The phytohormonal interactions betweenAzospirillum and wheat

A simple model system was designed to detect positive effects ofAzospirillum on the root growth of cereals. Cultures ofA. brasilense Sp7 andA. lipoferum Sp59 did not excrete gibberellins and cytokinins in the logarithmic and in the early stationary growth phase. Indoleacetic acid (IAA) was formed, however, only in the stationary phase of the cultures. The addition of D,L-tryptophan to the medium enhanced the formation of IAA. A further, still unidentified substance was produced byAzospirillum under denitrifying conditions in the logarithmic growth phase. The substance was almost twice as active as IAA in increasing the wet weight of wheat root segments. It is suggested that this unidentified substance is the major stimulus affecting the growth of cereals.Dedicated to Professor E.-G. Niemann, Hannover, on the occasion of his 60th birthday.