Nitrogen transport xylem

Plants also form the ureides allantoin and allantoic acid, and in some legumes, such as soy beans, these compounds account for 70-80% of the organic nitrogen in the xylem. They appear to function in nitrogen transport.337 As indicated in Fig. 25-18, the hydrolysis to glyoxylate, NH4+, and C02 follows a different pathway than in animals. See also Chapter 24, Section C. 

N2 Fixation The NH3 produced by the bacteroids has first to be exported via bacteroid and host membranes to cytosol of the nodule and then possibly assimilated within various organelles or cell types to form amino acids, amides, or ureides for export in the xylem. Considering the numbers of nitrogen atoms exported per compound, costs of 3, 2, and 1.75 ATP/nitrogen transported would appear to be reasonable estimates of the energy cost for the transport of amino acids, amides, and ureides, respectively. Added to this would be any costs in transmembrane transfers of ammonia from bacteroid to plant. 

There is a close relationship between the metabolism of the shoot and the root. It is generally accepted that the xylem forms the main path for upward movement of water and ions from the roots to the leaves. Most of the essential major elements are transported in the xylem as inorganic ions. Nitrogen may be transported along the xylem as N03 if it is present in the external solution as nitrate. However, the plant sap may also contain organic nitrogen compounds such as amino acids. In the xylem, heavy metals will usually only be transported if special chelates are formed, eg, by citrate (Streit and Stumm, 1993). Iron is taken up and transported more readily when supplied as a chelated complex, such as ferric ethylenediamine tetraacetate (FeEDTA) or as ferric diethylenetriamine pentaacetate (FeDTPA) (Wallace and North, 1953). Calcium may also be transported in a chelated form (Jacoby, 1966). 

The functions of potassium in the plant are manifold. This element serves to activate or catalyze a host of enzyme actions, to facilitate the transport of nutrients and assimilates in the xylem and phloem, to maintain the structural integrity of the plant cell, to regulate turgor pressure, to mediate the fixation of nitrogen in leguminous plant species, and to protect plants to some degree from certain plant diseases. 

Surprisingly little is known of the energy cost associated with the radial flow of inorganic nitrogen from the soil medium to the xylem, or with the transport of organic nitrogen through the xylem and phloem to the ultimate sites of... 

It is obviously difficult to separate the above transport costs concerned with nitrogen metabolism from general maintenance of solute gradients and membrane integrity for transport activity in xylem and phloem, and to distinguish these items in turn within the overall budget of tissue respiration. 

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