Ammonium and nitrate nutrition inPlantago lanceolata L. andPlantago major L. ssp.major

Abstract

With the aims (1) to test whether the different natural occurrence of two Plantago species in grasslands is explained by a different preference of the species for nitrate or ammonium; (2) to test whether the different occurrence is explained by differences in the flexibility of the species towards changes in the nitrogen form; (3) to find suitable parameters as a tool to study ammonium and nitrate utilization of these species at the natural sites in grasslands, plants of Plantago lanceolata and P. major ssp. major were grown with an abundant supply of nitrate, ammonium or nitrate + ammonium as the nitrogen source (0.5 mM). The combination of ammonium and nitrate gave a slightly higher final plant weight than nitrate or ammonium alone. Ammonium lowered the shoot to root ratio in P. major. Uptake of nitrate per g root was faster than that of ammonium, but from the mixed source ammonium and nitrate were taken up at the same rate. In vivo nitrate reductase activity (NRA) was present in both shoot and roots of plants receiving nitrate. When ammonium was applied in addition to nitrate, NRA of the shoot was not affected, but in the root the activity decreased. Thus, a larger proportion of total NRA was present in the shoot than with nitrate alone. In vitro glutamate dehydrogenase activity (GDHA) was enhanced by ammonium, both in the shoot and in the roots. In vitro glutamine synthetase activity (GSA) was highest in roots of plants receiving ammonium. . Both GDHA and GSA were higher in P. lanceolata than in P. major. The concentration of ammonium in the roots increased with ammonium, but it did not accumulate in the shoot. The concentration of amino acids in the roots was also enhanced by ammonium. Protein concentration was not affected by the form of nitrogen. Nitrate accumulated in both the shoot and the roots of nitrate grown plants. When nitrate in the solution was replaced by ammonium, the nitrate concentration in the roots decreased rapidly. It also decreased in the shoot, but slowly. It is concluded that the nitrogen metabolism of the two Plantago species shows a similar response to a change in the form of the nitrogen source, and that differences in natural occurrence of these species are not related to a differential adaptation of nitrogen metabolism towards the nitrogen form. Suitable parameters for establishing the nitrogen source in the field are the in vivo NRA, nitrate concentrations in tissues and xylem exudate, and the fraction of total reduced nitrogen in the roots that is in the soluble form, and to some extent the in vitro GDHA and GSA of the roots. 'Grassland Species Research Group. Pubi, no 118.