Methods of Studying Plant Growth Stimulating Pseudomonads: Problems and Progress

Abstract

The stimulation of plant growth by fluorescent pseudomonads has recently been reviewed (Schippers et al., 1985; Schippers et al., in press). Field- and pot experiments in the Netherlands strongly suggest that potato growth stimulation by seed tuber bacterization with pseudomonads selected for efficient siderophore mediated Fe³⁺-uptake, is most noticable in soils where frequent potato cropping causes a significant reduction in potato yield (Geels and Schippers, 1983b; Geels and Schippers, 1983c; Schippers et al., 1985). Such yield reductions were more severe with increasing potato cropping frequency in long term rotational experiments (Hoekstra, 1981; Lamers, 1981; Schippers et al., 1985). They are largely caused by as yet unknown harmful rhizosphere microorganisms (HMO) rather than by well-known soil-borne pathogens (Schölte, et al.,1985; Schippers et al., 1985). The yield reductions were reproduced in pot experiments which could be almost eliminated if potato tubers were treated with selected pseudomonads before planting (Geels and Schippers, 1983a; Geels and Schippers, 1983b). In pot experiments significant stimulation of potato growth by bacterization of tubers was only observed in soil from fields frequently cropped with potato and not in soil from fields with no recent history of potato cropping. Similarly, in field experiments yield increases by tuber bacterization were only obtained in soils frequently cropped with potato. In some experiments yield increases were detected only during the first three months following planting and differences between treated tubers and controls were not significant at harvest-time. However, the rifampicin resistant Pseudomonas isolates used could be reisolated from roots of treated plants until harvest although their numbers gradually declined during the season (Geels et al., submitted for publication). Failure to increase yield in the field is one of the major problems. This is possibly due to inadequate colonization by PGSP of root parts that are responsible for tuber development later in the season. Failure of HMO to develop their harmful activity could be another factor, because yield reductions in short rotations were not apparent to the same extent every season and in every field plot.