each unit of root which, in turn, reflected the decreased R:S and R:L of infected plants.