ACQUISITION AND ASSIMILATION OF GASEOUS AMMONIA AS REVEALED BY INTRACELLULAR PH CHANGES IN LEAVES OF HIGHER-PLANTS

Atmospheric ammonia (NH3) from various anthropogenic sources has becom e a serious problem for natural vegetation. Ammonia not only causes ch anges in plant nitrogen metabolism but also affects the acid-base bala nce of plants. Using the pH-sensitive fluorescent dyes pyranine and es culin, cytosolic and vacuolar pH changes were measured in leaves of C- 3 and C-4 plants exposed for brief periods to concentrations of NH3 in air ranging from 1.33 to 8.29 mu mol NH3 . mol(-1) gas (0.94-5.56 mg . m(-3)). After a lag phase, uptake of NH3 from air at a rate of 200 n mol NH3 . m(-2) leaf area . s(-1) into leaves of Zea mays L. increased pyranine fluorescence indicating cytosolic alkalinisation. The increa se was much larger in the dark than in the light. In illuminated leave s of the C-3 plant Pelargonium zonale L. and the C-4 plants Z. mays an d Amaranthus caudatus L., NH3-dependent cytosolic alkalinisation was p articularly pronounced when CO2 was supplied at very low levels (16 or 20 mu mol CO2 . mol(-1) gas, containing 210 mmol O-2 . mol(-1) gas). An increase in esculin fluorescence, which was smaller than that of py ranine, was indicative of trapping of some of the NH3 in the vacuoles of leaves of Spinacia oleracea L. and Z. mays. Photosynthesis and tran spiration remained unchanged during exposure of illuminated leaves to NH3, yielding an influx of 200 nmol NH3 . m(-2) leaf area . s(-1) for up to 30 min, the longest exposure time used. Both CO2 and O-2 influen ced the extent of cytosolic alkalinisation. At 500 mu mol CO2 . mol(-1 ) gas the cytosolic alkalinisation was suppressed more than at 16 or 2 0 mu mol CO2 . mol(-1) gas. The suppressing effect of CO2 on the NH3-i nduced alkalinisation was larger in illuminated leaves of the C-4 plan ts Z. mays and A. caudatus than in leaves of the C-3 giant P. zonale. A reduction of the O-2 concentration from 210 to 10 mmol O-2 . mol(-1) gas, which inhibits photorespiration, increased the NH3-induced cytos olic alkalinisation in C-3 plants. Suppression by CO2 or O-2 of the al kaline pH shift caused by the dissolution and protonation of NH3 in aq ueous leaf compartments, and possibly by the produc tion of organic co mpounds synthesised from atmospheric NH3, indicates that NH3 which ent ers leaves is rapidly assimilated if photosynthesis or photorespiratio n provide nitrogen acceptor molecules.