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Two aspects of C-4 plants : 1. effects of light intensity on photosynthesis : 2. A nitrogen-fixing association Hart, Alan Leedham

Abstract

Part 1 concerns the effect of light intensity on the leaves of Gomphrena globosa, a C-4 plant. Plants were grown under three light intensities. The structure and function of the photosynthetic apparatus of newly matured leaves were examined by a number of techniques. These were electron microscopy, infra-red fluorescence photomicrography, dye (TNBT) reduction and ¹⁴CO₂ feedings. Contrary to conclusions from the literature, light intensity did not affect the membrane configuration of the chioroplasts. Metabolite levels seemed to vary with light intensity. With respect to its photosynthetic properties G. globosa seems to occupy an intermediate position in a range of other C-4 plants. Part 2 describes some aspects of the relationship between a grass, Paspalum notatum, and a bacterium, Asotobacter paspali which is found mainly on the root surface of the grass. These organisms form a nitrogen fixing association in South-Eastern and Central Brazil. Both were studied separately because of the difficulty of establishing the association in the laboratory. An apparatus was built in which photosynthesis and root function of P. notatum could be monitored. ¹⁴CO₂ was fed to the leaves and ¹⁴C appearing in the roots and root exudate was analyzed. P. notatum appears to exude sufficient substrate to support nitrogen fixation by A. paspali but the amount and nature of exudate proved difficult to ascertain. The association is found in acid soils but nitrogen fixation by A. paspali is inhibited by low pH. It was found that roots of P. notatum were able to neutralize increases in acidity in their environment. A low pH inhibited N₂[C₂H₂]-fixing activity of A. paspali grown in continuous culture at high and low concentrations of carbohydrate and oxygen. Other nitrogen fixing bacteria, e.g. Beijerinckia spp and Derxia gummosa, are found in relatively low numbers in the rhizosphere of P. notatum. Nitrogen fixation by A. paspali may be less prone to oxygen inhibition than it is in these other bacteria, thus allowing A. paspali to compete more efficiently for carbohydrate. It seems that A. paspali lives with P. notatum because of the favourable pH that is maintained in the root environment. The carbohydrate and oxygen concentration are probably also favourable for nitrogen fixation. How can the association be easily established in the laboratory? Before further research can be done, a way must be found to answer this question.

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