Potassium efficiency mechanisms of wheat, barley, and sugar beet grown on a K fixing soil under controlled conditions

Plant species differ in their potassium (K) efficiency, but the mechanisms are not clearly documented and understood. Therefore, K efficiency of spring wheat, spring barley, and sugar beet was studied under controlled conditions on a K fixing sandy clay loam. The effect of four K concentrations in soil ranging from low (5 and 20 muM K) to high (2.65 and 10 mM. K) on plant growth and K uptake was investigated at 3 harvest dates (14, 21, and 31 days: after sowing). The following parameters were determined: shoot dry matter (DM), K concentration in shoot dry matter, root length (RL), root length/shoot weight ratio (RSR), shoot growth rate/average root length ratio (GR(S/)aRL), K influx, and soil solution K concentrations. When proved to have a higher agronomic K efficiency than barley and sugar beet indicated by a greater relative yield under deficient conditions. As compared to both cereals, sugar beet was characterized by higher K concentrations in the shoot dry matter, only 30-50 % of the root length, 15-30 % of the RSR and a 3 to 6 times higher GR(S)/aRL. This means that the shoot of sugar beet had a 3 to 6 times higher K demand per unit root length. Even at low K concentrations in the soil solution, sugar beet had a 7 to 10 times higher K influx than the cereals, indicating that sugar beet was more effective in removing low available soil K. Wheat and barley, were characterized by slow shoot growth, low internal K requirement, i.e. high K utilization efficiency, and high RSR, resulting in a low K demand per unit root length. At low soil K concentrations, both cereals increased K influx with an indication of adaptation to K deficiency. The mechanism of this adaptation merits closer investigation. Model calculations were performed to estimate the K concentration difference between the bulk soil and the root surface (DeltaC(L)) needed to drive the measured K influx. For the two cereals, the calculated DeltaC(L) was smaller than the K concentration in the soil solution, but for sugar beet, Delta(CL) was up to seven times higher. This indicates that sugar beet was able to mobilize K in the rhizosphere, but the mechanisms responsible for this mobilization remain to be, studied.