Determination of kinetic parameters for potassium uptake by wheat at different growth stages

Root influx parameters (Vmax and Km) of wheat (Triticum aestivum var. HD-2285) were determined at different stages of crop growth, viz. CRIS-Crown Root Initiation Stage, MTS-Maximum Tillering Stage, FLS-Flag Leaf Stage and DFS-Dough Formation Stage. Wheat was grown in sand medium with Hoagland nutrient solution (all nutrients except potassium). Plants were taken out from sand culture at 22, 41, 69, and 87 days after germination (DAG) and placed in a specially designed assembly of flowing solution in laboratory and greenhouse. The nutrient solution allowed to flow into the culture vessel in a regulated manner by means of a separating funnel. Fresh Hoagland solutions having five different concentration of K+ (0.051, 0.154, 0.256, 0.333, and 0.410 mM) were added drop-wise on the funnels at the rate of about 35 μL s-1 and continuously supplied in five different assemblies. Root influx parameters (Vmax and Km) were determined employing a simple solution culture technique, by measuring the depletion of potassium (input-output concentration) by wheat roots in five nutrient media differing in potassium concentration only. Vmax and Km were calculated with the help of coefficients obtained by fitting the solution K+ concentration and K+ uptake rate data on the Michaelis-Menten equation. The Vmax and Km values were decreased with the age of wheat crop. The Vmax was 48.7 nmol m-2 s-1 at 22 DAG and decreased to 19.4 nmol m-2 s-1 at 41 DAG, 5.90 nmol m-2 s-1 at 69 DAG and finally decreased to 4.21 nmo1m-2 s-1 at 87 days of crop growth; corresponding Km values were 0.299, 0.254, 0.176 and 0.146 mM, respectively.