Fluorescent pseudomonad populations in modified rhizosphere atmospheres

Populations of four isolates of fluorescent pseudomonads in the rhizosphere of tomato (Lycopersicon esculentum cv. Earlypak) and cucumber (Cucumis sativus cv. Straight 8) seedlings were measured 48 h after exposure of roots to four soil atmospheres containing O2-to-CO2 concentrations of 210-to-0.3 (210 ml 1−1 O2, 0.3 ml 1−1 CO2, ambient concentrations), 180-to-30, 150-to-60 or 120-to-90. While populations varied depending on the bacterial isolate, the plant genus and the composition of the soil atmosphere, they were generally greater under modified soil atmospheres (atmospheres containing lower-than-ambient concentrations of O2 and higher-than-ambient concentrations of CO2) than under a soil atmosphere containing the ambient concentration of O2 and CO2. The populations of three of four test isolates in tomato rhizosphere and of two of three isolates in cucumber rhizosphere were between 0.44 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 significantly (P < 0.05) greater under modified soil atmospheres than under the ambient atmosphere. These results suggest that the reported inconsistency in biocontrol activity of several fluorescent pseudomonad isolates may in part be due to the effect of soil atmospheric composition on the ability of these bacteria to establish in the rhizosphere.