Localization and Capacity of Proton Pumps in Roots of Intact Sunflower Plants

Proton extrusion by roots of intact sunflower plants (Helianthus annuus L.) was studied in nutrient solutions or in agar media with a pH indicator. Proton extrusion was enhanced by either iron deficiency, addition of fusicoccin, or single salt solutions of ammonium or potassium salts. The three types of proton extrusion differ in both localization along the roots and capacity. From their sensitivity to ATPase inhibitors it seems justified to characterize them as proton pumps driven by plasma membrane APTases.Enhanced proton extrusion induced by preferential cation uptake from (NH(4))(2)SO(4) or K(2)SO(4) was uniformly distributed over the whole root system. In contrast, the enhancement effect of fusicoccin was confined to the basal root zones and that of iron deficiency to the apical root zones. Also the rates of proton extrusion per unit of root fresh weight differed remarkably and increased in the order: Fusicoccin << K(2)SO(4) < (NH(4))(2)SO(4) < iron deficiency.Under iron deficiency the average values of proton extrusion for the whole root system are 5.6 micromoles H(+) per gram fresh weight per hour; however, for the apical root zones values of about 28 micromoles H(+) can be calculated. This high capacity is most probably related to the iron deficiency-induced formation of rhizodermal transfer cells in the apical root zones. It can be assumed that the various types of root-induced acidification of the rhizosphere are of considerable ecological importance for the plant-soil relationships in general and for mobilization of mineral nutrients from sparingly soluble sources in particular.