2-[micro]m membrane filter) L-TRP was also added at 1 g/kg soil.
Of the rhizobacterial isolates, 83% (50 of 60) collected from the rhizosphere of various plants produced auxins in the absence of L-TRP, whereas 100% produced auxin in the presence of L-TRP but efficiency varied greatly (data not shown).
Application of L-TRP stimulated auxin biosynthesis several-fold in both rhizosphere and non-rhizosphere soils.
Auxin biosynthesis was stimulated by L-TRP application.
As observed in other soils, L-TRP application to cotton rhizosphere and non-rhizosphere soils stimulated auxin biosynthesis several-fold.
HPLC demonstrated the presence of several auxin compounds (IAA intermediates) derived from L-TRP, and IAA and indole-3-acetamide (IAM) were identified as the major L-TRP-derived microbial products both in broth medium and in soil.
In vitro studies on auxin biosynthesis demonstrated that a majority of the rhizobacteria was capable of producing auxins in the broth medium; however, different rhizobacteria varied greatly in their auxin-producing ability, and amendment of the culture with L-TRP had a strong stimulatory effect on auxin biosynthesis.
This study also revealed that inocula were less effective in promoting auxin production in rhizosphere soil than in bulk (non-rhizosphere) soils in both the presence and absence of L-TRP.
Addition of L-TRP to different soils inoculated with various rhizobacteria also effectively stimulated auxin (IAA-equivalents) production relative to unamended (no TRP) soil inoculated with the same rhizobacteria, most likely due to L-TRP serving as an auxin precursor.