Macronutrients: Functions; Deficiency & Toxicity Symptoms (Lecture 15)

B.  Phosphorus (P):

1) Soil Relations

  – Mineral apatite [Ca5F(PO4)3]

– Relatively stable in soil

– Has a low mobility

  2) Plant Functions

#Component of nucleic acid (DNA, RNA), phospholipids, coenzymes, high-energy phosphate bonds  (ADP,  ATP)

#Seeds are high in P

#Stimulates early growth and root formation

#Hastens maturity

#Promotes seed production and makes plants hardy.

Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F−and Cl− ions, respectively, in the crystal.

3) Deficiency and Toxicity

P is mobile in plant tissues (Deficiency occurs in

older leaves) 

Deficiency:

vdark, purplish color on older leaves

vSmall root growth,

vSpindly stalk,

vDelayed maturity,

vDying of tips of older leaves

vPoor fruit and seed development

Excess P:

causes deficiency symptoms of Zn, Cu, Fe, Mn

 

4) Fertilizers

  – Superphosphates

Single superphosphate (8.6% P)

Triple superphosphate (20% P):

– Ammonium phosphate:  (NH4)2PO4, NH4HPO4

– Bone meal

– Available forms:  PO43-,  HPO42-,  H2PO4-

P absorption influenced by pH

C.  Potassium (K):

1) Soil Relations

  – Present in large amounts in mineral soil

– Low in organic soils

  2) Plant Functions

  – Activator of many enzymes

– Regulation of water movement across membranes and through stomata  (Guard cell functions)

– Improves plant’s ability to resist disease and cold,

– aids in the production of carbohydrates.

 

3) Deficiency and Toxicity

  Deficiency:

– Leaf margin necrosis and browning

– Older leaves are more affected

          – Slow growth,

– Margins on leaves develop a scorched effect

starting on the older leaves,

– Weak stalk,

– Shriveled seed or fruit.

  Toxicity:  Leaf tip and marginal necrosis

4) Fertilizers

  – Potassium chloride (KCl)

– Potassium sulfate (K2SO4)

– Potassium nitrate (KNO3)

D.  Magnesium (Mg)

1) Soil Relations

  – Present in soil as an exchangeable cation (Mg2+)

– Similar to Ca2+ as a cation

  2) Plant Functions

  – Core component of chlorophyll molecule

– Catalyst for certain enzyme activity

– aids in the translocation of starch within the plant,

– essential for formation of oils and fats

Effects of Mg deficiency on photosystems:

Visual symptoms: Brown spottings and necroses appear when deficiency is severe or under high light intensity.

• The behavior of PSII and PSI was assessed using direct and modulated fluorescence measurement.
• An early effect of Mg deficiency on PSI oxidation rate could be identified before any decrease in the rate of PSII electron transport or any chlorophyll degradation in sugar beet leaves.
• Simultaneously, sucrose accumulates in source leaves upon Mg deficiency as an early response.
• High sucrose levels are known to down-regulate the chloroplast electron chain transport.
• The delay of photosynthesis regulation in Mg-deficient plants may provoke an unbalance between light and dark reactions, so inducing oxidative stress and further leading to necrogenesis on leaves.

3) Deficiency and Toxicity

  – Deficiency: 

vInterveinal chlorosis on mature leaves

vleaves abnormally thin

vtissue may dry and die

vleaves have tendency to curve upward

(Mg is highly mobile)

  – Excess:  Causes deficiency symptoms of Ca, K

  4) Fertilizers

  – Dolomite (mixture of

CaCO3·MgCO3)

– Epsom salt (MgSO4)

– Magnesium nitrate [Mg(NO3)2]

– Magnesium sulfate (MgSO4)

 

 

E.  Calcium (Ca):

1) Soil Relations

  – Present in large quantities in earth’s surface (~1% in top soils)

– Influences availability of other ions from soil 

  2) Plant Functions

  – essential to healthy cell walls and root structure.

– Involved in cell membrane function

– Aids in the movement of carbohydrates in plants

– Largely present as calcium pectate in middle lamella

Calcium pectate is immobile in plant tissues which give stability to plant cell walls.   

3) Deficiency and Toxicity

  – Deficiency symptoms in young leaves and new shoots (Ca is immobile)

• Stunted growth,
• Leaf distortion,
• Margins of younger leaves scalloped
• Necrotic spots,
• Shoot tip death
• Blossom-end rot in tomato
• Blossoms shed prematurely
• Weak stalk or stem structure.
• Terminal bud dies under severe deficiency

 

– No Ca toxicity symptoms have been observed

4) Fertilizers

  – Agricultural meal (finely ground CaCO3·MgCO3)

– Lime (CaCO3), Gypsum (CaSO4)

– Superphosphate

– Bone meal

-organic P source

 

F.  Sulfur (S):

1) Soil Relations

  – Present in mineral pyrite (FeS2), sulfides (S-mineral complex),   sulfates (involving SO4-2)

– Mostly contained in organic matter

– Acid rain provides sulfur

  2) Plant Functions

  – Aids in the formation of oils and parts of protein

– Component of amino acids (methionine, cysteine)

– Constituent of coenzymes and vitamins

– Responsible for pungency and flavor (onion, garlic, mustard)

 

3) Deficiency and Toxicity

  – Deficiency:  

• light green or yellowing on new growth
•     (S is immobile)
• In some plants older tissue may be affected also
• Small spindly plants
• retarded growth
• delayed maturity.
• Interveinal chlorosis on corn leaves

  – Toxicity: not commonly seen

 

4) Fertilizers

  – Gypsum (CaSO4)

– Magnesium sulfate

(MgSO4)

– Ammonium sulfate

[(NH4)2SO4]

– Elemental sulfur (S)