Manganese dynamics in the rhizosphere and Mn uptake by different crops evaluated by a mechanistic model

Understanding of the mechanisms of Mn supply from the soil and uptake by th e plants can be improved by using simulation models that are based on basic principles. For this, a pot culture experiment was conducted with a sandy clay loam soil to measure Mn uptake by summer wheat (Triticum aestivum L. c v. Planet), maize (Zea mays L. cv. Pirat) and sugar beet (Beta vulgaris L. cv. Orbis) and to simulate Mn dynamics in the rhizosphere by means of a mec hanistic model. Seeds of three crops were sown in pots containing 2.9 kg so il in a controlled growth chamber. Root and shoot weight, Mn content of pla nts, root length and root radius were determined 8 (13 days in case of suga r beet) and 20 days after germination. Soil and plant parameters were deter mined to run nutrient uptake model calculations. Manganese content of the s hoot varied from 25 mg kg(-1) for sugar beet to 34 mg kg(-1) for maize. Sug ar beet had the lowest root length/shoot weight ratio but the highest relat ive shoot growth rate, resulting in the highest shoot demand on the root. T his is reflected by the Mn influx which was 0.9 x 10(-7), 1.7 x 10(-7) and 2.5 x 10(-7) nmol cm(-1) s(-1) for wheat, maize and sugar beet, respectivel y. Nutrient uptake model calculations predicted similar influx values. Init ial Mn concentration of 0.2 mu M in the soil solution decreased to only 0.1 6 mu M for wheat, 0.13 mu M for maize and 0.11 mu M for sugar beet at the r oot surface. This shows that manganese transport to the root was not a limi ting step. This was confirmed by the fact that an assumed 20 times increase in maximum influx (I-max) increased the calculated Mn influx by 3.7 times. Sensitivity analysis demonstrated that for controlling Mn uptake the initi al soil solution concentration (C-Li), the root radius (r(0)), I-max and th e Michaelis constant (K-m) were the most sensitive factors in the listed or der.