PARENT SESSION
Oral Session #43: Soil Ecology: Mycorrhizae and Roots. Presiding: W. Stock.
Wednesday, August 8, 2001. 8:00 AM to 12:15 PM. Hall of Ideas F.

The soil biotic community increases the infectivity of Frankia.

Zimpfer, Jeff¹, Kaelke, Christopher¹, Smyth, Charles¹, Dawson, Jeffrey¹, ¹

ABSTRACT- Actinorhizal plants in symbiosis with the soil diazotroph Frankia fix atmospheric nitrogen. However, little is known about factors influencing Frankia infectivity. We hypothesized that the soil biotic community and Casuarina tissue extracts would both increase Casuarina nodulation where a Frankia isolate was added to the soil. Soil with a relatively low Frankia infectious capacity was taken from beneath mature Casuarina trees and incubated in sealed, sterile jars. In a factorial experiment jars with 25 g soil received the following treatments: +/- steam pasteurization, +/- an added Frankia isolate, and +/- the addition of aqueous Casuarina tissue extract. Infectivity was greatest in the treatments where a Frankia isolate was added to soil with an intact biotic community (11,000 infective units g^-1soil). The highest concentration of tissue extract decreased infectivity, while the lower concentration increased infectivity, but did not contribute significantly to overall variation in nodulation in the model. The infectious capacity of soil with its intact biotic community and an added Frankia isolate was three times greater than the sum of the infectious capacity of the soil alone and the infectious capacity of the same Frankia isolate added to pasteurized soil. This indicates a synergism between the biotic community and Frankia with respect to host infection.

KEY WORDS: Frankia, Casuarina, infectivity, actinorhizal