Potassium uptake efficiency mechanisms and root exudates of different crop species

Abstract

Listen

Translate article

Potassium uptake efficiency is the ability of plants to take up sufficient K under low soil K availability. Plant species differ in their K uptake efficiency. This study was done with the objective to investigate the possible mechanisms responsible for the differences in K uptake efficiency of crop species. Potassium uptake efficiency and K dynamics in the rhizosphere of maize, wheat and sugar beet were evaluated by a pot experiment which was conducted on K deficient soil with and without K fertilization. Sugar beet and wheat can take sufficient K under low soil K supply and therefore are uptake efficient for K. High K uptake efficiency in wheat was mainly due to its large root system. Sugar beet has few roots, but it could acquire more K per unit shoot dry weight, because of higher K influx. The nutrient uptake model (NST 3.0) could satisfactorily predict K influx in all the crops under high K supply, however under low K supply; the model prediction was 0.64, 0.68 and 0.31 times the measured K influx for maize, wheat and sugar beet, respectively. The severe under prediction in case of sugar beet indicated that processes not considered in the model were important for the high K uptake efficiency. Results of sensitivity analysis showed that initial soil solution K concentration (CLi) is the most important parameter responsible for the differences in the measured and calculated K influx of wheat, maize and sugar beet. However, the mechanisms responsible for increasing CLi in rhizosphere of different plant species is not clear yet, whether it is due to the capacity of plant root to release some organic compounds, which can solubilize K from the non-exchangeable fraction of soil or it is due to the indirect effect of higher Imax (maximum K uptake capacity) of the root and/or root hairs. To study the root exudation pattern, wheat and sugar beet plants were grown in quartz sand supplied with modified Hoagland nutrient solution of low and high K levels at two growing conditions, one in screen house under natural environmental conditions and another in growth chamber under control conditions. Root exudates were collected by percolation method. Root exudation rate was many-fold higher under low K compared to high K supply in both the crops and was higher in young plants and at natural sun light, perhaps due to higher light intensity in the screen house. HPLC analysis of the root exudates showed that exudation rate of organic acids, amino acids and sugars was higher under low K supply in both the crops and it was higher in wheat compared to sugar beet. Arginine was the amino acid detected only in root exudates of sugar beet. The results of mobilization of K in a K fixing soil by amino acids, as found in root exudates showed that total K desorbed by Arginine was the highest. Arginine might work like long chain n-alkyl ammonium compound, which could widen the interlayer of clay mineral resulting in a higher soil solution K concentration. Though amino acids can desorb K in K fixing soil, but degree of desorption does not seem to be sufficient to explain the differences in soil solution K concentration in the rhizosphere of wheat and sugar beet grown on low K soil. Non-targeted metabolite profiling was done by separating the root exudates collected from plants grown in the growth chamber by HPLC coupled with ESI-MS. Several signals and change in intensity of certain signals specific for root exudates from K deficient plants were found. Signal corresponding to m/z value 475 was relatively stronger under low K supplied sugar beet. From KEGG data base, one of the possible structures for m/z 475 was Amastatin (C21H38N4O8), which resembles to n-alkyl ammonium compound in chemical structure. Further investigation is needed to identify the compounds corresponding to the signals and to study their effect in desorbing K in low K soil.