Freezing of healthy and rust (Puccinia lagenophorae) infected leaves of Senecio vulgaris was compared calorimetrically by thermal analysis. In fully expanded leaves the threshold freezing temperature was in the range −6.8 to −8.4 °C in controls but −3.0 to −5.1 °C in leaves with sporulating rust sori. Comparable values in expanding leaves were −5.0 to −8.9 °C and −3.9 to −6.7 °C for healthy and rusted tissues, respectively. The bulk tissue freezing point was between −1.0 and −4.0 °C in both fully expanded and expanding healthy leaves, and was increased by infection by between +0.2 and 2.5 °C. Whereas healthy leaves supercooled by 3.1−5.8 °C, rusted leaves supercooled by only 1.8−4.9 °C Supercooling of control leaves was reduced by dusting with aeciospores, particularly when leaves were wounded to simulate the rupture of the surface caused by sporulation, but wounding alone had no significant effect. Supercooling of distilled water was also significantly reduced by aeciospores, suspended at a concentration of 105 spores ml−1.
It is concluded that rust-induced changes in leaf freezing in S. vulgaris grown in controlled environments were due to an increase in the number of sites for ice nucleation, caused by the presence of the aeciospores, and increased penetration of ice into internal tissues, resulting from damage to the cuticle and epidermis. Although data for frost resistance obtained in the growth-room are similar to previous field observations, the role of the above mechanisms under field conditions remains unproven.