Nitrogen

Question 1

What are the main functions of nitrogen in the plant?

Answer 1

1. 1-5% of the DW of plants
2. Part of the amino acids, proteins, nucleic acids, chlorophylls, phytohormones, coenzymes, vitamins, secondary metabolites
3. 25% of the photosynthesis energy is used to acquire N by the roots (most plants depend on other N compounds for their growth)
4. N is mostly acquired by plants in the nitrate (NO) and ammonium (NH) forms and for a small part as small organic molecules (e.g. amino acids, urea)

Question 2

What are the main source of N in the atmosphere, terrestrial and aquatic ecosystems?

Answer 2

• ATMOSPHERE: 99.99% N. × 0.01% N-compounds
• AQUATIC System: 95.2% N. 4,8 % N-compounds
• TERRESTRIAL SYSTEM 93,5% organic compounds 6,3% inorganic compounds

Question 3

How does nitrogen deficincy show?

Answer 3

1. N is a very mobile element
2. plants are typically stunted with narrow leaves
3. chlorosis caused by N deficiency typically begins in the older leaves as N is remobilized to younger leaves
4. At the field scale, N-deficient crops appear pale green or even yellow
5. plants suffering from N deficiency mature earlier, and the vegetative growth stage is often shortened

Question 4

What factors infulence the ammout of nitrogen in the soil solution?

Answer 4

1. Mineralization and/or humification of organic matter
2. NO₃ leaching
3. NO₃ denitrification
4. NH_⁴⁺ fixation
5. NH₃ volatilization
6. Nutritional uptake
7. Input of N fertilizers
8. Ninput from atmospheric events
9. Nitrogen fixation

Question 5

What is the HATS and what is the LATS?

Answer 5

• Plants have evolved mechanisms to modulate their N acquisition efficiency in response to availability and form of external N as well as to plant N demand during their life cycle
  1. The high-affinity transport systems (HATS) operate at low concentrations (<0.5 mM) of externalion availability
  2. At higher concentrations(>0.5 mM) uptake is primarily via the low-affinity transport systems (LATS), allowing large influxes of substrate at high substrate availability

Question 6

What type of nitrate transporters are there?

Answer 6

1. There are two types of nitrate transporters
2. NRT1 (low affinity transport) and NRT2 high affinity t.
3. NRT1 and NRT2 transport nitrate across the plasma mebrane in symport with protons which requires ATP
4. transport against an electrochemical potential gradient (because the negatively charged nitrate ion has to overcome both the negative plasma-membrane potential as well as an uphill concentration gradient)
5. Nitrate content- External 1-4nM Internal 5-30nM

Question 7

What are the machanism involved in the nitrogen uptake?

Answer 7

• The nitrate uptake system is retro-regulated
• The nitrate system is genetically and phenptypically regulated

1. internal concentration of nitrate
2. internal concentration of ammonium
3. internal concentration of amino acids (glutamine)
4. activation of efflux mechanisms

Question 8

What happens after the initial uptake of Nitrogen?

Answer 8

1. Once absorbed in the cytosol, nitrate moves radially across the different cell types of the root
2. passes the endodermal Casparian strip
3. to be transported to the shoot
4. nitrate is loaded from the symplast of the stele cells into the apoplast of the xylem for long-distance transport via the transpiration stream

Question 9

On what does the Ammonium concentration in the cytosol depend upon?

Answer 9

• Generally the concentrations of ammonium in the cytosol ranges from 1 to 30 mM
• Excessive accumulation may lead to necrosis of plant tissue

Ammonium concentration in the cytosol is a function of

1. influx into cells and efflux of ammonium to the apoplast
2. compartmentation of ammonium into vacuoles,
3. ammonium assimilation in the cytoplasm or plastids

Question 10

Can amino acids be a potential source of nitrogen?

Answer 10

1. Peptides and proteins are broken down to amino acids in the soil by proteases released by soil microorganisms
2. The concentration of free amino acids in agricultural soils is in the range of 1 to 100 uM
3. Amino acids may constitute a significant part of the N absorbed by plants in terrestrial ecosystems, especially under low N conditions and in high organic matter containing soils (cold climate)
4. Amino acids uptake by plants is in strong competition with microbes

Question 11

Urea uptake and metabolism?

Answer 11

1. In agriculture, urea is used as N fertilizer and is also a naturally occurring and readily available N-source in soils
2. Urea is hydrolysed to ammonium in the soil by the enzyme urease produced by soil microorganisms, but plants can also take up urea directly
3. most plants have a single urease gene (except soybean multible)

Question 12

What urea transporters are there at a molecular level?

Answer 12

Hight affinity transporters (HATS)

1. AtDUR3 characetrized at physiological and molecular level (Liu et al., 2003; Kojima et al., 2007)

• Identified by its similarity to urea transporter ScDUR3
• Na+ solute symporter superfamily
• present 14 transmembran domains and 694aa
• Urea/H+ symporter with a K3-4 M
• Localized at PM of N-starved roots

2. OsDUR3 recently characterized in rice

Question 13

What are the grow efffects of different N-sources?

Answer 13

Question 14

What is the fundamental difference in the metabolism of assimilated nitrate and ammonium?

Answer 14

1. Nitrate readily mobile in the xylem and can also be stored in the vacuoles of roots, shoots and storage organs ⇒to be incorporated into organic compounds, nitrate has to be reduced to ammonium (NH₄)
2. Ammonium is normally directly incorporated into organic compounds in the roots, although some NH₄₊ may also be translocated to the shoot

• Relevance of reduction and assimilation of nitrate for plants similar to reduction and assimilation of CO₂
• Nitrogen assimilation is intricately regulated (necessary to integrate environmental signals with carbon metabolism)

Question 15

How is the nitrate reduction being mediated?

Answer 15

Mediated by two enzymes:

1. nitrate reductase: catalyses the two electron reduction of nitrate to nitrite (NO_^2-)
2. nitrite reductase: transforms nitrite to ammonium in a six electron transfer process

Question 16

What is the function of the Glutamine synthease in the ammonium assimilation?

Answer 16

1. GS exists in multiple enzyme forms located in the cytosol and in plastids
2. Cytosolic GS has multiple metabolic functions such as assimilation of ammonium into glutamine for transport and distribution throughout the plant.
3. During leaf senescence cytosolic GS fulfils a key function in the assimilation and recycling of ammonium generated from various catabolic processes

Question 17

What are the two phases of continious N-deficiency?

Answer 17

First phase:

1. leaf elongation rate is reduced without affecting photosynthesis
2. root growth is maintained or even stimulated by transport of assimilated carbon to the roots (lower shoot/root biomass ratio)
3. N compounds, particularly nitrate, are mobilized in order to maintain N metabolism and the capacity to take N from the soil is increased

Second phase upon continious starvation:

1. the break-down of leaf nucleic acids and proteins is triggered
2. leaf senescence breakdown of Rubisco leads to a decrease in the maximum photosynthetic capacity of the plant
3. ultimately inhibiting whole plant growth

Question 18

How does the root sytem change in response to the N-supply?

Answer 18

1. A uniformly high nutrient supply supresses root branching
2. When N is limited, plants respond by enhancing lateral root development into N-rich patches
3. nitrate stimulates lateral root elongation
4. ammonium triggers the initiation of lateral root growth

Question 19

Are there synergies between Ammonium and Nitrate nutrition?

Answer 19

Generally:

1. plants that are adapted to soils which are acid (calcifuge species) or have a low redox potential (e.g., wetlands) have a preference for ammonium
2. plants adapted to calcareous, high pH soils (calcicole species) utilize nitrate preferentially

→highest growth rates and plant yields are obtained by combined supply of both ammonium and nitrate

Question 20

What are the up and downsides of ammonium/nitrate nutrition?

Answer 20

Ammonium is preferentially taken upwhen supplied in equimolar concentrations with nitrate, particularly when:

1. the N supply is low
2. the temperatures decrease (and below 5°C uptake of ammonium can still proceed, while that of nitrate ceases)

Nitrate has the advantage of:

1. allowing more flexible distribution of assimilation between roots and shoots
2. can be stored in higher amounts than ammonium in the vacuoles.

Question 21

How does the supply of ammonium or the supply of nitrate affect the rizosphere pH?

Answer 21

Ammonium:

• the assimilation of ammonium in roots produces about one proton per molecule of ammonium
• ammonium supply may reduce rhizosphere pH through a net excretion of protons

Nitrate:

• nitrate supply may increase rhizosphere pH through a net uptake of protons from the rhizosphere

⇒implications for the availability of other nutrients such as P and micronutrients

Question 22

What are the symptoms of ammonium toxicity?

Answer 22

• Symptom: leaf chlorosis, staunted growth and eventually necrotic leafs and plant death

Generally, compared to nitrate-fed plants,

1. there is an accumulation of ammonium ions, inorganic anions such as chloride, Sulphate and phosphate as well as of amino acids,
2. there is a reduction in the concentration of the essential cations such as K, Ca’ and Mg” as Well as organic acids such as malate

Question 23

List of all the different nitrogen fertilizers and there characteristics (N-slides page34-36)

Question 24

What is the nitrogen use efficency?

Answer 24

NUE = the ratio between the total biomass of output (e.g., grain yield) and the N input (e.g., N-supplied in fertilizers and/or residual N present in the soil).

NUE is divided into two components:

1. Nuptake efficiency (NupE; the ability of the plant to remove N from the soil) and
2. the utilization efficiency (NutE; the ability to use N to produce biomass or grain yield)

In crops, and particularly in cereals, NUE is estimated to be less than 50%. The resulting N losses from agricultural land give rise to soil and water pollution and global Warming through emissions of nitrous oxide

Question 25

Why is the NUE especially in cereals so low?

Answer 25

1. Denitrification: 10-20%
2. Runoff: 1-10%
3. Volatilization: up tp 40%
4. Leaching: 20-50%

Question 26

How could the use efficency of different N-sources be influenced?

Answer 26

1. Water soluble humic fractions can affect NO_3- uptake
2. Cadmium inhibits the induction of high affinity nitrate uptake in maize roots
3. An inadequat fe-supply can limit the aquistion of NO_3- in dicots