pH regulation in anoxic rice coleoptiles at pH 3.5: biochemical pHstats and net H+ influx in the absence and presence of NO3-

During anoxia, cytoplasmic pH regulation is crucial. Mechanisms of pH regulation were studied in the coleoptile of rice exposed to anoxia and pH 3.5, resulting in H+ influx. Germinating rice seedlings survived a combination of anoxia and exposure to pH 3.5 for at least 4 d, although development was retarded and net K+ efflux was continuous. Further experiments used excised coleoptile tips (7-10 mm) in anoxia at pH 6.5 or 3.5, either without or with 0.2 mM NO3-, which distinguished two processes involved in pH regulation. Net H+ influx (mu mol g(-1) fresh weight h(-1)) for coleoptiles with NO3- was similar to 1.55 over the first 24 h, being about twice that in the absence of NO3-, but then decreased to 0.5-0.9 as net NO3- uptake declined from similar to 1.3 to 0.5, indicating reduced uptake via H+-NO3- symports. NO3- reduction presumably functioned as a biochemical pHstat. A second biochemical pHstat consisted of malate and succinate, and their concentrations decreased substantially with time after exposure to pH 3.5. In anoxic coleoptiles, K+ balancing the organic anions was effluxed to the medium as organic anions declined, and this efflux rate was independent of NO3- supply. Thus, biochemical pHstats and reduced net H+ influx across the plasma membrane are important features contributing to pH regulation in anoxia-tolerant rice coleoptiles at pH 3.5.