Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2008
Publication Date: 9/12/2008
Citation: Funnell-Harris, D.L., Pedersen, J.F., Marx, D.B. 2008. Effect of sorghum seedlings and previous crop, on soil fluorescent pseudomonas spp.. Plant and Soil Journal 311:173-187. Interpretive Summary: Sorghum has the capacity to inhibit growth of weeds and pathogens, which makes it a good choice in cropping systems. One possible reason for this ability is that some sorghums produce large quantities of the inhibitory compound, sorgoleone. In growth chamber assays, seedlings of two different sorghum genotypes that produced different levels of sorgoleone and a wheat cultivar were grown. Effects of these three plant genotypes on plant growth and on numbers of soil-associated fungi and bacteria were assessed. Changes in numbers of fluorescent pseudomonads, a group of bacteria that can protect plants against pathogens, also were examined. A subset of these fluorescent pseudomonads was screened for production of four antibiotics that may be involved in plant protection. When wheat seedlings were planted in soils previously planted with sorghum, there were differences between the two different sorghum genotypes regarding numbers of fungi and fluorescent pseudomonads. The types of fluorescent pseudomonads that were associated with soils previously planted with the two sorghum genotypes, also were different. This is the first time differences between two sorghums, with respect to types of soil microorganisms, has been reported. Also, this is the first evidence that when wheat seedlings are grown in soil previously planted with sorghum, the sorghum genotype differentially affected characteristics of soil-associated fluorescent pseudomonads.
Technical Abstract: Aims: The hypotheses that sorghum seedlings [Sorghum bicolor (L.) Moench] varying in sorgoleone production will differentially modify soil microorganisms and will effect subsequent planting of wheat (Triticum aestivum L.) seedlings were tested. Methods: Wheat cultivar Lewjain, and sorghum genotypes Redlan (high sorgoleone) and RTx433 (low sorgoleone), were planted into soils previously planted with wheat or sorghum in a growth chamber. Bacterial, fungal, and fluorescent Pseudomonas spp. numbers (cfu) were determined. Pseudomonads were screened for production of four antibiotics. Pasteurized soils were inoculated with rifampicin-marked strains of Pseudomonas fluorescens then planted with Lewjain, Redlan and RTx433 to assess rhizosphere and soil colonization. Results: Fungal and pseudomonad numbers were significantly affected by plant species, sorghum genotype and previous crop. Soils planted with RTx433 or Lewjain had significantly greater numbers of fungal cfu than soils planted with Redlan. When Lewjain seedlings were grown in soil previously planted with RTx433, there were greater numbers of fungal cfu than when Lewjain was planted into Redlan soil. Wheat planted into wheat soil resulted in significantly fewer numbers of pseudomonads than when planted into sorghum soil. Percentages of hydrogen cyanide-producing pseudomonads increased, especially when wheat seedlings were planted in wheat soil. Percentage of isolates with 2,4-diacetylphloroglucinol production (Phl) declined, except when Redlan was planted in Redlan soil, which resulted in increased Phl isolates. When rifampicin-marked P. fluorescens isolates were applied to soils, sorghum seedlings sustained rhizosphere and soil populations similar to those of wheat. Conclusions: There may be differences between sorghum genotypes, regarding associations with soil microorganisms, suggesting that sorghum genotypes may differentially affect numbers of soil microorganisms in cropping systems.