EFFECTS OF THIAMINE (VITAMIN-B1) APPLICATION ON BULK SOIL AND RHIZOSPHERE MICROORGANISMS AND ON ITS RELEASE FROM BEAN (PHASEOLUS-VULGARIS L) SEEDLINGS

In in vitro studies thiamine mononitrate (vitamin B1) supplementation (50-1000 mg I-1) significantly increased growth of total bacteria and fungi isolated from different soils when cultivated in selective growt h media at pH 5.0. At pH 7.0, however, only some of no significant eff ects were observed in total bacteria isolated from the same soils. Gro wth of Actinomycetes was slightly increased by thiamine at pH 5.0, but inhibited at pH 7.0 at high concentrations. In most soils tested, tot al numbers of bacteria, Actinomycetes and fungi (determined as cfu g(- 1) soil) were significantly higher in the rhizosphere of 10 d old bean (Phaseolus vulgaris L.) seedlings after seed treatment with thiamine mononitrate. No significant increases were observed in total Pseudomon as fluorescens isolated from the rhizosphere of vitamin-treated plants . In seed and root exudates of bean seedlings about 12 % of the origin al seed content of thiamine (0.9 mu g seed(-1)) were released after 5 days of germination. Thiamine applied to bean seeds (3 mM in the imbib ition solution) was taken up rapidly and in high amounts. Subsequently , 83 % of this amount was released from the thiamine-treated seedlings during the 12 days of growth. Within the first 5 days of germination, the treated seeds released about thousand times more thiamine into th e rhizosphere compared to untreated seeds. The results suggest that a large proportion of the thiamine applied as seed dressing was released from the developing bean seedlings and has contributed to the thiamin e supply of rhizosphere microorganisms.