N and C metabolism are closely intertwined. It starts with photosystem I (PSI) when a surplus of electrons to ferrodoxin not only reduces NADP to NADPH, which is used for C02 assimilation, but is also used for nitrate and sulfate reduction (Fig. 2.3.7A). This is important in the N and S cycles (see Chap. 18.104.22.168 and 22.214.171.124).
The main processes involved in the interaction between N and C cycles are (Fig. 2.3.7B):
• production of carbohydrates for root maintenance and growth;
• production of C skeletons for N assimilation;
influence of N-nutrition on protein metabolism;
influence of nitrate on the phytohormone balance (especially cytokinin); feedback coupling of protein availability and phytohormones on photosynthesis (30% of the N content of a leaf is in the C02-fixing enzyme Rubisco; see Chap. 126.96.36.199) and growth (principally of leaves); regulation of C-allocation by metabolites of the C cycle and nitrate; development is limited by self-shading.
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