Development of the coordinated response to decreasing Fe availability was studied in cucumber plants grown in nutrient solution (NS) over a range of FeIII-EDTA concentrations (from 0.1 to 80 μM). Physiological and biochemical parameters were evaluated in intact roots, root extracts and plasma membrane (pm) vesicles. Acidification of the NS was evident in plants grown at ≤ 1 μM FeIII-EDTA and inversely related to the external Fe concentration. FeIII-EDTA reduction by intact roots was also gradually depressed by increasing Fe supply. The rate of net nitrate uptake by the roots was directly related to the amount of FeIII-EDTA added to the NS. Activity of pmH+-ATPase was significantly higher in plants grown without added Fe as compared to those grown at 80 μM Fe. A lower increase, dependent on Fe concentration, was observed at 0.1, 1, 5 or 10 μM FeIII-EDTA. Activity of pmFeIII-EDTA reductase was also increased by Fe deprivation and strongly correlated with pmH+-ATPase activity. PEP-carboxylase activity gradually increased with decreasing Fe concentration in the NS. Changes in activity and amount of the enzyme showed a close correlation with parameters measured in intact roots (nitrate uptake, FeIII-EDTA reduction). Results show that the development of the Fe-deficiency response in cucumber roots can be finely tuned by the level of Fe supply. Adjustments to different levels of available Fe involve a correlated modulation of pm-associated enzymes. PEP-carboxylase activity appeared to be a suitable metabolic marker of the Fe nutritional status of the plant.
