Effect of powdery mildew infection on concentrations of apoplastic sugars in pea leaves

SUMMARY Intercellular washing fluid (IWF) was extracted by centrifugation from healthy and mildew‐infected pea leaves. The level of cytoplasmic contamination of the IWF was estimated to be 0.3% for healthy leaves and 0.7% in mildew‐infected leaves as assessed by the presence of malate dehydrogenase activity. The IWF was analyzed for sugars using gas liquid chromatography and the glucose oxidase assay. Changes in the concentrations of glucose, fructose and sucrose showed different trends over an infection time course. Glucose and sucrose concentrations fluctuated over the 7 d period following infection but almost always showed higher levels in infected tissues than in the controls. Fructose concentrations were low and showed a greater increase over 7 d in infected compared to healthy leaves. The pattern of efflux of sucrose from pre‐loaded healthy and mildew‐infected pea leaf discs suggests that the plasma membranes of cells from infected pea leaves might be more leaky than those of healthy pea leaves. The implications of apoplastic sugar levels for nutritional aspects of the plant‐pathogen interaction are discussed.