FLUORESCENT PSEUDOMONAD POPULATIONS IN MODIFIED RHIZOSPHERE ATMOSPHERES

Populations of four isolates of fluorescent pseudomonads in the rhizos phere of tomato (Lycopersicon esculentum cv. Earlypak) and cucumber (C ucumis sativus cv. Straight 8) seedlings were measured 48 h after expo sure of roots to four soil atmospheres containing O-2-to-CO2 concentra tions of 210-to-0.3 (210 ml l(-1) O-2, 0.3 ml l(-1) CO2, ambient conce ntrations), 180-to-30, 150-to-60 or 120-to-90. While populations varie d depending on the bacterial isolate, the plant genus and the composit ion of the soil atmosphere, they were generally greater under modified soil atmospheres (atmospheres containing lower-than-ambient concentra tions of O-2 and higher-than-ambient concentrations of CO2) than under a soil atmosphere containing the ambient concentration of O-2 and CO2 . The populations of three of four test isolates in tomato rhizosphere and of two of three isolates in cucumber rhizosphere were between 0.4 4 and 1.38-fold higher under test modified soil atmospheres than under the ambient atmosphere. The differences were significant (P < 0.05). Populations of indigenous fluorescent pseudomonads were also significa ntly (P < 0.05) greater under modified soil atmospheres than under the ambient atmosphere. These results suggest that the reported inconsist ency in biocontrol activity of several fluorescent pseudomonad isolate s may in part be due to the effect of soil atmospheric composition on the ability of these bacteria to establish in the rhizosphere. Copyrig ht (C) 1996 Elsevier Science Ltd