ISSN:
1399-3054
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Biology
Notes:
Prolonged application of ammonium as a source of nitrogen leads to serious physiological and morphological disorders in many plants, including mustard (Sinapis alba L,) seedlings (ammonium toxicity syndrome). Ammonium tolerance was previously observed in mustard seedlings in the presence of considerable amounts of nitrate in the medium. In the present study, the question was addressed as to what extent accumulation of nitrate and ammonium occurs in the mustard seedling and how this relates to ammonium toxicity and tolerance. Emphasis was on light control of accumulation in the attached cotyledons. Both NQ3 and NH4 became strongly accumulated in the mustard cotyledons once the concentration in the medium exceeded 1 mM, In the cotyledons, we measured concentrations 〉 30 mM in the case of nitrate and 〉 50 mM in the case of ammonium 4 days after sowing. Accumulation of inorganic nitrogen in the mustard cotyledons did not depend on photosynthesis nor on intact chloroplasts. However, the rate of nitrate accumulation was strongly stimulated by light, operating through phytochrome, while ammonium accumulation was not affected by light in short-term experiments, i,e, within 24 h and only weakly (and probably indirectly) in long-term light.We conclude that strong interaction between NQ3 and NHJ is characteristic for accumulation of inorganic nitrogen. In the presence of NQ3, accumulation of NH3 is diminished and, to a lesser extent, vice versa. Using [15N]-labelling it was found that incorporation of inorganic nitrogen into organic compounds was stimulated strongly whenever nitrate and ammonium were provided simultaneously. It appears from the data that accumulation of nitrate is strictly controlled by the mustard cotyledons while the accumulation of ammonium is not. The data indicate that the level of ammonium is governed primarily by the rate of ammonium assimilation. Stimulation of ammonium assimilation by simultaneously applied nitrate appears to explain nitrate-mediated ammonium tolerance in the mustard plant.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1399-3054.1990.tb09052.x
