ABSTRACT
Carbon and nitrogen are important components of plants: carbon amounts to 50% and nitrogen to 2-5% of the dry matter. Whereas carbon is assimilated as CO2 from the atmosphere, nitrogen can be taken up as ammonium and nitrate from the soil and as N2 and ammonia from the gas phase. Carbon reduction is restricted to the green parts of plants, but nitrogen incorporation in organic components can take place in the roots and in the leaves, depending on the nitrogen source, plant species, and nutrient availability. N2-utilization is only found in plants which are living in a symbiotic relationship with bacteria, such as Rhizobia or cyanobacteria. If leaves are able to fix more CO2 during the light period than they need for their metabolism, the surplus carbon is exported to support other tissues. Such leaves are called ‘source’ leaves. ‘Sink’ tissues such as growing leaves, flowers, seeds, roots and storage organs are net importers of photoassimilates (Ho, 1988). The mesophyll cell is the dominant cell type of leaves and it contains most of the chloroplasts. Other abundant tissues are epidermis cells and the sieve tube complex. The primary CO2 and nitrate fixation reactions and the syntheses of the transported assimilates occur in the mesophyll cells; these contain subcellular compartments which have different functions in carbon and nitrogen metabolism. Their export to the sink tissues proceeds by mass flow through the sieve tubes. The production and export of assimilates involves a co-operation of various cell types and requires several transfer steps across membranes. To understand the complex interactions between all these compartments, subcellular concentrations of intermediates and products must be known.
