Vergleichende Untersuchungen zur Photosynthese und Atmung der Blätter von Schwefelmangelpflanzen

Abstract

In the course of our investigations on sulfur metabolism in higher plants possible changes of photosynthesis and respiration induced by sulfur deficiency have been studied. Leaves of S-deficient and normal tomato plants were illuminated in the presence of 14CO2 for various times. After subsequent lyophilization, chloroplasts were isolated using a non-aqueous procedure which prevents leaching of hydrophilic substances out of the plastids. The time course of distribution of phosphoglycerate, sugar mono- and -diphosphates, and of malate between chloroplasts and the cytoplasm was calculated from the 14C-incorporation into the fractions obtained. Neither a significant difference in the 14C-fixation rate nor remarkable changes of sites of synthesis and of transport of photosynthetic products between chloroplasts and cytoplasm could be observed between normal and S-deficient leaves. However, in S-deficient leaves the percentage of the sugar monophosphates is higher than normal even after 7 min photosynthesis, the 14C-activity in starch is twice as high as in normal leaves, whereas sucrose was labelled only weakly. The respiration rate measured manometrically, is on a unit leaf area basis, similar to that of normal leaves; protein is not useful as a reference figure owing to the reduced protein content of the deficient leaves. Infiltration of discs of leaves with 14C-sucrose and subsequent measurement of the various 14C-labelled metabolites resulted in a strongly reduced labelling of citrate, malate, aspartate, and glutamate as compared with normal leave cells. Alanine was found to be more radioactive. Since in addition citrate was less radioactive after 4 min dark fixation of 14CO2 we suggest from our results that the citrate cycle may be inhibited directly or indirectly by S-deficiency and that direct oxidation may dominate in the degradation of glucose in the S-deficient leaf cell.