title: Effects of temperature on orthophosphate absorption by excised corn roots.
abstract: The uptake of orthophosphate ((32)P) by excised corn roots, Zea mays L. was studied using roots grown on 0.2 mm CaSO(4). Nine concentrations of KH(2)PO(4) from 1 to 256 mum were used at temperatures of 20 degrees , 30 degrees , and 40 degrees . Enzyme kinetic analysis was applied to the data obtained. Two apparent mechanisms (sites) of phosphate uptake were observed, 1 dominating at high P concentrations and 1 at low P concentrations. A Km of 1.36 x 10(-4) and a Vmax of 177 x 10(-9) moles per gram of roots per hour at 30 degrees was calculated for the mechanism dominating at high P concentrations. Similar calculations gave a Km of 6.09 x 10(-6) and a Vmax of 162 x 10(-9) moles per gram of roots per hour at 30 degrees for the mechanism dominating at low P concentrations. The Q(10) for both mechanisms was approximately 2. Calculation of thermodynamic values from the data gave DeltaF of - 5200 cal, DeltaH of - 950 to - 1400 cal, and a enthalpy of activation (A) of 10,300 to 13,800 cal per mole for the mechanism dominating at high P concentrations. Similar calculations from the data for the mechanism dominating at low P concentrations gave a DeltaF of - 7300 cal, DeltaH of - 10,700 to - 8200 cal, and a A of 9300 to 18,900 cal per mole. If the dual mechanism interpretation of this kind of data adequately describes this system, then both mechanisms of P absorption by corn roots involve chemical reactions.
authors.affiliation: Department of Agronomy, Cornell University, Ithaca, New York 14850.
authors.forename: O G
authors.initials: OG
authors.lastname: Carter
authors.forename: D J
authors.initials: DJ
authors.lastname: Lathwell