RESPONSE OF PRIMARY PLANT-METABOLISM TO THE N-SOURCE

Even when plant growth was not visibly affected, ammonium versus nitra te nutrition had distinct effects on some parts of plant metabolism. B arley seedlings growing on 3 mM ammonium rapidly accumulated ammonium up to 20 mM in the roots. In leaves, ammonium accumulation was observe d only when the pH of the nutrient medium was very low (pH 4). Yet eve n under the most extreme conditions there was no indication that plant s were suffering from uncoupling of ATP synthesis or from a lack of ca rbohydrates. Especially dramatic was the response of the organic acid content of pea and barley leaves: it decreased strongly within a few d ays upon transfer of plants from nitrate to ammonium-media, and this w as apparently not due to an inhibition of PEP carboxylase, which was r ather activated under ammonium nutrition. As malate dispappeared from leaves even when pea plants were transferred to an N-free medium, mala te degradation was not necessarily connected to increased amino acid s ynthesis, but eventually to a more rapid decarboxylation by malic enzy me. Also, malate degradation was not a response to ammonium; but rathe r to (the absence of) nitrate.