SOLUTE CONTENT AND ENERGY STATUS OF ROOTS OF BARLEY PLANTS CULTIVATEDAT DIFFERENT PH ON NITRATE-NITROGEN OR AMMONIUM-NITROGEN

Barley seedlings (Hordeum vulgare cv. `Gaulois') were precultured hydr oponically for 4d on nutrient solutions with nitrate (3 mM) as N-sourc e at pH 4.7, and were then transferred to solutions with ammonium or n itrate as N-source (3 mM), buffered and adjusted to different pH (4.0, 5.5, 6.8). Variations in shoot and root growth and solute contents we re examined and grouped into pH effects, ammonium effects and interact ions. Shoot biomass was not affected under all conditions. Root fresh weight was insensitive to the external pH when nitrate was the N-sourc e, but was drastically affected by a combination of ammonium and low p H. In contrast, root length was negatively affected by low pH per se. In nitrate-grown plants, ammonium levels in roots and shoots were low (0.5 to 1 mM). After transfer of plants to ammonium solution, roots ac cumulated ammonium within 24h about sixfold (18 mM) above the external concentration. At pH 5.5 or 6.8, but not at pH4, root ammonium conten ts decreased afterwards to a lower steady state value (10 mM). Leaves also accumulated ammonium, especially at the most acidic pH. Concentra tions of major inorganic cations in roots were markedly but differenti ally affected by acidic pH and ammonium. The magnesium content of root s was drastically decreased (from 18 to 2 mM) by ammonium nutrition, a nd this was independent of the external pH. In contrast, calcium level s in roots were decreased by low external pH, independent of the N-sou rce. Potassium levels in roots were rather insensitive to both low pH and ammonium. The pH of crude root homogenates was measured in order t o obtain at least crude information about trends in possible cellular pH changes, at an external pH of 4.0 and 5.5, the ph of the homogenate s was 5.8, and it increased to 6.3 when the external pH was 6.8. The p H of the homogenates was not affected by the N-source. In spite of the drastic effects of the N-source on the concentration of ammonium and magnesium in root tissues, ATP/ADP-ratios were not affected. Also, sug ar levels were unchanged or even increased. Thus, growth impairment co uld not be traced back to impaired carbohydrate or ATP-supply, which m ight occur as a consequence of ammonium accumulation or Mg2+-deficienc y.