Kinetics of P absorption were investigated in mycorrhizal (Gkmws fascadana) and nonmycorrhizal tomato (Lycopersicon escude_uu) roots to determine why increased ion absorption by mycorrhizae occurs. Initial rates of absorption of "2P were measured at 1 to 100 micromolar KH2PO4 (pH 4.6). Absorption rates of mycorrhizae were about twice those of control roots. Augustnsson-Hofstee analysis yielded two linear phases; V.. and K,. were calculated for each phase. In the low phase (I to 20 micromolar), V..,,S values for the mycorrhizal and nonmycorhizal roots were each 0.10 micromoles P per gram fresh weight per hour while K. values were 1.6 and 3.9 micromolar KH2PO4, respectively. For the high phase (30 to 100 micromolar), V,. vahes for mycorrhizal and nonmycorrhizal roots were 0.32 and 0.25 micromoles P per gram fresh weight per hour and K. values were 35 and 42 micromolar, respectively. These results indicate that at the lower phase concentrations, similar to those expected in most soil solutions, a major factor contributing to the increased uptake was an apparent greater affinty of the absorbing sites for H2PO4-(lower K.).Mycorrhizal plants commonly accumulate more P than do nonmycorrhizal plants, especially when P availability is limited (5,9). Increased absorption is usually attributed to increased surface area and increased soil exploration by the root-fungus association (7,17,19). To our knowledge, the kinetics of ion uptake by mycorrhizae has not been investigated. Information of this type is needed to determine whether enhanced absorption is in fact due solely to increased number of absorbing sites contributed by the fungus, or possibly to greater ion affinity of the fungal absorption sites, or a combination of both factors. Such a kinetic approach to examination of the absorption of P by mycorrhizae is reported here. Tomatoes, known to exhibit a mycorrhizal response in ion uptake (3,18), were used in the investigation. MATERIALS AND METHODS

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Subcellular particles capable of oxidizing various members of the tricarboxylic aci(l cvcle have been isolated from a nunmber of plant sources, as evidenced by the numerous references in various reviews (6. 8. 9, 11

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