Effect of nitrogen source on oxalate accumulation in Setaria sphacelata (cv. Kazungula)

Setaria sphacelata (cv. Kazungula) was grown under controlled environment conditions on sand provided with mineral nutrient solutions containing nitrate, ammonium, urea, or nitrate plus ammonium as sole nitrogen sources. Growth in urea or ammonium resulted in small but significant decreases in oxalate concentrations of foliage compared with growth in nitrate, but nitrogen source had little effect on plant growth rates. Similar levels of potassium and total nitrogen were found in foliage from all treatments. Nitrate concentration was highest in leaves of plants supplied nitrate plus ammonium and lowest in leaves of plants supplied only ammonium as the nitrogen source. Nitrate reductase activity was virtually absent from new fully expanded leaves from all treatments, but readily detectable in young regrowth when highest activities were associated with the nitrate plus ammonium treatment and lowest with ammonium alone. Leaves continued to accumulate oxalate after reaching maturity and it is considered that anions other than nitrate must be responsible for the cation excess which prompts the synthesis of this oxalate. No diurnal variation in oxalate content of controlled environment or field grown Setaria sphacelata was observed.