The influence of atmospheric NH3 on the apoplastic pH of green leaves: a non‐invasive approach with pH‐sensitive microelectrodes

Abstract

The apoplastic pH of intact green leaves of Bromus erectus was measured non‐invasively by inserting blunt microelectrodes through stomatal openings. After making electrical contact, the recorded signal was stable for hours, yielding a pH of 4.67±0.10. The leaves responded to ‘light‐off’ with an initial transient acidification and subsequent sustained alkalinization of 0.2–0.3 pH; ‘light‐on’ caused the opposite response. Flushing the leaves with 280 nmol NH3 mol−1 air within 18±6 s alkalinized the apoplast by 0.22±0.07 pH, followed by a slower pH increase to reach a steady‐state alkalinization of 0.53±0.14 after 19±7 min. This pH shift was persistent as long as the NH3 was flushed, and readily returned to its initial value after replacing the NH3 with clean air. The resultant [NH4+] increase within the apoplast was measured with a NH4+‐selective microelectrode. In the presence of 280 nmol NH3 mol−1 air, apoplastic NH4+ initially increased within 15±10 s to 1.53±0.41 mM, to reach a steady state of 1.62±0.16 mM after 27±7 min. An apoplastic buffer capacity of 6 mM pH−1 unit was calculated from the initial changes of pH and [NH4+], whereas the steady‐state values yielded 2.7 mM pH−1. Infiltrated leaves responded to NH4+ with concentration‐dependent depolarizations, the maxima of which yielded saturation kinetics indicating carrier‐mediated NH4+ uptake into adjacent cells, as well as a linear component indicating non‐specific transport. We infer that the initial alkalinization is due to rapid conversion of NH3 to NH4+, whereas the slower pH increase would be caused by regulatory processes involving both membrane transport, and (mainly) NH4+ assimilation. Possible consequences of the NH3‐induced pH shift for the development of plants growing in polluted areas are discussed.