Relationship between sorbitol and solute potential in apple shoots relative to fire blight symptom development after infection by Erwinia amylovora

The soluble carbohydrate (SC) content and solute potential (psi5) of individual leaves on apple shoots were determined to test the hypothesis that these parameters might be related to the progress of fire blight symptom development after infection by Erwinia amylovora. Experiments were based on the use of leaves and shoots of the same physiological age from 1-yr-old Jonathan and Delicious apple trees, which are susceptible and normally resistant to fire blight, respectively. After inoculation of the individual leaves with a bacterial suspension containing 10(8) cfu/ml, symptom severity declined with increasing leaf age. Inoculations of the first three apical leaves of Jonathan shoots and the first two leaves of Delicious shoots all led to the invasion and subsequent death of the supporting shoot. No infections were established on shoots of either variety when inoculations were made below the fifth leaf. High-pressure liquid chromatography analyses of SC from individual leaves showed that this progressive expression of resistance with leaf age paralleled an increase in sorbitol concentration and an increasingly negative psi5. In cultured apple shoots, tissue damage after tip inoculation progressed more rapidly in etiolated than in light-grown shoots. In both etiolated and light-grown cultured shoots, symptom severity declined with the accumulation of SC, particularly sorbitol, and increasingly negative psi5 when the carbon source in the medium, as sucrose or sorbitol, was increased. The psi5 of E. amylovora colonies in culture and of the bacterial ooze produced on inoculated immature pear slices is generally greater than the psi5 of young apple leaves but less than that of mature leaves. Thus, it appears that normal cell-maintenance functions in mature cells may confer resistance to damage by E. amylovora and that this mechanism is independent of the genetic resistance exhibited by the Delicious cultivar to fire blight in the field. This mechanism appears to explain why young tissues are more severely damaged by fire blight than mature tissues and provides a basis for understanding the renewal of infectious activity by the pathogen at canker margins in the spring.