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 (32)P were measured at 1 to 100 micromolar KH(2)PO(4) (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 KH(2)PO(4), 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 H(2)PO(4) (-) (lower K(m)).