THE PH REQUIREMENT FOR IN-VIVO ACTIVITY OF THE IRON DEFICIENCY-INDUCED TURBO FERRIC CHELATE REDUCTASE - A COMPARISON OF THE IRON-DEFICIENCY-INDUCED IRON REDUCTASE ACTIVITIES OF INTACT PLANTS AND ISOLATED PLASMA MEMBRANE FRACTIONS IN SUGAR-BEET

The characteristics of the Fe reduction mechanisms induced by Fe defic iency have been studied in intact plants of Beta vulgaris and in purif ied plasma membrane vesicles from the same plants. In Fe-deficient pla nts the in vivo Fe(III)-ethylenediaminetetraacetic complex [Fe(III)-ED TA] reductase activity increased over the control values 10 to 20 time s when assayed at a pH of 6.0 or below (''turbo'' reductase) but incre ased only 2 to 4 times when assayed at a pH of 6.5 or above. The Fe(II I)-EDTA reductase activity of root plasma membrane preparations increa sed 2 and 3.5 times over the controls, irrespective of the assay pH. T he K-m for Fe(III)-EDTA of the in vivo ferric chelate reductase in Fe- deficient plants was approximately 510 and 240 mu M in the pH ranges 4 .5 to 6.0 and 6.5 to 8.0, respectively. The K-m for Fe(III)-EDTA of th e ferric chelate reductase in intact control plants and in plasma memb rane preparations isolated from Fe-deficient and control plants was ap proximately 200 to 240 mu M. Therefore, the turbo ferric chelate reduc tase activity of Fe-deficient plants at low pH appears to be different from the constitutive ferric chelate reductase.