Effect of Root Exudates on Potassium Dynamics in Wheat and Sugar Beet Rhizosphere.
Kambiz Bazargan, Soil and Water Research Institute, P. O. Box: 14155-6185, Tehran, Iran, Norbert Claassen, Institut fur Agrikulturchemie, Carl-Sprengel-Weg 1, Gottingen, D 37075, Germany, and MJ Malakouti, Tarbiat Modares Univ, Soil and Water Research Institute, PO Box 14115-6185, Tehran, Iran.
Plant species and some cultivars of the same species differ in their ability to grow in low potassium soils. There is a big difference between wheat and sugar beet in potassium needs and uptake efficiency. Mathematical simulation model has been developed to estimate and evaluate nutrient uptake in different soil and plant conditions. Although the model contains different soil and plant parameters, it can not explain some of the differences in potassium uptake efficiency between wheat and sugar beet. The model estimates that the depletion distance near roots of different plants will be the same (the depletion distance is independent from the plant). If the depletion distance (volume) is the same, sugar beet has to have a special ability to release more potassium from the same soil volume than wheat. In this study we were looking for the difference between sugar beet and wheat root exudates under different soil available potassium levels and the effect of these exudates on soil potassium availability. The experiment was established in sand culture and treated with 3 different potassium concentrations in nutrient solution for wheat and sugar beet in 9 replications, under controlled conditions. Root exudates were collected from 54 pots after one and two months of growth by percolating the pots with distilled water for two hours followed by freezing and freeze-drying the collected solution samples. The analysis data showed that the amounts of exudates were different for wheat and sugar beets and that the levels of potassium treatments affected the rates of exudation per cm of the root length. Furthermore, potassium deficiency caused significant increases in the rates of root exudates per cm of root length in wheat and sugar beet. A qualitative analysis of organic acids in the root exudates indicated that potassium levels did not significantly affect the production of such acids by wheat roots, but that sugar beet produced significantly more malonic acid under potassium deficiency. The root exudates collected at three different potassium levels were added at the rates of 0.5, 1, and 2 mg per g of a soil with low available potassium which were then incubated for 6 hours at 22oC at a moisture content equal to the field capacity in a separate experiment to determine the effect of these exudates on soil potassium release into the soil solution. Since different exudates contained different amounts of potassium, the differences were made up by adding potassium chloride solutions to the soil samples along with the exudates. Finally, the analytical data revealed that all the exudates produced by the roots of wheat and sugar beets grown with different potassium treatments released more potassium into the solution than did distilled water alone. It showed that wheat and sugar beet root exudates have a meaningful effect on soil potassium release in soil solution. There was no any meaningful difference between wheat and sugar beet root exudates in this effect, but released potassium into soil solution increased meaningfully by increasing root exudates to soil ratio.