Water relations and growth of rust-infected groundsel (Puccinia lagenophorae Cooke; Senecio vulgaris L.) during and after exposure to freezing soil temperatures

Groundsel (Senecio vulgaris L.) was grown at three soil temperatures (ambient, and −2 and 6 °C) in an unheated glasshouse during winter in order to examine how a combination of unavailability of soil water due to freezing and of infection by rust (Puccinia lagenophorae Cooke) would affect water relations and growth. Measurements were continued after plants were transferred to a controlled environment room (10 ° C, 12 h light per day) for recovery.

Low soil temperatures for 2 or 6 weeks reduced leaf water potential (ψ₁) and, over the shorter period, rust exacerbated this reduction. Turgor potential (ψ_p) also fell during soil freezing, more so in rusted than control plants, but both regained turgor rapidly when freezing ended. However, rust prevented a return to pre-freezing ψ₁ during the recovery period. Rust reduced the gain in total dry weight (about 70% of which was leaf dry weight) in plants held at ambient soil temperatures and increased the loss of dry weight of plants held at low soil temperatures; loss of root tissue may have contributed to the inability of rusted plants to regain normal ψ₁ during recovery. In both healthy and rusted plants, specific leaf area (area per unit weight) tended to decrease during low temperature treatment and to increase during recovery. Rust inhibited the increase in leaf area during the recovery period, probably because it lowered ψ₁ and, consequently, increases in dry weight were inhibited by rust, particularly in plants previously held at −2 °C.