A photosynthetic and ¹⁵N investigation of the differential growth response of barley to nitrate, ammonium, and nitrate + ammonium nutrition

Summary

Barley plants grown on a mixed ammonium + nitrate feeding solution showed a far larger productivity (in terms of plant size and N content) than plants grown on either nitrogen source alone. Although ammonium-fed plants absorbed N very rapidly they were smaller and weaker than nitrate-fed or nitrate + ammonium-fed plants. Nitrate-fed plants, although robust, had a low N content compared with plants fed the other N sources.

¹⁵N assays of plant organs and xylem sap indicated compartmentation of N assimilation between root and shoot according to N source: nitrate in shoots, ammonium in roots, with both organs active in nitrate + ammonium assimilation. Photosynthetic rate determinations by IRGA indicated an enhanced C-fixation rate in whole plants receiving nitrate compared to ammonium-only fed plants, but measurements made on a unit leaf area basis did not confirm this, i.e. increased photosynthesis of nitrate-fed plants was due to their larger size. ¹⁴CO₂-feeding experiments showed that ammonium-fed plants allocated a greater proportion of newly assimilated C to nitrogen compounds (nitrogen C: non-nitrogen C, 1:6) than did nitratefed plants (nitrogen C: non-nitrogen C, 1:11) thus probably accounting for the larger size of the latter.