Intact spinach chloroplasts scavenge hydrogen peroxide with a peroxidase that uses a photoreductant as the electron donor, but the activity of ruptured chloroplasts is very low [Nakano and Asada (1980) Plant & Cell Physiol. 21: 1295]. Ruptured spinach chloroplasts recovered their ability to photoreduce hydrogen peroxide with the concomitant evolution of oxygen after the addition of glutathione and dehydroascorbate (DHA). In ruptured chloroplasts, DHA was photoreduced to ascorbate and oxygen was evolved in the process in the presence of glutathione. DHA reductase (EC 1.8.5.1) and a peroxidase whose electron donor is specific to L-ascorbate are localized in chloroplast stroma. These observations confirm that the electron donor for the scavenging of hydrogen peroxide in chloroplasts is L-ascorbate and that the L-ascorbate is regenerated from DHA by the system: photosystem I-*ferredoxin-*NADP^>glutathione. A preliminary characterization of the chloroplast peroxidase is given.

Hydrogen Peroxide is Scavenged by Ascorbate-specific Peroxidase in Spinach Chloroplasts

One of the first of the specialized agencies of the United Nations to become active, the Food and Agriculture Organization has elicited interest beyond the specialized field of agricultural economists. Attempting as it does to solve one of the very basic problems of the world, that of an adequate food supply, the organization represents a significant and hopeful international attempt to create a world in which there may actually exist “freedom from want.” The objectives of FAO, as formally expressed in the preamble to the constitution, read as follows:“The nations accepting this constitution being determined to promote the common welfare by furthering separate and collective action on their part for the purpose of raising levels of nutrition and standards of living of the people under their jurisdiction, securing improvements in the efficiency of the production of all food and agricultural products, bettering the conditions of rural populations, and thus contributing toward an expanding world economy, hereby establish the Food and Agriculture Organization of the United Nations.”

The Food and Agriculture Organization of the United Nations

1. Plant Nutrients. 2. The Soil as a Plant Nutrient Medium. 3. Nutrient Uptake and Assimilation. 4. Plant Water Relationships. 5. Plant Growth and Crop Production. 6. Fertilizer Application. 7. Nitrogen. 8. Sulphur. 9. Phosphorus. 10. Potassium. 11. Calcium. 12. Magnesium. 13. Iron. 14. Manganese. 15. Zinc. 16. Copper. 17. Molybdenum. 18. Boron. 19. Further Elements of Importance. 20. Elements with More Toxic Effects. General Readings. References. Index.

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Principles of plant nutrition

Coupling of Phosphorylation to Electron and Hydrogen Transfer by a Chemi-Osmotic type of Mechanism

Publisher Summary This chapter discusses the chemistry of submerged soils. The chemical changes in the submerged materials influence: (a) the character of the sediment or soil that forms, (b) the suitability of wet soils for crops, (c) the distribution of plant species around lakes and streams and in estuaries, deltas, and marine flood plains, (d) the quality and quantity of aquatic life, and (e) the capacity of lakes and seas to serve as sinks for terrestrial wastes. The single electrochemical property that serves to distinguish a submerged soil from a well-drained soil is its redox potential. The redox potential of a soil or sediment provides a quick, useful, semiquantitative measure of its oxidation–reduction status. Two recent developments have stimulated interest in the chemistry of submerged soils: the breeding of lowland rice varieties, with a high yield potential, and the pollution of streams, lakes, and seas, by domestic, agricultural, and industrial wastes. The chemistry of submerged soils is valuable: (a) in understanding the soil problems, limiting the performance of high-yielding rice varieties, and (b) in assessing the role of lake, estuarine, and ocean sediments as reservoirs of nutrients for aquatic plants and as sinks for terrestrial wastes.

The Chemistry of Submerged Soils

The findings with regard to the influence of the external hydrogen ion concentration on growth (2), particularly the extreme effects observed at pH 3 and 9, have led to a search for a possible explanation of the observed responses in terms of the physiological activities of the plant. A further investigation of the relation of external hydrogen ion concentration to the absorption of inorganic nutrients, which was already elucidated for several ions by Hoagland and Broyer (5), seemed particularly pertinent. A series of experiments was therefore initiated to explore the effect of external reaction, within a pH range of 3 to 9, on the absorption of the nutrient ions : Ca++, Mg++, K+, NO3and H2POr. In order to differentiate between the direct effects of external reaction

Hydrogen ion concentration in relation to absorption of inorganic nutrients by higher plants.

Evidence has been presented that a soluble fraction from R. rubrum cells contains two new primary carboxylation reactions which depend on the reducing power of ferredoxin: (a) pyruvate synthase which brings about a synthesis of pyruvate from acetyl-CoA and CO2 and (b) α-ketoglutarate synthase which brings about a synthesis of α-ketoglutarate from succinyl-CoA and CO2. The soluble fraction of R. rubrum cells contains also a series of other enzymes which, together with the ferredoxin-dependent enzymes, constitutes a reductive carboxylic acid cycle—a new cyclic pathway for CO2 assimilation that was first found in the photosynthetic bacterium, Chlorobium thiosulfatophilum.

Ferredoxin-dependent carbon assimilation in Rhodospirillum rubrum.

Vitamin K as a cofactor of photosynthetic phosphorylation

The b-type cytochromes of chloroplasts have heretofore been viewed as photosynthetic electron carriers that probably occupy an intermediate position in a light-induced electron flow. The oxidation-reduction of such intermediate electron carriers, being removed from the primary photochemical reaction linked to photon capture by chlorophyll, would be expected to show a temperature dependence. Evidence has now been obtained that cytochrome b559 is photooxidized at -189°C and that this photooxidation can be induced only by “short-wavelength” monochromatic light which activates the oxygen-evolving system in chloroplasts (photosystem II). In appears, therefore, that photooxidation of cytochrome b559 is closely linked with photon capture by the chlorophyll pigments characteristic of photosystem II.

http://www.pnas.org/content/63/3/956.full.pdf

Daniel I. Arnon

Papers

Hydrogen ion concentration in relation to absorption of inorganic nutrients by higher plants.

Ferredoxin-dependent carbon assimilation in Rhodospirillum rubrum.

Vitamin K as a cofactor of photosynthetic phosphorylation

LIGHT-INDUCED OXIDATION OF A CHLOROPLAST B-TYPE CYTOCHROME AT -189 degrees C.

Role of cyclic photophosphorylation in photosynthetic carbon dioxide assimilation by isolated chloroplasts.