Changes induced by Fe deficiency and Fe resupply in the organic acid metabolism of sugar beet (Beta vulgaris) leaves.

Abstract

The effects of iron deficiency and iron resupply on the metabolism of leaf organic acids have been investigated in hydroponically grown sugar beet. Organic acid concentrations and activities in leaf extracts of several enzymes related to organic acid metabolism were measured. Enzymes assayed included phosphoenol pyruvate carboxylase (PEPC; EC 4.1.1.31), different Krebs cycle enzymes: malate dehydrogenase (MDH; EC 1.1.1.37), aconitase (EC 4.2.1.3), fumarase (EC 4.2.1.2), citrate synthase (CS; EC 4.1.3.7) and isocitrate dehydrogenase (ICDH; EC 1.1.1.42), glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) and two enzymes related to anaerobic metabolism (lactate dehydrogenase [LDH]; EC 1.1.1.27, and pyruvate decarboxylase [PDC]; EC 4.1.1.1). Iron concentration in leaves was severely decreased by iron deficiency. Iron resupply caused an increase in iron concentrations, reaching levels similar to the controls in 96 h. Iron deficiency induced a 2.3-fold (from 16 to 37 mmol m-2) increase in leaf total organic acid concentration. Organic anion concentrations were still 4-fold higher than the controls 24 h after resupply and decreased to values similar to those found in the controls after 96 h. All measured enzymes had increased activities in extracts of iron-deficient leaves when compared to the controls and generally decreased to control values 24 h after iron addition. These data provide evidence that organic acid accumulation in iron-deficient leaves is likely not due to an enhancement in leaf carbon fixation. Instead, this accumulation could be associated with organic acid export from the roots to the leaves via xylem.