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The influence of Puccinia hordei (rust) infection on the root hydraulic conductivity (Lp) of excised roots of Hordeum distichum cv. Maris Mink (barley) was investigated using the Scholander pressure chamber technique. The solute potential of the root exudate was also measured to determine the contribution of the osmotic component to the total driving force for water flow across the roots. The root Lp of infected plants was significantly lower than control plants on days 5, 6 and 7 after infection. On days 4 and 8 after infection, Lp did not differ significantly. Root Lp increased with time for both control and infected plants from days 5 to 8 after infection. Root Lp varied between 1·4 and 3·6 × 10-5 cm s-1 MPa-1 for roots of infected plants, and between 2·3 and 4·0 × 10-5 cm s-1 MPa-1 for control plants. The solute potential of root xylem sap exudate was significantly higher (less negative) than that of control sap on days 6, 7 and 8 after infection, reflecting a decrease in the transport of solutes into the root xylem of roots of diseased plants. In control and infected plants, as the applied pressure increased, solute potential root exudate increased. These results are discussed in relation to the known susceptibility of rust infected plants to drought and the mechanisms by which rust infection increases such susceptibility.

The water balance of brown rust (Puccinia hordei) infected barley seedlings was investigated. It was found that with the progression of the disease, the infected leaves lost their ability to maintain a favourable water status. This was attributed to tears in the cuticle, which occurred from day 5 after infection. In addition, a decreasing responsiveness of leaf diffusion resistance to bulk leaf water content was observed, probably due to the increasing proportion of water loss via the tears in the cuticle. An increase in leaf dry weight per unit area which was not associated with an increase in tissue volume, was found in diseased leaves. These responses to infection were cumulative with disease development, with no sudden effect of sporulation. Leaf turgor and water potentials were found to be lower in diseased leaves than in control leaves.