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    Why does EAC have a maximum value of 0.75? What would be the likely effect on plant yield if this value declined to a very low level?

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    Plants require a number of different mineral elements for healthy growth, elements which may already be available in the soil or may need to be added in the form of fertilizers. Some of these elements are needed in relatively large amounts (the macro-nutrients) and others in very small amounts (the micro-nutrients). One of the most important macro-nutrients, one that is frequently in short supply, is nitrogen. This may seem strange when the atmosphere contains 80% nitrogen. Unfortunately this nitrogen is in a gaseous form (N2) that plants are unable to use. Plants must obtain nitrogen in the form of nitrate (NO3-) or ammonia (NH3) and these forms are much less abundant.

    There are organisms that can 'fix' or convert gaseous nitrogen into ammonia and they are referred to as 'nitrogen fixers'. They are able to make large amounts of the nitrogen in the air available to plants through this activity. All of these nitrogen-fixing organisms are prokaryotes; some are free-living in soil or water and others live in a symbiotic relationship with higher organisms. The well-studied symbiotic association exists between legumes (beans, peas) and the bacterium Rhizobium. The Rhizobium cells live in nodules on the roots of the plant. It is within the bacteria that live inside these nodules that the following reaction occurs:

    N2 + 8H+ + 8e- + 16ATP 2NH3 + H2 + 16ADP + 16Pi

    This reaction is catalyzed by the enzyme, nitrogenase. As you can see from this equation, the reduction of N2 to ammonia is a very expensive process energetically. It requires a large number of electrons and ATP molecules to reduce a single molecule of N2. The process is made more expensive by an unavoidable side reaction which simultaneously reduces protons (H+) to molecular H2.
    The important parameter of nitrogen fixation is the Electron Allocation Coefficient (EAC) which describes the allocation of electrons between proton (H+) reduction and N2 reduction.
    At least 25% of total electron flow through the nitrogenase enzyme is used to reduce H+ to H2 but this value is not constant. Often more than 25% of the electrons are used to reduce H+, which is energetically very wasteful for the plant.

    EAC is an indicator of the efficiency of nitrogenase activity. Higher values of the EAC indicate that a higher proportion of nitrogenase activity is being used to provide ammonia to the plant. If this value is very low, it means that the plant is not getting sufficient amount of Nitrogen which is a macro-nutrient.
    Once nitrogen is taken up by plant roots, it moves freely within the plant until it is converted into proteins which are the important building substances from which the living material or protoplasm of every plant cell is made. Chlorophyll, the green coloring matter of leaves, also contains nitrogen. This green pigment enables the plant to transfer energy from sunlight by photosynthesis. Therefore, the nitrogen supply to the plant influences the amount of protein, protoplasm and chlorophyll formed. In turn, this influences cell size and leaf area, and thus photosynthetic activity.

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