Replacement of nitrate by ammonium as the nitrogen source increases the salt sensitivity of pea plants. II. Inter‐ and intracellular solute compartmentation in leaflets

Pea plants (Pisum sativum L. cv. ‘Kleine Rheinländerin’) grown on ammonium or nitrate as the sole nitrogen source were treated with 50 mol m−3 NaCl. Four days after salt addition, ammonium‐grown plants developed the first visible damage symptoms (wilting of leaflets, starting from the margins). Salt‐treated, nitrate‐grown plants were not affected during the experimental period. In order to obtain a better understanding of this differential salt sensitivity, we investigated the inter‐ and intracellular ion compartmentation of leaflets under both nutritional conditions by analysing ion concentrations in the apoplastic space, in chloroplasts and in protoplasts. When the leaves of nitrate‐ and ammonium‐grown plants had attained similar sodium and chloride contents (after different times of exposure to salinity), the latter had a considerably lower chloroplastic chloride (and also sulphate) concentration. The results suggest that the intracellular compartmentation capacity of ammonium‐grown plants is considerably lower than that of nitrate‐grown plants. Ion toxicity appeared to initiate breakdown of metabolism in parts of the mesophyll tissue of ammonium‐grown plants, causing an abrupt release of solutes into the apoplast, which coincided with the appearance of visible damage. Although the ammonium concentrations in leaves increased dramatically in the later phases of damage development, they were too low to cause the collapse of electrochemical gradients at the time at which damage became visible. Thus, the reason for a lower compartmentation capacity under ammonium nutrition remains as yet unclear.