文档介绍：Part 3 Plant Mineral Nutrition
Overview of nitrogen in the biosphere and in plants
The nitrogen cycle
Nitrogen deficiency phenotype in leaves of sugar beet. Nitrogen deficiency is often associated with uniform yellowing of older leaves.
Overview of N uptake by a nonnodulated plant (left), and by a nodulated plant with N-fixing symbionts (right).
Table Rates of natural and anthropogenic nitrogen fixation
Source Amount of N fixed
Lighting <10 Tg/year
Biological N-fixation in terrestrial systems 90-140 Tg/year
Biological N-fixation in marine systems 30-300 Tg/year
N fertilizer synthesis 80 Tg/year
Fossil bustion >20 Tg/year
* Tg =1012 g.
Biological nitrogen fixation
Nitrogen fixation reduces nitrogen gas to ammonia, at the cost of ATP and reducing equivalent.
N2 + 16 ATP + 8 e - + 8 H→ 2 NH3 + H2 + 16 ADP + 16 Pi
Eukaryotes cannot utilize dinitrogen, but some prokaryotes are able to catalyze the enzymatic reduction of pound to ammonia.
Nitrogen fixation is sensitive to oxygen.
Enzymology of nitrogen fixation
Table Substrates and products of nitrogenase
Substrate Product
N2 NH3
H+ H2
N2O N2, - NH3, CH4
C2H2 C2H4, C2H6
Symbiotic nitrogen fixation
Some vascular plants establish nitrogen-fixing symbioses
There are three major types of nitrogen-fixing symbioses.
1) Gram-negative bacteria, the rhizobia, form associations with numerous legume host plants (Fabaceae) and at least one non legume, Parasponia(Ulmaceae).
2) Gram-positive actinomycete genus ( Frankia) and a diverse group a dicots, generally trees or woody shrubs from about 60 genera in 9 families, including alder(Alnus), myrtle(Myrica), Casuarina, and Ceanothus.
3) Symbioses exist between cyanobacteria and a diverse array of plants: dicots (.,Gunnera), cycads, ferns,liverworts, and hornworts. Azolla, a water fern , associates symbiotically with the cyanobacterium .