Rhizosphere Ph And Cation-Anion Balance Determine The Exudation Of Nitrification Inhibitor 3-Epi-Brachialactone Suggesting Release Via Secondary Transport

Biological nitrification inhibition (BNI) of Brachiaria humidicola has been attributed to nitrification-inhibiting fusicoccanes, most prominently 3-epi-brachialactone. However, its release mechanism from B. humidicola roots remains elusive. Two hydroponic experiments were performed to investigate the role of rhizosphere pH and nutritional N form in regulating 3-epi-brachialactone release by B. humidicola and verify the underlying release pathway. Low rhizosphere pH and NH4+ nutrition promoted 3-epi-brachialactone exudation. However, the substitution of NH4+ by K+ revealed that the NH4+ effect was not founded in a direct physiological response to the N form but was related to the cation-anion balance during nutrient uptake. Release of 3-epi-brachialactone correlated with the transmembrane proton gradient Delta pH and NH4+ uptake (R-2 = 0.92 for high similar to 6.8 and R-2 = 0.84 for low similar to 4.2 trap solution pH). This corroborated the release of 3-epi-brachialactone through secondary transport, with the proton motive force (Delta P) defining transport rates across the plasma membrane. It was concluded that 3-epi-brachialactone release cannot be conceptualized as a regulated response to soil pH or NH4+ availability, but merely as the result of associated changes in Delta P.