DINITROGEN FIXATION BY THE VIETNAMESE SOY BEAN (GLYCINE-MAX) CV DH4 INOCULATED BY BRADYRHIZOBIUM-JAPONICUM USDA-138 (G3) IS ENHANCED UPON TRANSFER TO ACID-MEDIUM

Four symbiotic associations obtained by inoculation of soybean (Glycin e max L) cv DH4 (from Vietnam) and 171-16, with Rhizobium SMX11, a fas t-growing strain from the Mekong valley and G3 (Bradyrhizobium japonic um USDA 138), a slow-growing strain, were grown on N-free solution at pH 6.3. Acid treatments were applied by transfer of 30-d-old plants wi th well-developed nodules onto solutions at pH 5.3 and 4.3. The effect of pH was studied after 2 wk by measuring biomass (plant dry weight) and N2 fixation (organic N per plant). Three different responses to ac id pH were observed with no apparent connection with individual respon ses to acid pH of plants and rhizobia when grown asymbiotically: SMX11 symbiosis was resistant to acid pH whatever the host soybean, while G 3/171-16 symbiosis was sensitive to acid pH, and G3/DH4 preferred pH 4 .3. The non-inoculated soybeans 171-16 and DH4 cultured on NO3- grew r espectively faster and more slowly at pH 4.3 than at pH 6.3. Maximum g rowth rate of the free rhizobia was at pH 5.3 and 6.3 for G3 and SMX11 respectively. Thus, the effects of growth pH have no relationship wit h the rhizobia or the plant genotype considered alone. They likely dep end on the genetic control developed by the symbiotic conditions. When inoculated in soybean DH4 and grown on acid medium, G3 finds the most favorable cellular conditions for the expression of its N2 fixation c apacity. The G3/DH4 symbiotic association not only tolerates but prefe rs growing on acid medium. In conclusion it appears advisable to test the effect of inoculation of the vietnamese soybean DH4 with the strai n G3 in the field, especially in the Mekong delta where soils are know n to be very acid.