Kinetics of phosphorus absorption by mycorrhizal and nonmycorrhizal tomato roots

Kinetics of P absorption were investigated in mycorrhizal (Glomus fasciculatus) and nonmycorrhizal tomato (Lycopersicon esculentum) roots to determine why increased ion absorption by mycorrhizae occurs. Initial rates of absorption of ³²P were measured at 1 to 100 micromolar KH₂PO₄ (pH 4.6). Absorption rates of mycorrhizae were about twice those of control roots. Augustinsson-Hofstee analysis yielded two linear phases; V_max and K_m were calculated for each phase. In the low phase (1 to 20 micromolar), V_max values for the mycorrhizal and nonmycorrhizal roots were each 0.10 micromoles P per gram fresh weight per hour while K_m values were 1.6 and 3.9 micromolar KH₂PO₄, respectively. For the high phase (30 to 100 micromolar), V_max values for mycorrhizal and nonmycorrhizal roots were 0.32 and 0.25 micromoles P per gram fresh weight per hour and K_m 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 affinity of the absorbing sites for H₂PO₄⁻ (lower K_m).