CAHNRS and WSU Extension

Wheat and Small Grains


Mobility of Macronutrients in Plants

  • Mobile: Nitrogen (N), Phosphorus (P), Potassium (K), Magnesium (Mg)
  • Immobile: Calcium (Ca), Sulfur (S)

Nitrogen (N)

N Functions in Plants

  • N is constituent of amino acids, proteins, nucleic acids (DNA and RNA), chlorophyll, and coenzymes
  • N promotes vigorous, succulent growth
  • Excess N can delay maturity

N Deficiency in Plants

  • N is mobile in plants, so deficiencies show up on older plant tissue first, then proceed to newer leaves
  • N deficiencies appear as chlorosis on leaves
  • For corn, chlorosis appears as an inverted “V” down the midrib
  • Other plants show general chlorosis pattern
Wheat Examples Canola Examples
Wheat_N(1) Wheat_N(2) Canola_N(1) Canola_N(2)

Consequence of Over-application of N

  • Excessive aerial shoot growth
  • Poor root and lateral shoot growth
  • Reduced carbohydrate reserves
  • Higher disease incidence
  • Higher risk to lodging
  • Poor tolerance to heat, cold, drought, traffic, and other environmental stresses

Phosphorus (P)

P Functions in Plants

  • P main function is energy storage and transfer in plants. ADP and ATP are used to store and transfer energy in plants, liberating 12,000 cal/mol.  Almost all metabolic reactions require ADP and ATP
  • P also is an important structural component of nucleic acids, coenzymes, nucleotides, etc.
  • P is considered essential for seed formation. Large quantities of P are found in seeds and fruits
  • Good P nutrition strengthens structural tissues such as wheat straw to prevent lodging

P Deficiency in Plants

  • P is mobile in plants, so deficiency symptoms occur in the older leaves first
  • Overall stunted growth
  • Purple coloring of leaves and leaf margins (it is not reliable for diagnosis because this symptom can be also caused by other stresses, such as herbicide injury and cold injury)
Wheat Examples Canola Examples 
Wheat_P(2) Wheat_P(1) Canola_P(2) Canola_P(1)

Uptake of P by plants

  • Plant roots absorb P from the soil solution. Most of the absorption occurs in two locations on the root: near the tip of the root and at the root hairs
  • Transport of P to the roots occurs by mass flow and diffusion

Potassium (K)

K Functions in Plants

  • K is essential to photosynthesis because involved in:
    • ATP synthesis
    • Production and activity of specific photosynthetic enzymes
    • CO2 absorption through leaf stomates
    • Maintenance of electroneutrality during photophosphorylation in chloroplasts
  • K is involved in activation of more than 80 enzymes
  • K provides the osmotic “pull” that draws water into roots
  • Sufficient K improves the quality of fruits and vegetables
  • No organic compound of K in plants. K exists solely as K+ in solution or bound to negative charges on tissue surfaces

K Deficiency in Plants

  • K is mobile in plants, so deficiency symptoms occur in the older leaves first. Exception: K deficiency can occur in new leaves of fast‑maturing crops (e.g. cotton and wheat)
  • Wilting, yellow leave margins (chlorosis) that often become necrotic (dying).
  • Weak root system
  • Slow growth
  • Decreased tillering
  • Brittle stalks, result in plant lodging
 Wheat Example  Canola Examples
Wheat_K(1) Canola_K(1) Canola_K(2)

Sulfur (S)


S Functions in Plants

  • S is an essential component of S-containing amino acids: cystine, cysteine, and methionine
  • S is needed for synthesis of chlorophyll and coenzyme A, is important for oxidation and synthesis of fatty acids and amino acids
  • S is a component of ferredoxins, an Fe‑S protein in chloroplasts. Ferredoxin is important in NO2 and SO42- reduction and N2 assimilation by root nodule bacteria
  • S is responsible for the characteristic taste and smell of mustard and onion plants

S Deficiency in Plants

  • Adequate S improves crop quality for ruminants by narrowing N/S ratio to 9:1 to 12:1 needed for effective use of N by rumen microorganisms
  • S deficient plants produce less protein and accumulate nonprotein N as NH2 and NO3 → leaf NO3 accumulates under S deficiency reducing food quality
Wheat Examples Canola Examples
Wheat_S(1) Wheat_S(2) Canola_S(1) Canola_S(2)

Calcium (Ca)


Ca Functions in Plants

  • Ca is essential for cell elongation and division
  • Ca is important for structure and permeability of cell membranes
  • Ca enhances uptake of NO3 and, thus, influences N metabolism
  • Ca regulates cation uptake. For example, K+ = Na+ uptake under low Ca2+ , but K+ >> Na+ uptake under adequate Ca2+

Ca Deficiency in Plants

  • Ca2+ deficiency results in poor development of terminal buds of shoots and apical tips of roots (e.g. no plant growth)
  • Low Ca2+ weakens cell membranes, increasing cell wall permeability, resulting in loss of cell contents
  • Ca uptake is depressed by high concentrations of: K, Mg, Mn, and Al
    Wheat Example     Canola Example
Wheat_Ca Canola_Ca

Ca and Soil Structure

  • Ca helps to keep clay particles in soils flocculated, and creates good soil structure. Cations, mainly Al, Fe and Ca, create bridges between negatively charged clay particles
  • If Na is the dominant cation, the force of the monovalent Na cation is not sufficient to create a bridge between clay particles, and particles do not flocculate well resulting in poor soil structure

Magnesium (Mg)


Mg Functions in Plants

  • Mg2+ is a primary constituent of chlorophyll, comprising about 15-20% of total Mg2+. Without chlorophyll, photosynthesis would not take place
  • Mg is a structural component in ribosomes, which are important for protein synthesis. Under Mg2+ deficiency, protein N decreases and non-protein N increases in plants
  • Mg is required for normal function of nearly every phosphorylating enzyme in carbohydrate metabolism. Most energy transfer reactions involving ATP require Mg2+.  Since the fundamental process of energy transfer occurs in photosynthesis, glycolysis, the Krebs cycle, and respiration, Mg2+ is important throughout plant metabolism

Mg deficiencies are likely to occur with these conditions

Most forage legumes have a high demand for Mg compared with grass crops. Usually needs the following factors and a crop that has a high demand for Mg:

  • Acidic, sandy, highly leached soils with low CEC
  • Calcareous soils with low Mg levels
  • Acidic soils receiving high rates of lime with low Mg content
  • High rates of K or ammonium fertilization (maybe)
    Wheat Example
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