Nitrate and Ammonia Assimilation by Plants
Nitrogen fixation is confined to selected microbes and plants. But all plants require nitrogen because it has a role to play in the general metabolism. Therefore, plants which do not fix nitrogen, use other combined nitrogen sources such as nitrate and ammonia for carrying on metabolic activity.
Nitrate is absorbed by most plants and reduced to ammonia with the help of two different enzymes. The first step conversion of nitrate to nitrite is catalyzed by an enzyme called nitrate reductase. This enzyme has several other important constituents including FAD, cytochrome, NADPH or NADH and molybdenum.
NO3- + NADH + H+ → NO2- + NAD+ + H2O
The overall process of nitrate reduction take place in the cytosol and is an energy Plants and animals dependent reaction. The enzyme nitrate reductase has been studied in many plants and it is observed that the enzyme is continuously synthesized and degraded. The enzyme nitrate reductase is inducible. This means that increase in nitrate concentration in the cytosol induces more of nitrate redutase to be synthesized.
However, when excess NH4+ is produced then it has a negative effect on the synthesis of nitrate reductase. In plants, it has also been observed that light also increases nitrate reductase when nitrate is available.
In the second step the nitrite so formed is further reduced to ammonia and this is catalyzed by the enzyme nitrite reductase. Nitrite present in the cytosol is transported into chloroplast or plastids where it is reduced to ammonia.
NO2- + 3NADPH + 3H+ → NH3 + 3NADP+
The enzyme nitrite reductase is able to accept electrons from sources such as NADH, NADPH or FADH2. Besides, reduced ferredoxin has also been shown to provide electrons to nitrite reductase for reducing nitrite to ammonia. Ammonia so formed has to be utilized quickly by plants because accumulation of ammonia has a toxic effect. Some plants including algae leach out excess ammonia which can further be oxidized to nitrite and nitrate by microorganisms in the soil or water.