Effects of temperature on orthophosphate absorption by excised corn roots.
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.