Groundsel (Senecio vulgaris), healthy or infected with rust, Puccinia lagenophorae, was grown at a range of nutrient concentrations in sand culture. Specific absorption rates calculated on the basis of root dry weight (SARW) were greater in rusted than control groundsel for nitrogen, potassium and phosphorus. While the magnitudes of these stimulations varied, they occurred across the whole range of nutrient concentrations. By contrast, specific absorption rate on the basis of root length (SARL) were little changed by rust at any external nutrient concentration; SARL for phosphate and potassium were slightly reduced when nutrients were freely available.
Water flux per unit dry root weight and length was stimulated by rust because transpiration per unit leaf area was more rapid in infected plants after fungal sporulation. However, water-flux and the rate of uptake of nutrients were correlated only when expressed on the basis of root weight and increased transpiration did not appear to be the mechanism underlying increased root activity. Rather, increased SARW for N, P and K could very largely be attributed to increased shoot demand per unit root, which resulted from the higher shoot: root (S: R) ratios of infected individuals. Changes in S: R accounted for 92, 81 and 57% of total variation in SARW for K, P and N respectively. Greater values for SARW were possible because specific root length (SRL) increased, producing more functional root per unit root weight. The lack of stimulation in SARL in response to rust could be explained since the higher SRL of infected plants resulted in stable values of shoot weight per unit root length, i.e. shoot demand was not increased by infection on this basis.