As a rule, ion uptake by plant cells and roots has features of saturation kinetics. This is in accordance with the assumption of control, as for example by the number of binding sites of ions (carriers, permeases), or the capacity of the proton efflux pumps, in the plasma membrane and tonoplast. Solute transport across membranes is carrier mediated transport. Protein macromolecules integrated into the membrane matrix seems to be the carriers. The carrier-mediated process is subject to kinetics assumig that the number of carriers (binding sites) in the membranes is limited. Kinetics of ion transport through a membrane is considered equivalent to relationship between an enzyme and its substrate. To obtain plants of different P status, two genetically diverse Brassica cultivars (P-tolerant 'Con-1' and P-sensitive 'Gold Rush') were grown for several weeks in nutrient solution culture media. P-uptake kinetics of the roots with intact plants in short-term experiments by monitoring P depletion in culture media revealed that P-tolerant 'Con-1' cultivar had favorable characteristics for P-uptake because of high I(max) or V(max) and low K(m) or 1/2 I(max) value than P-sensitive 'Gold Rush' cultivar. By plotting relative growth rate (RGR) and internal P-concentration (PNC) among P-tolerant (group I; Brown Raya, Con-1, Rainbow, Dunkled and Peela Raya) and P-sensitive (group II; Toria, Sultan Raya, B.S.A, Toria Selection and Gold Rush) cultivars revealed that group I cultivars showed large metabolic fraction and small structural fraction than group II cultivars which provided basis for P-stress tolerance.
Membrane binding sites
I(max) or V(max), K(m) Pi-uptake rate