How Plants Uptake Nitrogen?

Forms of Nitrogen Available to Plants

Plants have evolved remarkable adaptability in acquiring nitrogen, capable of utilizing various forms present in their environment:

Inorganic Forms

• Nitrate (NO₃^-): The predominant form of nitrogen taken up by plants, especially in agricultural soils. Nitrate is highly mobile in the soil and readily available for root absorption [Miller & Cramer, 2005].
• Ammonium (NH₄⁺): Another form of nitrogen absorbed by plants, although its uptake is typically less prevalent than nitrate. Ammonium binds to soil particles and is less prone to leaching [Britto & Kronzucker, 2002].
• Nitrite (NO₂^-): A less common form of nitrogen absorbed by plants, nitrite can be converted to nitrate or ammonium in the soil or within the plant itself [Crawford & Glass, 1998].

Organic Forms

• Urea (CO(NH₂)₂): A widely used fertilizer, urea serves as a readily available organic nitrogen source. It can be directly absorbed or broken down into ammonium in the soil [Havlin et al., 2014].
• Amino Acids: Plants can absorb certain amino acids through the leaves and directly from the soil, especially in environments with high microbial activity [Marschner, 2012]. Some of the commonly absorbed amino acids include: Glycine, Glutamic acid, Alanine, Proline, Arginine, Aspartic acid, Phenylalanine.
• Hydrolysates: Hydrolysates are products of protein hydrolysis, containing a mixture of amino acids and peptides. They can be applied to the soil or directly to the leaves of plants, providing a readily available source of nitrogen [Shaviv, 2000].
• Other Organic Compounds: Plants can also absorb nitrogen from other organic compounds, such as nucleic acids, chlorophyll, and heme, which are released into the soil by decomposing plant and animal matter [Brady & Weil, 2008].

Mechanisms of Uptake

Root Uptake: The Primary Pathway

Nitrogen is primarily absorbed by plants through their root systems, mainly through the root hairs, which significantly increase the surface area available for nutrient uptake. These tiny, filamentous extensions of the root cells significantly increase the surface area available for nutrient uptake, facilitating efficient contact with the surrounding soil particles and the nitrogen compounds they contain. This intimate contact allows the transfer of nitrogen ions from the soil into the root cells [Marschner, 2012]. A well-developed root system is crucial for efficient nitrogen absorption, especially as plants grow and their nitrogen demands increase.

Steps Involved in Root Uptake:

1) Root Hair Absorption: Nitrogen ions, such as nitrate (NO₃^-) and ammonium (NH₄⁺), diffuse from the soil solution into the root hair cells. These cells possess specialized transport proteins, including nitrate transporters and ammonium transporters, that actively facilitate the movement of nitrogen ions across the cell membrane [Crawford & Glass, 1998].

2) Root Cell-to-Cell Transport: Once inside the root hair cells, nitrogen ions are transported to the root cortex, where they may undergo further transformations or be loaded into the vascular tissues for transport to other parts of the plant. This transport occurs through a combination of diffusion and active transport mechanisms, involving various transporter proteins and channels located in the cell membranes [Taiz & Zeiger, 2010].

It's worth noting that while the majority of nitrogen absorption happens through root hairs, even in the absence of prominent root hairs, other epidermal cells can contribute to nitrogen uptake through several mechanisms:

• Direct Absorption: The epidermal cells themselves possess transporter proteins in their plasma membranes, enabling them to directly absorb nitrate and ammonium ions from the soil solution. Though they lack the extensive surface area of root hairs, the combined action of numerous epidermal cells can still lead to substantial nitrogen uptake.
• Lateral Movement: If nitrogen is absorbed by root hairs or other specialized epidermal cells at a certain point, it can move laterally through the apoplast (cell walls and intercellular spaces) or symplast (cytoplasm connected via plasmodesmata) to reach nearby epidermal cells. This lateral movement effectively expands the zone of nutrient uptake beyond the immediate vicinity of the specialized absorbing cells.
• Root Tip Absorption: The root tip, where cell division and growth are active, is another important site for nutrient uptake, including nitrogen.
  The newly formed epidermal cells in this region are often more permeable and can readily absorb nutrients from the soil.
• Mycorrhizal Associations: Many plants form symbiotic relationships with mycorrhizal fungi, which effectively extend the root system and enhance nutrient uptake, including nitrogen. The fungal hyphae can access nitrogen from soil zones beyond the reach of the plant's roots and transfer it to the epidermal cells.

The relative contribution of these different mechanisms varies depending on the plant species, its developmental stage, and environmental conditions. However, they collectively demonstrate the adaptability of plant roots in acquiring essential nutrients like nitrogen even in the absence of prominent root hairs.

Factors Governing Nitrogen Uptake

• Soil pH: The pH of the soil significantly impacts the availability of different nitrogen forms. Ammonium uptake is favored in slightly acidic soils, while nitrate is more readily absorbed in neutral to slightly alkaline conditions.
• Soil Moisture: Adequate moisture is essential for nutrient transport, facilitating the movement of nitrogen ions to the root surface.
• Plant Species and Growth Stage: Different plants exhibit varying nitrogen requirements and uptake capacities. Additionally, young, actively growing plants typically absorb nitrogen more rapidly than mature ones.

Leaf Absorption: The Supplemental Route

In addition to root uptake, plants can also absorb nitrogen through their leaves, a process known as foliar absorption. This pathway is especially crucial for plants growing in nutrient-deficient soils or when root growth is restricted. The process of foliar nitrogen absorption in crops involves several key steps:

1) Deposition and Retention: The first step is the successful deposition and retention of the nitrogen-containing solution onto the leaf surface. Factors influencing this include droplet size, leaf surface characteristics (e.g., waxiness, hairiness), and environmental conditions (e.g., wind, humidity) [Fernández & Eichert, 2009].
2) Penetration: Once deposited, the nitrogen compounds need to penetrate the leaf surface. This can occur through two main pathways [Fernández & Eichert, 2009]:

• Cuticular Pathway: The cuticle is a waxy layer on the leaf surface that acts as a barrier. However, some nitrogen compounds can directly penetrate the cuticle through diffusion or by dissolving in the cuticular wax [Fernández & Eichert, 2009].
• Stomatal Pathway: Stomata are tiny pores on the leaf surface primarily involved in gas exchange. Some nitrogen compounds can enter the leaf through these pores, although this pathway is generally considered less significant for foliar absorption compared to the cuticular pathway [Fernández & Eichert, 2009].

3) Absorption and Transport: Once inside the leaf, the nitrogen compounds are absorbed by the leaf cells. This process may involve active transport mechanisms, requiring energy expenditure by the plant [Fernández & Eichert, 2009]. After absorption, the nitrogen is transported to other parts of the plant via the phloem, where it can be utilized for various metabolic processes, such as protein synthesis and growth [Fernández & Eichert, 2009].

Factors Affecting Foliar Absorption

Nitrogen Form: Different forms of nitrogen (e.g., urea, ammonium, nitrate) have varying absorption rates. Urea is generally considered to be readily absorbed, while the absorption of ammonium and nitrate can be influenced by factors such as pH and the plant's nutritional status [Fernández & Eichert, 2009].
Leaf Age and Surface Characteristics: Young leaves generally have a thinner cuticle and are more permeable to foliar-applied nutrients compared to older leaves [Fernández & Eichert, 2009]. Leaf surface characteristics, such as the presence of hairs or wax, can also affect absorption [Fernández & Eichert, 2009].
Environmental Conditions: Environmental factors, such as temperature, humidity, and light intensity, can significantly influence foliar absorption. High humidity and moderate temperatures generally favor absorption [Fernández & Eichert, 2009].