Root uptake of N-containing and N-free low molecular weight organic substances by maize:: A 14C/15N tracer study

Most studies showing potential organic nitrogen uptake were conducted with amino acids They. conclude that, in some ecosystems, amino acids significantly contribute to the N demand of plants and that roots have special transporters to re-uptake amino acids released into the rhizosphere. However, the relevance of the uptake of organic N compounds can only be evaluated by comparing the uptake of N-containing and N-free organic substances. We compared the uptake of alanine, glucose and acetate labelled with C-14 by maize. Additionally, the N uptake was estimated by N-15 labelled alanine and KNO3. We found a similar uptake of C-14 from alanine, glucose and acetate, amounting for the whole plant less than 1% of C-14 input. These results show that maize did not prefer N-containing to N-free organic substances. The uptake of N-15 by maize exceeded that of C-14 (10- to 50-fold), irrespective of the N-15 source. However, plant uptake of nitrate (23.6-35.2% of N-15 input) always exceeded the uptake of N from alanine (9.6-28.8%). The uptake of organically bound N by maize growing in soil occurred mainly by transpiration flow - as dissolved organics. The contribution of specific amino acid transporters was minor. (C) 2008 Elsevier Ltd. All rights reserved.