Soil humic substances stimulate proton release by intact oat seedling roots

The effect of a soil humic fraction (HS) on proton extrusion into deionized water by intact oat seedling roots was studied. In the presence of HS, at concentration of 10 mu g organic carbon (C) mL(-1), a clear stimulation of acidification of the outer medium by the roots was observed after three to four hours of incubation. The addition of 0.5 mM vanadate to the solution bathing the roots drastically reduced the net proton extrusion, either in the presence or absence of HS, suggesting the involvement of the plasma membrane H+-ATPase in the stimulation of the acidification of the outer medium by oat roots. The release of potassium (K) from the roots into deionized water was also monitored concomitantly to the proton extrusion. The loss of endogenous K from the roots was similar in the presence or absence of HS, while the recovery of the cation was slower in the presence of the humic fraction. However, after reabsorption of the released K, no net acidification was observed in control roots, while HS-treated roots significantly decreased the pH of the deionized water. The addition of 3 mM K to the external medium greatly enhanced the proton release from roots, while the presence of humic substances reduced the magnitude of the stimulation by K. When K was supplied at a concentration closer to that encountered in the soil (<0.1 mM), MS significantly stimulated proton release. The effect of HS on root extracellular acidification supports the idea of a role of soil humic substances on plant nutrition via interaction with cell membrane functions.