Scientific registration n o : 275 Symposium n o : 43 Presentation : poster. SADANA Upkar Singh (1), CLAASSEN Norbert (2)

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1 Scientific registration n o : 275 Symposium n o : 43 Presentation : poster Manganese uptake by different crops and its dynamics in the rhizosphere evaluated by a mechanistic model Absorption du manganèse par différentes espèces cultivées et évaluation de sa dynamique dans la rhizosphère par un SADANA Upkar Singh (1), CLAASSEN Norbert (2) (1) Department of Soils, Punjab Agricultural University, Ludhiana , India (2) Insit. f. Agrikulturchemie, Georg-August-Universität, v. Siebold-Str.6, Göttingen, Germany Introduction Nutrient uptake and plant growth is influenced by conditions occurring at the soilroot interface. These conditions vary from site to site and with plant species. Processes and associated theory governing nutrient uptake from soil, however, should be applicable to all soils and plant species. Halstead and Barber (1968) studied Mn uptake by corn seedlings and found a high correlation between Mn uptake and the calculated Mn reaching the roots by diffusion. Several workers have developed nutrient uptake models based on ion transport from the soil to roots by means of mass flow and diffusion and nutrient uptake following Michaelis-Menten kinetics (Barber and Cushman, 1981; Claassen, 1990; Claassen and Barber, 1976; Nye, 1984). These models have been validated for predicting P and K uptake by different crops (Brouder and Cassman, 1994; Chen and Barber, 1990; Claassen et al., 1986; Föhse et al., 1991; Shenk and Barber, 1980; Silverbush and Barber,1983; Van Rees et al., 1990). Little effort has been made to evaluate these models for predicting Mn uptake by crop plants. In the present investigation nutrient uptake model NST 3.0 of Claassen (1990) which also takes into account the contribution of root hairs has been used to evaluate Mn uptake and Mn depletion in the rhizosphere of summer wheat, maize and sugar beet. Materials and methods A pot culture experiment was conducted to determine Mn uptake by summer wheat (Triticum aestivum L. cv. Planet), maize (Zea mays L. cv. Pirat) and sugar beet (Beta vulgaris L. cv. Orbis) and to measure soil and plant parameters for nutrient uptake model calculations. Surface soil (0-30 cm) of Angelberg sandy clay loam soil from Bavaria, Germany having ph 7.5, 33% clay, 31% silt and 3.8% organic carbon was used for the pot experiment. A basal dose of 100 mg N kg -1 soil as Ca(NO 3 ) 2 and 200 mg P kg -1 soil as 1

2 Ca(H 2 PO 4 ) 2.H 2 O was applied. Seeds of the three crops were sown in pots containing 2.9 kg soil in a controlled growth chamber (light/dark regime of 16/8 hours at 25/18 o C, relative humidity 60/75%, light intensity 41 W m -2 PAR). The treatments were replicated 6 times in a completely randomized design. Three replications were used for first and second harvest, respectively. Total number of plants grown for first and second harvest were 10 and 3 of sugar beet, 15 and 4 of wheat and 10 and 3 of maize, respectively. Root and shoot weight, Mn content of plants, root length and root radius were determined 8 (13 days in case of sugar beet) and 20 days after germination. Root radius was measured with a microscope and root length was measured following the method of Newman (1966). Manganese influx was calculated using the formula of Williams (1948). Soil and plant parameters used for nutrient uptake model calculations (Table 1) were determined as described below : I max : I n measured x 50. The factor 50 results from the fact that Mn concentration of plants could be about 50 times higher as measured in the experiment. This also means that the influx could have been 50 times of that measured. Table1: Plant and soil parameters used for nutrient uptake model calculations Plant and soil parameters Crop Wheat Maize Sugar beet I max, 10-5 nmol cm -2 s K m, nmol cm C lmin, nmol cm r 0, 10-2 cm v 0, 10-7 cm s r 1, 10-2 cm k, d RL 0, cm C Li, nmol cm θ, cm 3 cm f, D L, 10-6 cm 2 s b,

3 C Lmin : the minimum soil solution concentration at which net influx equals zero. This value was taken zero in the model calculations because the plants can reduce Mn concentration close to zero. K m : the Michaelis constant, is the difference between concentration at which influx is half the I max and C Lmin. The value of 10 nmol cm -3 was taken from Kochian, C li : the initial Mn concentration of the soil solution. Soil solution was obtained by displacement technique of Adams (1974) and Mn was measured by atomic absorption spectrophotometer. Mean half distance between neighboring roots (r 1 ), water influx (v o ) and the relative root growth rate (k) was calculated as described by Claassen et al. (1990). The value of impedance factor (f) was taken from Kaselowsky (1990). Results and discussion The results pertaining to dry matter yield, Mn content and root length of wheat, maize and sugar beet at first and second harvest are presented in table 2. The dry matter yield of wheat, maize and sugar beet was 31, 71 and 54 mg plant -1 at first harvest and it increased by 5.6, 10.2 and 6.6 times at second harvest, respectively. Manganese content of the shoot varied from 25 mg Kg -1 for sugar beet to 34 mg kg -1 for maize. Amongst the Table 2 Dry matter yield, Mn content and root length of wheat, maize and sugar beet crop Days after germination Dry matter yield Mn content Root length mg plant -1 mg kg -1 cm Wheat Miaze Sugar beet three crops sugar beet had the lowest root/shoot ratio of 55 m g -1 but the highest relative shoot growth rate of 3.1 x 10-6 s -1 which resulted in the highest shoot demand on root (Table 3). This is reflected in the Mn influx which was about 3 times higher for sugar beet than for wheat with lowest shoot demand on root. Recent version of nutrient uptake model NST 3.0 was run using the measured soil and plant parameters. The model calculations, based on ion transport to the root by mass flow and diffusion and ion uptake following Michaelis-Menten kinetics, predicted closely the Mn influx (Table 3). These results were surprising since some parameters, mainly I max and K m were far fetched and their size has a large effect on the uptake of Mn from the soil.

4 I max was derived from the measured influx multiplied by 50, but this factor would have been somewhat higher or lower. The value of K m was taken from Kochian (1991) who reviewed solution culture experiments, where the short term measured influx was often so high that had it lasted longer would have caused Mn toxicity. However, the close agreement between measured and calculated Mn influx suggests that the calculated values of Mn depletion in the rhizosphere are realistic. The results showed that the initial Mn concentration of 0.2 µm in the soil solution decreased to only 0.16 µm Table 3: Root-shoot relations and Mn influx in roots of wheat, maize and sugar beet Crop Root/shoot Shoot Shoot demand Mn influx ratio growth rate on root * Observed Calculated m g s mg s -1 cm nmol cm -1 s -1 Wheat Maize Sugar beet * amount of shoot produced per second per unit of root for wheat, 0.13 µm for maize and 0.11 µm for sugar beet at the root surface after 11 days of uptake. This indicates that Mn transport to the root was not a limiting step i.e. Mn transport could have been higher if the plants had decreased the Mn concentration to a lower value at the root surface. This was confirmed by the fact that an assumed 20 times increase in Mn uptake capacity (I max ) increased the calculated Mn influx from 1.6 x 10-7 to 6 x 10-7 nmol cm -1 s -1. The model calculations, therefore, show that under the soil conditions used the plant determines the amount of Mn absorbed since the supply by the soil could have been much higher. Sensitivity analysis was performed to evaluate the effect of each soil and plant parameter on Mn uptake, considering all parameters to be independent of one another. Simulations of Mn uptake were done by varying each parameter between 0.5 and 2.0 times its measured value. While each parameter was changed, the remaining parameters were held constant at initial values. Results indicated that C Li, r 0, I max and K m were the most sensitive factors controlling Mn uptake. When C Li and r 0 were increased by a factor of 2, Mn uptake increased by a factor of 1.9 and 1.8, respectively. However, increasing I max or decreasing K m by a factor of 2, had a similar effect and Mn uptake increased by by a factor of 1.6. On the other hand buffer power, b, had no effect on calculated Mn uptake and similarly r 1 had no effect because there was no inter root competition. The results suggest that at low Mn concentration in the soil, increasing C Li would be helpful in ameliorating Mn deficiency in low Mn soils. The other possibility is the breeding of varieties having thicker roots but plants would have to spend much more carbon for producing thicker roots. The third possibility would be if roots could decrease Mn concentration at the root surface to a lower value by either increasing I max or decreasing K m, thereby, increasing the concentration gradient and so the transport to the root surface. Further studies should be conducted in this direction.

5 Acknowledgements The first author thanks the Alexander von Humboldlt-Stiftung for the award of a post doctoral fellowship during the tenure of which the present study was conducted. This study was carried out in Freising while the second author was at the Institute of Plant Nutrition, Technical University, Munich. References Adams, F. (1974) : Soil solution. In: E.W. Carson (ed.) The Plant Root and its Environment. University Press of Virginia, Charlottesville, V.A. pp Barber, S.A. and Cushman, J.H. (1981): Nitrogen uptake model for agronomic crops. In: J.K. Ishandar (Ed.) Modeling Waste Water Renovation - Land Treatment. Wiley nter- Science, New York, pp Brouder, S.M. and Cassman, K.G. (1994): Evaluation of a mechanistic model of potassium uptake by cotton in vermiculitic soil. Soil Sci. Soc. Am. J. 58, Chen, J.H. and Barber, S.A. (1990): Soil ph and phosphorus and potassium uptake by maize evaluated with an uptake model. Soil Sci. Soc. Am. J. 54, Claassen, N. (1990): Die Aufnahme von Nährstoffen aus dem Boden durch die höhere Pflanze als Ergebins von Verfügbarkeit und Aneignungsvermögen. Severin-Verlag, Göttingen, Germany. 327 p. Claassen, N. and Barber, S.A. (1976). Simulation model for nutrient uptake from soil by a growing plant root systems. Agro. J. 68, Claassen, N., Syring, K.M. and Jungk, A. (1986): Verification of a mathematical model by simulating potassium uptake from soil. Plant and Soil 95, Föhse, D., Claassen, N. and Jungk, A.(1991): Phosphorous efficiency of plants. II. Significance of root radius, root hairs and cation -anion balance for phosphorous index in seven plant species. Plant and Soil 132, Halstead, E.H. and Barber, S.A. (1968): Manganese uptake attributed to diffusion from soil. Soil Sci. Soc. Am. Proc. 32: Kaselowsky, J. (1990): Wirkung von Lagerungsdichte und Wassergehalt des Bodens auf die Verfügbarkeit von Phosphat und Kalium sowie das Nährstoffaneignungsvermögen von Pflanzen. Diss., Universität Göttingen. Kochian, L.V. (1991): Mechanisms of micronutrient uptake and translocation in plants. In : Morvedt, J.J., Cox, S.R., Shuman, L.M. and Welch, R.M. (Eds.). Micronutrients in Agriculture. 2nd edition. Soil Sci. Soc. Am. Inc. Medison, Wisconsin, U.S.A. Newman, E.J. (1966): A method of estimating the total root length in sample. J. Appl. Ecol. 3, Nye, P.H. (1984): On estimating the uptake of nutrients solubilized near roots or other surfaces. J. Soil Sci. 35, Shenk, M.K. and Barber, S.A. (1980): Potassium and phosphorous uptake by corn genotypes in the field as influenced by root characteristics. Plant and Soil 54, Silberbush, M. and Barber,S.A. (1983): Prediction of phosphorus and potassium uptake by soybeans with a mechanistic mathematical model. Soil Sci. Soc. Am. J. 47, Van Rees, K.C.J., Comerford, N.B. and McFee, W.W. (1990): Modeling potassium uptake by slash pine seedlings from low-potassium-supplying soils. Soil Sci. Soc. Am. J. 54, Williams, R.F.(1948) : The effect of phosphorus supply on the rates of intake of

6 phosphorus and nitrogen upon certain aspects of phosphorus metabolism in gramineous plants. Aust. J. Sci. Res. (B) 1, Keywords : Beta vulgaris, uptake, mechanistic model, Triticum aestivum, Zea mays, manganese Mots clés : Beta vulgaris, absorption, modèle mécaniste, Triticum aestivum, Zea mays, manganèse