Nitrogen is an element fundamental to all life, forming the backbone of many biological structures. Plants require significant amounts of nitrogen for healthy growth and reproduction. Plants absorb nitrogen from the soil primarily as nitrate and ammonium, the ionized, water-soluble form of ammonia. The direct answer to whether plants use ammonia is yes, but they must manage the uptake and internal conversion of these sources efficiently.
The Plant’s Fundamental Need for Nitrogen
Nitrogen is a constituent of the most basic building blocks of a plant cell, accounting for up to six percent of a plant’s total dry weight. It is an essential component of all amino acids, which link together to create proteins. These proteins serve as structural components within cells and as enzymes that catalyze nearly all biochemical reactions necessary for life.
Beyond proteins, nitrogen is also a necessary part of nucleic acids, specifically DNA and RNA, which hold the genetic code and direct the processes of growth and development. Nitrogen is central to the structure of the chlorophyll molecule. Without sufficient nitrogen, a plant cannot effectively perform photosynthesis, which impacts the creation of the carbohydrates needed for energy and structure.
Ammonia Versus Nitrate as Nitrogen Sources
Plants can absorb nitrogen from the soil in two main inorganic forms: the positively charged ammonium ion (\(\text{NH}_4^+\)) and the negatively charged nitrate ion (\(\text{NO}_3^-\)). The choice between these two forms represents a trade-off between energy expenditure and toxicity risk. Ammonium is already in a reduced chemical state, meaning plants can incorporate it directly into organic molecules with minimal metabolic effort.
Nitrate, however, is in a highly oxidized state and requires a two-step enzymatic reduction process to convert it into ammonium before it can be used. This conversion demands energy from the plant. Environmental conditions heavily influence which form is more readily available; for instance, ammonium tends to be the dominant form in acidic or waterlogged soils, while nitrate is more prevalent in well-aerated, neutral to alkaline soils. Furthermore, the uptake of ammonium by roots causes the release of protons (\(\text{H}^+\)) into the surrounding soil, which tends to lower the \(\text{pH}\) in the immediate root zone.
Assimilating Ammonia Inside Plant Cells
Once the ammonium ion is absorbed by the roots, or is generated internally from the reduction of nitrate, it must be rapidly converted into organic compounds. This immediate conversion is necessary because free ammonium is toxic to plant cells. The primary mechanism for this detoxification and assimilation is the Glutamine Synthetase/Glutamate Synthase (GS/GOGAT) cycle.
The first enzyme in this pathway, Glutamine Synthetase (GS), catalyzes the reaction that combines ammonium with the amino acid glutamate, using energy from ATP to form glutamine. This step efficiently traps the ammonium into a usable organic compound. The second enzyme, Glutamate Synthase (GOGAT), then transfers the nitrogen group from glutamine to a molecule of 2-oxoglutarate.
This transfer reaction produces two molecules of glutamate. One molecule is used to restart the GS cycle, and the other is a building block for the synthesis of all other amino acids and nitrogen-containing compounds. The entire GS/GOGAT cycle ensures that the toxic ammonium is quickly sequestered and incorporated into the first stable organic compounds, which can then be transported throughout the plant.
The Dangers of Ammonia Overload
Although ammonium is an energetically efficient form of nitrogen for assimilation, its concentration must be controlled to prevent toxicity. Ammonia toxicity occurs when the rate of ammonium uptake exceeds the plant’s capacity to assimilate it via the GS/GOGAT cycle. This excess assimilation process generates protons, leading to an internal acidification stress within the plant cell.
High ammonium levels can also disrupt the uptake of other positively charged nutrients, such as calcium and magnesium, leading to nutrient deficiencies. Symptoms of ammonia toxicity include a restriction in root growth, which impairs the plant’s ability to absorb water and nutrients. Above-ground, symptoms manifest as chlorosis, or the yellowing of leaves, and a general stunting of the plant’s overall growth. This condition is often exacerbated by cool soil temperatures, which slow down the microbial conversion of ammonium to nitrate in the soil, leaving more ammonium available for plant uptake.