Plants as sources of airborne bacteria, including ice nucleation-active bacteria.

Vertical wind shear and concentration gradients of viable, airborne bacteria were used to calculate the upward flux of viable cells above bare soil and canopies of several crops. Concentrations at soil or canopy height varied from 46 colony-forming units per m over young corn and wet soil to 663 colony-forming units per m over dry soil and 6,500 colony-forming units per m over a closed wheat canopy. In simultaneous samples, concentrations of viable bacteria in the air 10 m inside an alfalfa field were fourfold higher than those over a field with dry, bare soil immediately upwind. The upward flux of viable bacteria over alfalfa was three- to fourfold greater than over dry soil. Concentrations of ice nucleation-active bacteria were higher over plants than over soil. Thus, plant canopies may constitute a major source of bacteria, including ice nucleation-active bacteria, in the air.

Relationship between Ice Nucleation Frequency of Bacteria and Frost Injury.

Not every cell of a given bacterial 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) was found to vary 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 degrees C on nutrient agar containing glycerol. The relationship between nucleation frequency and frost injury was examined by subjecting corn seedlings to -4 degrees C immediately after they were sprayed with bacterial suspensions with different nucleation frequencies and by following both 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 degrees C was a function of the number of ice nuclei active at that temperature on the leaves. The number of ice nuclei, in turn, is the product of the nucleation frequency and population size of ice-nucleation-active bacteria present on the leaves.