Calcium (Ca) oxalate crystals occur in many plant species and in most organs and tissues. They generally form within cells although extracellular crystals have been reported. The crystal cells or idioblasts display ultrastructural modifications which are related to crystal precipitation. Crystal formation is usually associated with membranes, chambers, or inclusions found within the cell vacuole(s). Tubules, modified plastids and enlarged nuclei also have been reported in crystal idioblasts. The Ca oxalate crystals consist of either the monohydrate whewellite form, or the dihydrate weddellite form. A number of techniques exist for the identification of calcium oxalate. X-ray diffraction, Raman microprobe analysis and infrared spectroscopy are the most accurate. Many plant crystals assumed to be Ca oxalate have never been positively identified as such. In some instances, crystals have been classified as whewellite or weddellite solely on the basis of their shape. Certain evidence indicates that crystal shape may be independent of hydration form of Ca oxalate and that the vacuole crystal chamber membranes may act to mold crystal shape; however, the actual mechanism controlling shape is unknown.

Calcium oxalate crystals in plants

Sucrose synthesis from acetate in the germinating castor bean: kinetics and pathway.

A series of in situ productivity experiments were carried out on four freshwater reservoirs. Carbon fixation into particulate matter was estimated from the $^{14}$C content of phytoplankton collected on membrane filters. Inorganic $^{14}$C was removed from the filtrate by air bubbling at pH 4 and aliquots of the resulting solution of extracellular organic $^{14}$C were dried on metal disks for radioactivity estimation. Extracellular release expressed as a percentage of the total carbon fixation increases with depth towards the bottom of the photic zone and in bright sunlight there may also be an increase near the surface. The surface increase in percentage extracellular release (p.e.r.) is associated with the inhibition of photosynthesis by high light intensities. The increase in p.e.r. with depth is associated with dark fixation of inorganic carbon. In the dark p.e.r. is always higher than in uninhibiting light. The inhibition of dark fixation by previous exposure to bright sunlight indicates that 90% or more of the fixation in dark bottles is by algae. Laboratory experiments with phytoplankton and with axenic cultures of Chlorella pyrenoidosa confirm these relationships. For the light bottles there is a highly significant correlation between p.e.r. at the depth of maximum photosynthesis and for the photic zone as a whole. From this relationship it is possible to estimate extracellular release per unit area from a determination at one depth only. When phytoplankton populations are diluted with filtered water p.e.r. increases with dilution, the increase being most marked when the population density is reduced by more than 90%. A similar relationship holds over a range of population densities in the in situ experiments. The population density of phytoplankton was estimated by pigment analysis-chlorophylls a, b and c, and plant carotenoids. With each of these pigments there is a highly significant inverse correlation between pigment concentration and the logarithm of p.e.r. The results indicate that less productive waters have the higher relative release of extracellular products. Productivity estimation methods that measure carbon fixation into particulate matter only may underestimate primary production by up to 30%. The increase in p.e.r. with decreasing population density suggests the operation of an equilibration system. The time-course curves of extracellular release give no clear indication of the nature of this equilibration. Nevertheless, it was established that one extracellular product (glycollic acid) is taken up by phytoplankton and/or bacteria in the water by a process of diffusion that is independent of light. The $^{14}$C-labelled extracellular products of three phytoplankton populations were separated by paper chromatography. All three were found to contain glycollic acid, polysaccharide, and various amino acids.

Release of Dissolved Organic Material from the Cells of Phytoplankton Populations

The relationship of glycolic acid to respiration and photosynthesis in tobacco leaves.

The discovery and elucidation of 8-carboxylation of pyruvate 8-Carboxylases of plants . . . . . . ( I ) Oxaloacetic decarboxylase . . . . . (2) Malic enzyme . . . . . . (3) Phosphoenolpyruvate carboxylase . , . . (4) Phosphoenolpyruvate carboxykinase . . . The role of pyruvate carboxylation in plant metabolism . (I) Respiration . . . . . . . . (2) Photosynthesis . . . . . . . (3) Crassulacean acid metabolism . . . . . (4) Fat metabolism . . . . . . . (5) Carbon dioxide fixation by roots Conclusions . . . . . . . . . Summary . . . . . . . . . References . . . . . . . . . Addendum . . . . . . . . . . . . . . 215 . 216 . 220 . 220 . 220 . 223 . 226 . 227 . 227 . 232 . 239 . 244 . 247 . 248 . 248 249 254

Pyruvate carboxylation and plant metabolism.

The importance of organic acids as photosynthetic intermediates, first suggested by Liebig, has been subject to speculation for over100 years. The free organic acids or their phosphate esters still are included in the most recent schemes for photosynthetic assimilation of carbon dioxide (8, 12). Recent advances in our knowledge of the path of carbon in photosynthesis have been described so adequately (3, 8, 9, 12), that any extensive review of the subject here is unwarranted. BENNET-CLARK (2) recently has reviewed the literature on the organic acids of plants. Our early attempts to use higher plants for the biosynthesis of C14-labeled organic acids (7) revealed that equilibrium in C14 distribution among the organic acids was not established even many hours after exposure to C1402. The observed concentration of C14 in malic acid suggested the possible importance of this acid as an early photosynthetic product. Accordingly, the distribution of C14 among several organic acids from plants supplied C1402 was investigated.

/pdf/photosynthesis-and-metabolism-of-organic-acids-in-higher-4byyyms9sy.pdf

Photosynthesis and metabolism of organic acids in higher plants

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Photosynthesis and metabolism of organic acids in higher plants