Nitrogen assimilation
Nitrogen assimilation is the formation of organic
Nitrogen assimilation in plants
Plants absorb nitrogen from the soil in the form of nitrate (NO3−) and ammonium (NH4+). In aerobic soils where nitrification can occur, nitrate is usually the predominant form of available nitrogen that is absorbed.[1][2] However this is not always the case as ammonia can predominate in grasslands[3] and in flooded, anaerobic soils like rice paddies.[4] Plant roots themselves can affect the abundance of various forms of nitrogen by changing the pH and secreting organic compounds or oxygen.[5] This influences microbial activities like the inter-conversion of various nitrogen species, the release of ammonia from organic matter in the soil and the fixation of nitrogen by non-nodule-forming bacteria.
Ammonium ions are absorbed by the plant via
Nitrate reduction is carried out in two steps. Nitrate is first reduced to nitrite (NO2−) in the cytosol by nitrate reductase using NADH or NADPH.[7] Nitrite is then reduced to ammonia in the chloroplasts (plastids in roots) by a ferredoxin dependent nitrite reductase. In photosynthesizing tissues, it uses an isoform of ferredoxin (Fd1) that is reduced by PSI while in the root it uses a form of ferredoxin (Fd3) that has a less negative midpoint potential and can be reduced easily by NADPH.[13] In non photosynthesizing tissues, NADPH is generated by glycolysis and the pentose phosphate pathway.
In the chloroplasts,
pH and Ionic balance during nitrogen assimilation
Every nitrate ion reduced to ammonia produces one OH− ion. To maintain a pH balance, the plant must either excrete it into the surrounding medium or neutralize it with organic acids. This results in the medium around the plants roots becoming alkaline when they take up nitrate.
To maintain ionic balance, every NO3− taken into the root must be accompanied by either the uptake of a cation or the excretion of an anion. Plants like tomatoes take up metal ions like K+, Na+, Ca2+ and Mg2+ to exactly match every nitrate taken up and store these as the salts of organic acids like
Plants that reduce nitrates in the shoots and excrete alkali from their roots need to transport the alkali in an inert form from the shoots to the roots. To achieve this they synthesize malic acid in the leaves from neutral precursors like carbohydrates. The potassium ions brought to the leaves along with the nitrate in the xylem are then sent along with the malate to the roots via the phloem. In the roots, the malate is consumed. When malate is converted back to malic acid prior to use, an OH− is released and excreted. (RCOO− + H2O -> RCOOH +OH−) The potassium ions are then recirculated up the xylem with fresh nitrate. Thus the plants avoid having to absorb and store excess salts and also transport the OH−.[18]
Plants like castor reduce a lot of nitrate in the root itself, and excrete the resulting base. Some of the base produced in the shoots is transported to the roots as salts of organic acids while a small amount of the carboxylates are just stored in the shoot itself.[19]
Nitrogen use efficiency
Nitrogen use efficiency (NUE) is the proportion of nitrogen present that a plant absorbs and uses. Improving nitrogen use efficiency and thus fertilizer efficiency is important to make agriculture more sustainable, can increase NUE.
Nitrogen use efficiency can be measured at various levels: the crop plant, the soil, by fertilizer input, by ecosystem productivity, etc.[24] At the level of photosynthesis in leaves, it is termed photosynthetic nitrogen use efficiency (PNUE).[25][26]
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