CHARACTERISTICS OF FE-DEFICIENCY-INDUCED ACIDIFICATION IN SUBTERRANEAN CLOVER

Iron-deficiency-induced acidification is one of the important reaction s of plant Fe-deficiency-stress response, but the overall understandin g of this reaction is limited. The characteristics of Pc-deficiency-in duce acidification of subterranean clover (subclover) (Trifolium brach ycalycinum Katzn. and Morley cv. Koala) were studied in this paper. Pl ants were grown hydroponically under -Fe conditions, and Fe-deficiency -induced acidification was determined using pH-stat, back-titration an d chemical equilibrium procedures. Fe-deficiency-induced acidification was undetectable during the first day after Fe-deficiency stress init iation, but the maximum acidification rate was attained by the second day, when plants exhibited visual chlorosis symptoms. The acidificatio n rate was relatively constant with increasing Fe-deficiency chlorosis , suggesting that a critical level of Fe deficiency was needed to trig ger acidification, but that once the acidification process was initiat ed, the intensity of acidification was independent of severity of Fe d eficiency. Net H+-release (PR) rate determined using a chemical equili brium method and net acidity release (AR) rate determined using a back -titration method were practically identical, indicating that Fe-defic iency-induced acidification involved almost entirely the release of fr ee H+, not organic acid. In the assay temperature range of 5 to 35 deg rees C, PR rate was highest at about 20 degrees C. Net acidity release rate was almost totally inhibited at pH Values less than or equal to 4.5 and increased with increasing assay pH up to pH 9. The pH effect o ccurred within 30 min of incubation initiation, implying that the effe ct of pH is probably on the activity of H+ transport through the plasm a membrane, not on the quantity of responsible protein(s). Cations wer e required in the incubation solution for Fe-deficiency-induced acidif ication. Divalent cations in the assay solution resulted in a higher A R rate than monovalent cations, and essential cations resulted in a hi gher AR rate than non-essential cations, indicating that the relative effectiveness of cations is related to the efficiency of their absorpt ion by plant roots. These results are discussed in relation to their p ractical significance and the mechanisms of Fe-deficiency-induced acid ification.