Active Extrusion ofProtons into Deionized WaterbyRoots of Intact MaizePlants

Theinvestiptions werefocussed onthequestion astowhether roots ofintact maize plants (ZeamaysL.cvBlizzard) release protons into deionized H20.Plants inthesixtoseven leaf stage depressed thepHof deionized H20from6toabout 4.8during anexperimental period of4 hours. Onlyone-third oftheprotons released could beascribed tothe solvation ofCO2inH20.Themaincounter anions released wereCl-, NO3-, andS042-. Atlowtemperature (2°C), theH'release wasvirtually blocked while arelatively high amountofK'wasreleased. Thepresence ofK+,Nat, Ca2+, andMg2+intheexternal solution increased theH+ secretion significantly. Addition ofvanadate totheouter medium inhibited theH+release while fusicoccin hadastimulating effect. Substituting the nutrient solution ofdeionized H20resulted inasubstantial increase of themembrane potential difference from-120to-190millivolts. The experimental results support theconclusion that theH+release byroots ofintact maize plants isanactive process driven byaplasmalemmalocated ATPase. Since thenetH release wasnotassociated withanet uptake ofK+,itisunlikely tooriginate fromaK+/H+ antiport. species. Suchacapability wouldenable roots tomobiize plant nutrients fromasoil initially free ofsoluble nutrients. Protons extruded byaplasmalemma-located H+pumpmaybeadsorbed inthefree space byindiffusible anions where theymayreplace other cations. Therefore, arelease ofH+fromroots into the outer mediumimplies that thequantity ofH+extruded ishigh enough toovercome theH+buffer poweroftheapoplast. Toourknowledge, ithasnotyetbeenproven that changes in thepHofnutrient solutions withwholeplants arecaused by ATPase activity. Alsothequestion astowhether plant roots can secrete H+into deionized H20hasnotbeeninvestigated except forworkbyMengel andMalissiovas (18) whofound that roots ofintact vine plants werecapable ofreleasing H+into deionized H20.TheH+secretion washigher inlight thanatdark. The authors suggested that this H+release wasbrought about bya plasmalemma-located ATPase. Direct evidence forthis assumption, however, wasnotprovided. Inthefollowing, experimental results arepresented showing that roots ofentire maize plants areable tosecrete H+into deionized H20without aconcomitant netuptake ofcations. Evidence isprovided that this H+extrusion isassociated with an ATPase.