TRANSPIRATION AND THE WATER RELATIONS OF FABA BEAN ( VICIA FABA ) INFECTED BY RUST ( UROMYCES VICIAE‐FABAE )

S ummary Transpiration and the water relations of faba bean ( Vicia faba L.) were studied in healthy tissues of plants infected by rust ( Uromyces viciae‐fabae (Pers.) Shroet). Water relations measurements on infected plants were made either on uninfected, healthy halves of otherwise infected lower leaves or on wholly uninfected upper leaves and, thus, they excluded the direct influence of fungal tissues. Rust caused increased transpiration from infected tissues after sporulation but, even in well‐watered plants, infection of proximal halves lowered leaf water potentials and inhibited transpiration from distal regions of the same leaf. The normal relationship between leaf water potential and stomatal diffusion resistance was not affected in the uninfected distal part of the leaves. Relationships between soil water potential and leaf water potential were affected by rooting patterns. Increased transpiration caused by rust accelerated drying of the soil, and consequent lowering of leaf water potentials, when plants were grown singly in small unwatered pots. When healthy and rusted plants grew together in pairs in the same small unwatered pots of soil, and therefore at the same soil water potentials, healthy regions of proximally rusted leaves, and healthy leaves near the shoot apex of rusted plants, had lower leaf water potentials and were less turgid than comparable tissues of healthy plants. However, rust did not impair the ability of these same tissues to recover normal leaf water potentials and turgor when plants were rewatered. When healthy and rusted plants shared soil columns that allowed unrestricted downward growth of roots and soil was cooled to temperatures similar to those in the field, roots grew faster in well watered soil than in unwatered soil. Rust inhibited the growth of roots in the mid‐depth (mid soil water potential) region of unwatered soil columns. Although the pattern of water potentials and changes in turgor that developed in unwatered plants after 15 d was similar to that seen previously in unwatered plants grown in small pots, differences in root growth would probably give healthy plants and increasing advantage over rusted plants in water uptake during longer periods of drought.