SUCCINIC AND LACTIC ACIDS AS PLANT-GROWTH PROMOTING COMPOUNDS PRODUCED BY RHIZOSPHERIC PSEUDOMONAS-PUTIDA

In greenhouse trials, root growth of Asparagus officinalis L. increased up to 30% when roots of 3-week-old seedlings were dipped in the culture filtrate of Pseudomonas putida RSA9, a strain isolated from rhizosphere soil of asparagus and antagonistic to the crown rot pathogen Fusarium moniliforme. The culture filtrate was extracted with ethyl acetate at pH 3, and the extracts were fractionated on a column of octadesylsilica gel. The active fraction was found to be a 45:55 mixture of succinic and lactic acids. Root mass increased 40% when the roots of the seedlings were treated with a 1:1 mixture of the acids at 10 ppm. The results provide an explanation for the plant growth promoting effects of some rhizobacteria; the bacteria may secrete organic acids, such as succinic and lactic acids, and these acids may increase plant growth under conditions in which the populations of pathogens are reduced.