Modulation of NO3- uptake by water-extractable humic substances: involvement of root plasma membrane H(+)ATPase

The effect of a water extractable humic substances fraction (WEHS) on nitra te uptake and plasma membrane (pm) H+-ATPase activity of maize roots was in vestigated. Four days old maize root seedlings were exposed for 4 to 24 h t o a nutrient solution containing 200 mu M nitrate in the absence or presenc e of 5 mg org. C.L-1 WEHS. Plants exposed to nitrate developed a higher cap acity to absorb the anion (induction): the net uptake rate progressively in creased up to 12 h of contact with the solution; thereafter, a decline was observed. When WEHS was present together with nitrate in the nutrient solut ion, the induction of nitrate uptake was evident and maximal already 4 h af ter starting the treatment. The rate of net nitrate uptake decreased only s lightly during the remaining period (4-24 h). Stimulation of net nitrate up take rate was also observed when WEHS was added to a nitrogen- or nitrate-f ree nutrient solution or to a 5 mM CaSO4 solution. The activity of pmH(+)-A TPase raised upon exposure of the roots to nitrate with the same pattern ob served for nitrate uptake. The contemporary presence of nitrate and WEHS ca used a further stimulation of the pmH(+)-ATPase activity after 4 h treatmen t. An increase in the enzyme activity was also observed when plants were tr eated for 4 h in the presence of WEHS in CaSO4, nitrogen- or nitrate-free s olutions. However, when nitrate was present the enhancement was even greate r. Results support the idea that the plasma membrane proton pump might be o ne of the primary targets of the action of humic substances on plant nutrie nt acquisition. A role of WEHS in the modulation of nitrate uptake via an i nteraction with the pm H+-ATPase is also discussed.