Allelochemicals Biologica... - Name

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ROBERT J. KREMER
seeds and seedlings with AB can be expressed graphically (Figure 1) to emphasize
the following key areas for consideration: a) biology and ecology of weeds; b) growth
of the crop within the established cultivation system; c) characterization of AB,
including those that are selected and introduced as biological control agents and those
that are indigenous to the soil environment; and d) the wide range of cultural practices
available for implementation of biological control within the agroecosystems.
After recognizing the need for consideration of ecological interactions, a program
for development of selected AB or management of the indigenous bacterial
population can be undertaken
3. CHARACTERIZATION OF ALLELOPATHIC BACTERIA
Kremer et al. (1990) identified colonizing ability, chemotactic response, and mode of
action to be vital characteristics for the successful development of rhizobacteria as
weed biocontrol agents. Bacteria that can rapidly colonize the root will likely be a
successful biocontrol agent. Migration towards the seed or root is the first step in
colonization, illustrated by movement of rhizobacterial isolates through 2-cm of soil
towards velvetleaf seeds (Begonia and Kremer, 1994). As the seedling develops,
movement of bacteria along roots and within the rhizosphere is influenced by root
binding sites, amounts of organic material present, type of root (i.e., seminal vs.
nodal roots), water movement through soil and along roots, and soil texture (Bolton
et al., 1993). Compared to other bacterial groups present in non-rhizosphere soil,
gram-negative bacteria readily colonize the rhizosphere, partly due to their metabolic
diversity (Nehl et al., 1997). Pseudomonads are particularly adapted for rhizosphere
colonization because of the ability to utilize diverse carbon sources present in root
exudates. Observations with scanning electron microscopy reveal the intimate
relationship of rhizoplane colonization by selected AB (Figure 2).
Figure 2. Root surface of a two-week old velvetleaf root colonized by Pseudomonas
fluorescens strain 239 cells (arrow) aligned in intercellular spaces of root epidermal cells.
Magnification is X 6,000. Scanning electron micrograph from Begonia et al. (1990).