Groundsel (Senecio vulgaris L.), healthy or infected with the rust fungus Puccinia lagenophorae was grown in sand and fed with a complete nutrient medium diluted to give a range of concentrations. Analysis of bulked, dried tissues of the plant showed that under nutrient-rich conditions rust infection resulted in increased concentrations of total (Kjeldahl) nitrogen and potassium but had little effect on phosphorus concentration. Thus, despite reduced dry weight growth, total plant nitrogen contents were no less in rusted than control plants. Although total contents of phosphorus and potassium were reduced by rust, effects were probably related to loss of these nutrients in fungal spores.
Interactions between rust infection and nutrient supply were significant but differed between nutrients: rust caused increased nitrogen concentrations only under nutrient-rich conditions but increased phosphorus concentrations only when nutrient supply was limited. Increased concentrations were not confined to infected tissues. Mechanisms underlying rust-nutrient interactions appear to be complex and to depend inter alia on the partitioning and recycling of the particular nutrient within the plant. Rust-induced increases in potassium concentration occurred under both high and low nutrient conditions but were confined to infected tissues. Potassium accumulation in nutrient deficient conditions was probably due to increased transpirational flux into infected tissues, but under nutrient-rich conditions reduced potassium export appeared to assume greater significance.
The possible implications of the changed nutrient relations for the wider interactions of rust-infected plants in natural vegetation are discussed.