Influence of nitrate and ammonium on critical nitrogen deficiency concentrations and mineral composition of Dupontia fisheri grown hydroponically in a controlled environment

Dupontia fisheri plants, derived from a clone, were propagated in plant growth chambers by the open‐pot nutrient solution technique, with vermiculite as the solid phase. The plants were illuminated continuously at 21, 500 lux (2,000 f.c.) by a combination of fluorescent and incandescent lamps. Air temperature was kept constant at 20°C. The plants, after transplanting to 20‐liter pots (closed‐pot system), were nourished by a modified half‐strength Hoagland solution, supplied with a one time addition of nitrogen at the rate of zero, 0.25, 0.5, 1.0, 2.0, 4.0 and 8.0 me/1 derived from (NH₄)₂SO₄, Ca(NO₃)₂ or NH₄NO₃. They were harvested 49 days after transplanting at a time when those in the three lowest treatments were distinctly deficient in nitrogen. Critical nitrate‐N values (the concentration at a 10% reduction in vegetative growth) were found to be identical, at 100 μg/g (dry basis), in the stem, blade‐1 and blade‐3 tissues, and those for total‐N at 0.901, 2.251, and 2.501, respectively.

Absence of nitrate in stem tissue indicated a nitrogen deficiency while the total‐N value indicated the degree of deficiency: the lower the value the greater the deficiency. Nitrogen also influenced the mineral composition of stem and blade tissues directly, mainly by ionic competition, and possibly indirectly, by decreasing dry matter content as the plants became less deficient in nitrogen. Transitions from nitrogen deficiency to sufficiency caused relatively large changes in the concentration of other nutrients in both stems and blades, but sometimes in opposite directions. For example, soluble‐P and total‐P in stems increased dramatically with increases in total‐N, but decreased greatly in the blade‐1 and blade‐3 tissues. Potassium, on the other hand, increased greatly in all tisues with increases in total‐N. These effects were much smaller for phosphorus with ammonium‐N as a nitrogen source than with nitrate, but for potassium there was no appreciable effect of nitrogen source in stems, a larger effect in blade‐1 and an erratic effect in blade‐3. Additionally, there were rather large decreases in manganese concentration with increases in nitrogen while effects on other nutrients were either small (Mg and Zn) or not significant (Ca, Fe, Cu and Na). All values were above critical concentrations.