Non-ionic nitrogen nutrition of plants : nutrient uptake and assimilation and proton extrusion during utilization of urea or symbiotically fixed nitrogen

Abstract

This thesis encompasses six papers, dealing with mainly ionic balance aspects of non-ionic nitrogen nutrition of plants. In most cases urea nutrition or symbiotic N 2 -fixation were compared with NH 4+ - or NO 3- -supply with respect to nutrient uptake and assimilation.From ionic balance and proton release data it was established that maize and sugar-beet plants are able to absorb urea as an undestructed molecule. Results of xylem sap analyses learned that urea, like NH 4+ , is almost quantitatively metabolized in the roots.Complete ionic uptake balances, including direct measurements of respective H + - and OH - /HCO 3- -release from the roots of N 2 -fixing and NO 3- -supplied pea plants are presented. Excess nutrient cation over anion uptake and hence H + -release by N 2 -fixing plants increased at higher pH of the nutrient solution. When such plants were grown in soil, cation uptake also exceeded anion uptake, but root growth was severely reduced at low soil pH. This effect could be eliminated completely by liming. Root growth was not inhibited when NO 3- was the form of N-nutrition.In soils, mineralized N may confuse the comparison between NO 3- -nutrition and N 2 -fixation. It is suggested that the relative contribution of N 2 -fixation to the total N- accumulation in plants reflects the point of time at which ( 15 N-)NO 3- in the soil was depleted and the N 2 -fixing process started.Different ionic uptake patterns of plants in relation to the form of nitrogen nutrition necessarily invoke essential differences in both inorganic and organic chemical composition of the xylem sap of these plants. Complete xylary ionic balances and data about partitioning of the nitrogenous compounds In xylem saps allowed the conclusion that N 2 -fixing pea plants belong to the group of amidetransporting legumes and that in NO 3- -supplied pea plants no phloem transport of cation-organate is necessary for. the regulation of Intracellular pH and electroneutrality