Nitrogen source effects on rhizosphere pH and nutrient accumulation by Pacific Northwest conifers

Growth responses ofCasuarina cunninghamiana to inoculation withFrankia are described in unsterilized field soils at three sites. At Mt Crawford, South Australia, seedlings of three provenances ofC. cunninghamiana were inoculated with a singleFrankia source just prior to planting out. Forty-four months after planting, inoculation had more than doubled wood production by twoC. cunninghamiana provenances, whilst a third provenance grew poorly and did not respond to inoculation. In Zimbabwe, seedlings of one provenance ofC. cunninghamiana were inoculated in the nursery with one of four differentFrankia strains. In an N deficient soil at Kadoma, three of theseFrankia increased tree height 14 months after planting by between 50% and 70% in comparison to the uninoculated seedlings. The fourthFrankia strain resulted in increased tree height to three times that of the uninoculated controls and up to double that of the other threeFrankia strains. At Gympie, Queensland, Australia, seedlings ofC. cunninghamiana raised open-rooted in a nursery bed were inoculated withFrankia seventeen weeks before planting out. During the 22 months following planting in the field, tree growth was limited by soil P status and there was no response in tree height or stem diameter to inoculation withFrankia or to N fertilizer unless P was applied. In the presence of added P there was a significant response both toFrankia inoculation and to N fertilizer. This positive interaction between P application and N treatment was reflected in wood volumes-inoculated trees and those trees supplied N fertilizer produced 34% and 95% more wood volume than did the uninoculated trees. These results demonstrate the potential to increase the productivity of Casuarina plantings by inoculation withFrankia and by alleviation of P deficiency.     

Frankia in the rhizosphere of nonhost plants: A comparison with root-associated N2-fixing Enterobacter,Klebsiella and Pseudomonas

Bacterial growth in the rhizosphere and resulting changes in plant growth parameters were studied in small aseptic seedlings of birch (Betula pendula and B. pubescens) and grasses (Poa pratensis and Festuca rubra). The seedlings were inoculated with three Frankia strains (Ai1a and Ag5b isolated from native Alnus root nodules and Ai17 from a root nodule induced by soil originating from a Betula pendula stand), and three associative N₂-fixing bacteria (Enterobacter agglomerans, Klebsiella pneumoniae and Pseudomonas sp., isolated from grass roots). Microscopic observations showed that all the Frankia strains were able to colonize and grow on the root surface of the plants tested without addition of an exogenous carbon source. No net growth of the associative N₂-fixers was observed in the rhizosphere, although inoculum viable counts were maintained over the experimental period. Changes in both the biomass and morphology of plant seedlings in response to bacterial inoculation were recorded, which were more dependent on the plant species than on the bacterial strain.     

Sucrose synthase and enolase expression in actinorhizal nodules ofAlnus glutinosa: comparison with legume nodules

Two different types of nitrogen-fixing root nodules are known — actinorhizal nodules induced byFrankia and legume nodules induced by rhizobia. While legume nodules show a stem-like structure with peripheral vascular bundles, actinorhizal nodule lobes resemble modified lateral roots with a central vascular bundle. To compare carbon metabolism in legume and actinorhizal nodules, sucrose synthase and enolase cDNA clones were isolated from a cDNA library, obtained from actinorhizal nodules ofAlnus glutinosa. The expression of the corresponding genes was markedly enhanced in nodules compared to roots. In situ hybridization showed that, in nodules, both sucrose synthase and enolase were expressed at high levels in the infected cortical cells as well as in the pericycle of the central vascular bundle of a nodule lobe. Legume sucrose synthase expression was studied in indeterminate nodules from pea and determinate nodules fromPhaseolus vulgaris by usingin situ hybridization.     

Genetic heterogeneity among Frankia isolates from root nodules of individual actinorhizal plants

Abstract Genetic variations among selected Frankia isolates from nitrogen-fixing root nodules harvested from an individual actinorhizal plant ( Elaeagnus angustifolia L. or Shepherdia argentea Nutt.) were estimated by restriction fragment analysis of their total genomic DNA. The presence of plasmids and their restriction enzyme patterns were used as additional criteria. Certain isolates from separate nodules on the same plant were found indistinguishable, being probably clones of the same strain. An endophytic passage of a strain isolated from S. argentea on another host plant, Hippophaë rhamnoides L., did not modify the structural characteristics of the genome in the reisolates obtained. However, in some cases, especially when restriction endonucleases cleaving Frankia DNA into relatively small fragments were used, multiple infection of the actinorhizal plants with different Frankia strains and the presence of more than one strain in a nodule were demonstrated. Some aspects of variability in natural populations of Frankia are discussed.