Effects of Iron Deficiency on the Composition of the Leaf Apoplastic Fluid and Xylem Sap in Sugar Beet. Implications for Iron and Carbon Transport

The effects of iron deficiency on the composition of the xylem sap and leaf apoplastic fluid have been characterized in sugar beet (Beta vulgaris Monohil hybrid). pH was estimated from direct measurements in apoplastic fluid and xylem sap obtained by centrifugation and by fluorescence of leaves incubated with 5-carboxyfluorescein and fluorescein isothiocyanate-dextran. Iron deficiency caused a slight decrease in the pH of the leaf apoplast (from 6.3 down to 5.9) and xylem sap (from 6.0 down to 5.7) of sugar beet. Major organic acids found in leaf apoplastic fluid and xylem sap were malate and citrate. Total organic acid concentration in control plants was 4.3 mM in apoplastic fluid and 9.4 mM in xylem sap and increased to 12.2 and 50.4 mM, respectively, in iron-deficient plants. Inorganic cation and anion concentrations also changed with iron deficiency both in apoplastic fluid and xylem sap. Iron decreased with iron deficiency from 5.5 to 2.5 μM in apoplastic fluid and xylem sap. Major predicted iron species in both compartments were $[\text{FeCitOH}]^{-1}$ in the controls and $[\text{FeCit}_{2}]^{-3}$ in the iron-deficient plants. Data suggest the existence of an influx of organic acids from the roots to the leaves via xylem, probably associated to an anaplerotic carbon dioxide fixation by roots.