Barley seedlings (Hordeum vulgare cv. ‘Gaulois’) were precultured hydroponically for 4d on nutrient solutions with nitrate (3 mM) as N-source at pH 4.7, and were then transferred to solutions with ammonium or nitrate 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 were examined and grouped into pH-effects, ammonium effects and interactions.
Shoot biomass was not affected under all conditions. Root fresh weight was insensitive to the external pH when nitrate was the N-source, but was drastically affected by a combination of ammonium and low pH. 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 accumulated ammonium within 24 h about sixfold (18 mM) above the external concentration. At pH 5.5 or 6.8, but not at pH 4, root ammonium contents decreased afterwards to a lower steady state value (10 mM). Leaves also accumulated ammonium, especially at the most acidic pH.
Concentrations of major inorganic cations in roots were markedly but differentially affected by acidic pH and ammonium. The magnesium content of roots was drastically decreased (from 18 to 2 mM) by ammonium nutrition, and this was independent of the external pH. In contrast, calcium levels in roots were decreased by low external pH, independent of the N-source. Potassium levels in roots were rather insensitive to both low pH and ammonium.
The pH of crude root homogenates was measured in order to 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 homogenates was 5.8, and it increased to 6.3 when the external pH was 6.8. The pH 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, sugar levels were unchanged or even increased. Thus, growth impairment could not be traced back to impaired carbohydrate or ATP-supply, which might occur as a consequence of ammonium accumulation or Mg2+-deficiency.