Changes in the concentrations of malic acid, citric acid, calcium and potassium in the leaves during the growth of normal and iron-deficient mustard plants (Sinapis alba)

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

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.

The effect of illumination of the malic acid content and anion/cation balance of mustard leaves (Sinapis alba)

1. Mustard plants have been grown under conditions in which the length of artificial day could be controlled. 2. Leaf samples were analysed for malic acid and citric acid, and for a number of inorganic anions and cations. A simple method is described by which sap was obtained from 0.5g. samples of leaves. 3. In days of 16hr. or more, malic acid was accumulated; the chief cation accumulated was calcium. 4. When the day-length was reduced the malic acid content decreased considerably but the calcium content remained the same. There was little change in the pH value of the sap, the balance of anions and cations having been maintained mainly by increases in citrate and nitrate contents. Analyses of the whole leaf still showed some deficiency in anion after sodium, potassium, calcium, magnesium, nitrate, sulphate, inorganic phosphate, chloride, malate and citrate had been accounted for. 5. Analyses at shorter intervals revealed a large diurnal variation in malic acid content, which increased during the first 5-6hr. of the light period, and fell during darkness. 6. The significance of these findings is discussed, and it is suggested that malic acid accumulation is a by-product of photosynthesis, calcium being taken up irreversibly to maintain anion/cation balance, and hence creating a continuing need for anions to balance it.