Effects of Temperature on Orthophosphate Absorption by Excised Corn Roots

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Abstract
The uptake of orthophosphate (32p) by excised corn roots, Zea mays L. was studied using roots grown on 0.2 m[image] CaSO4. Nine concentrations of KH2PO4 from 1 to 256 [mu][image] were used at temperatures of 20[degree], 30[degree], and 40 . 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 177x 10-9 moles per gram of roots per hr. at 30[degree] was calculated for the mechanism dominating at high P concentrations. Similar calculations gave a Km of 6.09 x 10-6 and a Vman of 162 x 10-9 moles per gram of roots per hour at 30[degree] for the mechanism dominating at low P concentrations. The Q10 for both mechanisms was approximately 2. Calculation of thermodynamic values from the data gave [DELTA]F of [long dash]5200 cal, [DELTA]H 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 [DELTA]F of [long dash]7300 cal, [DELTA] H of [long dash]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.