Effects of Temperature on Orthophosphate Absorption by Excised Corn Roots ` 0

The uptake of orthophosphate (32P) by excised corn roots, Zea mays L. was studied using roots grown on 0.2 mm CaSiO4. Nine concentrations of KH2PO4 from 1 to 256 /uM were used at temperatures of 200, 300, and, 400. Enzyme kinetic analysis was applied to the data obtained. Two apparent mechanisms (sites) o,f 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 300 was calculated for the mechanism dominating at high P concentrations. Similar calctlations gave a Km of 6.09 X 10-6 anda Vmax of 162 X 10-9 moles per gram of roots per ihour at 300 for the mechanism dominating at low P concentrations. The Qlo for both mechanisms was approximately 2. Calculation of thermodynamic values from the data gave AF of -5200 cal, AH 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 AF of -7300 cal, AH 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.