Stomatal Closure in Plants Infected with Mycoplasmalike Organisms

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Abstract
Stomatal closure, as indicated by abnormally high diffusive resistance of leaf surfaces (Rl, s .cntdot. cm-1) and by leaf surface replicas, was a consistent symptom of elm yellows (phloem necrosis) in Ulmus americana and U. rubra, ash yellows (ash witches'' broom) in Fraxinus americana, infection by the elm yellows agent or the ash yellows agent in Madagascar periwinkle (Catharanthus roseus), X-disease in Prunus virginiana, and corn stunt in Zea mays. An abnormal increase in Rl preceded or coincided with the earliest foliar symptoms of elm yellows and ash yellows and is therefore useful for detection of these mycoplasmal infections. In each infection studied, the degree of stomatal closure increased as foliar symptoms became more intense. Xylem pressure potentials less negative than normal were consistently associated with elevated Rl in U. americana, but Rl was the more sensitive indicator of infection.

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