Relationship between Ice Nucleation Frequency of Bacteria and Frost Injury

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Abstract
Not every cell of a given bacterial [Pseudomonas syringae, Erwinia herbicola, P. syringae pathovar coronafaciens] isolate that has ice-nucleating properties can serve as an ice nucleus at any given time and temperature. The ratio between the number of ice nuclei and number of bacterial cells in a culture (i.e., nucleation frequency) varied with incubation temperature, growth medium composition, culture age and genotype. Optimal conditions for ice nucleus production in vitro included incubation of the bacterial cells at 20 to 24.degree. C on nutrient agar containing glycerol. The relationship between nucleation frequency and frost injury was examined by subjecting corn seedlings to -4.degree. C immediately after they were sprayed with bacterial suspensions with different nucleation frequencies, and by following ice nucleus concentration and bacterial population size on leaves of corn seedlings as a function of time after bacterial application. The amount of frost injury to growth chamber-grown corn seedlings at -4.degree. C was a function of the number of ice nuclei active at that temperature on the leaves. The number of ice nuclei is the product of the nucleation frequency and population size of ice-nucleation-active bacteria present on the leaves.