PHOSPHORUS INFLUX OF SUGAR BEET, MAIZE AND GROUNDNUT IN A LOW P OXISOL Reza Khorassani, Bernd Steingrobe and Norbert Claassen Department of Crop Sciences - plant nutrition of Georg-August-University of Goettingen Carl-Sprengel Weg 1, 37075 Göttingen, rkhoras@gwdg.de, Plant species differ in P efficiency, i. e., they differ in their ability to grow under low P condition. For example, maize was more efficient than groundnut at low P (Bhadoria et al., 2004). An important parameter determining the P efficiency is the P influx (Bhadoria et al., 2002). At limited P supply, P transport in soil to the root surface is mainly by diffusion, and is therefore dependent on the P concentration gradient (dCL/dr ) in soil solution. Theoretically, plants can increase the concentration gradient either by (a) stronger decrease of the concentration at the root surface, (b) increase of the soil solution concentration or by both of them. This study was carried out to look for differences in P influx of maize, sugar beet and groundnut, when grown on a soil with low P supply and to investigate how these differences came about. Sugar beet, maize and groundnut were grown in a growth chamber on a fossil Oxisol containing P mainly bound to Fe and Al. Three P levels of 30, 100 and 1000 mg kg-1 soil were adjusted by applying Ca(H2PO4)2.H2O. Three harvests were performed at 20, 35 and 50 days after sowing. At the lowest P level, P influx of sugar beet was 4.510-14 mol cm-1 s-1, which was around 4 and 45 time higher than for maize and groundnut, respectively. Also, sugar beet was more efficient in taking up P than two other species in all treatments. There was a large increase in P uptake at second harvest for sugar beet. The P influx was reduced in later stage of growth for all three plants. Buhse (1992) obtained the Michaelis-Menten parameters for sugar beet grown in nutrient solution and found a minimum concentration (CLmin) of 0.15 µmol L-1 at 25oC. Furthermore, from his data it can be deduced that a concentration of 1 µmol L-1 at the root surface would be necessary to secure the above mentioned influx of 4.510-14 mol cm-1 s-1. However, measured P concentration in soil solution at low P supply was much less and with maximum 0.34 µmol L-1 close to CLmin. Hence, sugar beet could not increase the P influx in soil by lowering the concentration at the root surface, but only by increasing P concentration in the rhizosphere. In sugar beet, there is no arbuscular mycorrhizae in root to increase surface area to take up P and increase P solubility in soil. Therefore, probably chemical mobilization by root exudates was the most important factor to explain the high P influx in sugar beet. This is a reason to choose sugar beet for future studies on the effect of root exudates on P solubility in soil. References: 1- Bhadoria P. S., Steingrobe B., Claassen N. 2002. Phosphorus efficiency of wheat and sugar beet seedlings grown in soils with mainly calcium, or iron and aluminium phosphate. Plant and Soil 246, 41-52. 2- Bhadoria P. S., El Dessougi H., Liebersbach H., Claassen N. 2004. Phosphorus uptake kinetics, size of root system and growth of maize and groundnut in solution culture. Plant and Soil 262, 327-336. 3- Buhse J. 1992. Wirkung der Wurzelraumtemperatur auf das Phosphataneignungsvermögen von Pflanzen und die Phosphatverfügbarkeit im Boden. Dissertation der Georg-August-Universität zu Göttingen.

Keywords