Methods for the determination of chlorophyll were compared in Amaranthus tricolor, Kochia childsii and Chloris gayana. From sequential extr-action data, 96% ethanol appeared to be more efficient than 80% acetone in extracting chlorophyll from these plants.The chlorophyll alb ratio was significantly lower in sodium-deficient compared to normal C4 plants. Of the group I elements, only sodium, irrespective of the salt supplied to deficient cultures, restored the chlorophyll alb ratios to the value observed in normal plants. The concentration of sodium required to increase the chlorophyll alb ratio in leaves of sodium-deficient plants was similar to that required to bring about the growth responses. The increase of the chlorophyll alb ratio occurred at an early stage during recovery from sodium deficiency preceding the increase in chlorophyll concentration and the growth response. It is therefore likely that the low chlorophyll alb ratio may be intrinsically associated with the condition of sodium deficiency.
PEP formation from pyruvate was determined in mesophyll chloroplasts mechanically isolated from sodium-deficient and sodium-replete plants of the NADP malic enzyme-type C4 species, Kochia trichophylla. An extremely sensitive method for estimating PEP was developed which permitted determination of picomole quantities of PEP in small samples taken sequentially from the mesophyll chloroplast suspension concurrently with observations on oxygen evolution. It was shown that PEP formation requires light and depends upon the intactness of the chloroplasts. The rate of formation of PEP from pyruvate increased in the presence of the Hill reagent, oxaloacetate, thus indicating its dependence on non-cyclic photophosphorylation for the supply of ATP required in the conversion of pyruvate to PEP. The optimum inorganic phosphate concentration for PEP formation was approximately 16 mM. The rates of oxygen evolution and PEP formation were equivalent at concentrations of pyruvate up to 20 mM, suggesting tight coupling between electron transport and phosphorylation. In both Kochia trichophylla and the NAD malic enzyme-type, Panicum miliaceum, the rates of PEP formation were greater in the mesophyll chloroplasts from sodium-replete than from sodium-deficient plants.Chloroplasts resuspended in 'sodium-free'media containing less than 30 IJ.M (0.7 ppm) sodium showed reduced rates of PEP formation compared with chloroplasts resuspended in media to which 1.0 mM (23 ppm) sodium had been added. Both media contained 10 mM 'sodium-free' KCI. These results indicate that sodium ions may be required to maintain the functional integrity of the mesophyll chloroplasts and that irreversible damage is sustained when sodium is absent during their isolation.
Changes in C4 pathway intermediates in response to sodium nutrition, consistent with the hypothesis that there is a limitation in the conversion of pyruvate to phosphoenolpyruvate (PEP) in the mesophyll cells in sodium-deficiency, were observed in the C4 species, Kochia childsii, Chloris gayana, Amaranthus edulis, Amaranthus tricolor and Atriplex spongiosa. In the C3 species Lycopersicon esculentum (tomato), no differences were observed when grown with or without added sodium. Of the group 1 elements, only sodium, irrespective of the salt supplied to deficient cultures, effected these changes in the C4 species. In the light, concentrations of aspartate, PEP and 3-phosphoglycerate (3-PGA) were lower and those of pyruvate and alanine were greater in sodium-deficient than normal plants. In the dark, concentrations of aspartate, pyruvate, alanine, PEP and 3-PGA were similar in sodium-deficient and normal plants. In the C4 species Atriplex spongiosa, the concentration of sodium required to bring about these changes corresponded to that required for growth responses. Rapid increases in the concentrations of malate, PEP and 3-PGA and decreases in pyruvate and alanine were observed following the direct application of sodium to leaves.
The activitiy of carbonic anhydrase was determined in sodium-deficient and sodium-sufficient plants of Kochia childsii and Amaranthus tricolor. The enzyme extracted from leaves of sodium-deficient plants was found to be twice as active as that from leaves of control plants when expressed on a fresh weight or protein basis and three times as active on a chlorophyll basis. It was concluded that carbonic anhydrase activity was unlikely to limit the rate of photosynthesis in sodium-deficient C4 plants.
The concentration of alanine in the leaves of representatives of each of the three types of C4 plants was found to be consistently greater in sodium-deficient than in sodium-sufficient plants. The sodium treatment, however, did not affect the alanine concentration in the leaves of tomato, a C3 plant or in the roots of Amaranthus tricolor, a C4 plant. The alanine concentration of leaves of A. tricolor decreased rapidly upon the addition of sodium to cultures of sodium-deficient plants. The concentration of sodium required to give this response was approximately 50 �M, which was similar to that needed for the growth response. When plants that had been previously illuminated were placed in the dark, the alanine concentration increased rapidly, but was lowered again to the original concentration on return to the light. These changes in the alanine concentrations of leaves of C4 plants in response to sodium and light treatments are discussed in relation to the C4 photosynthetic system.
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