Phosphorus

Question 1

What can you say about phosphorus in the soil?

Answer 1

• P Total = 100 - 3000 mg Kg^-1 (0.02 - o.15%)
• P in solution 0.01 -3.0 mg L^-1
• From 20 to 80% of total P in the soil is present as organic forms

In the soil is present as:

1. Ionic inorganic forms or organic compounds in solution
2. Adsorbed on the surfaces of inorganic constituents
3. Crystalline and amorphous P minerals
4. Part of the organic matter

Question 2

How does the P-influx into the root cells work?

Answer 2

1. P is strongly bound to the soil solution
2. Absorbed as H₂PO₄
3. Cotransport with H⁺: 2-4 H+ per H₂PO₄^-/HPO₄^2-
4. Genes PT1 and PT2 encoding high-affinity P-transporters (Km 1-5mM)
5. Expressed preferentially in root epidermis and root hairs

Question 3

What is the P requiremnt for optimal grwoth?

Answer 3

• The P requirement for optimal growth is in the range of 3 to 5 mg g^-1 dw (0.3-0.5%) during the vegetative stage of growth

Question 4

What happens to the phosphate after the uptake?

Answer 4

Phosphate either remains:

1. as inorganic phosphate (Pi)
2. it is esterified through a hydroxyl group to a carbon chain (C-O-P) as a simple phosphate ester (e.g., sugar phosphate) or attached to another phosphate by the energy-rich pyrophosphate bond (P)~(P) (e.g., in ATP)
3. as the relatively stable diester (C-(P)-C) (in this association phosphate forms a bridging group connecting units to more complex or macromolecular structures)

Question 5

What is the function of phosphate in macromolecular stuctures?

Answer 5

• In both DNA and RNA, phosphate forms a bridge between ribonucleoside units to form macromolecules
• the bridging form of P diester is also abundant in phospholipids of biomembranes

Question 6

What are phospahte esters?

Answer 6

1. Most phosphate esters are intermediates in metabolic Pathways of biosynthesis and
2. phosphate esters (C-(P)) and energy-rich phosphates (P)-(P)) represent the metabolic energy of cells
3. Up to 50 esters formed from phosphate and sugars and alcohols have been identified
4. glucose-6-phosphate and phosphoglyceraldehyde are most abundant

Question 7

What´s the role of phosphorous in the energy transfer?

Answer 7

1. The energy required for biosynthesis of starch or ion uptake is supplied by an energy-rich intermediate or coenzyme, predominantly ATP
2. This energy can be transferred with the phosphoryl group in a phosphorylation reaction to another compound which results in the activation (priming reaction) of this compound

Question 8

What is the ATPases?

Answer 8

1. The ATPases is mediating the hydrolysis and thus the energy transfer
2. It is affected by other nurtrients like Mg, Ca and K

Question 9

Where is P stored and compartmentalized?

Answer 9

1. the vacuole acts as storage pool, or non-metabolic pool, of P
2. at adequate P supply ca. 85-95% of the total P of the cell is located in the vacuoles as Pi
3. In contrast, in leaves of P-deficient plants most Pi is found in the cytosol and chloroplasts, i.e. in the metabolic pool

Question 10

When is Pi released from the vacuoles into the cytosol?

Answer 10

As P is in many enzyme driven reactions either a substrate or an end-product (e.g., ATP-ADP + Pi) controlling some key enzyme reactions, its compartmentation is therefore essential for the regulation of metabolic pathways in the cytosol and chloroplasts

Question 11

What are symptoms of P deficiency?

Answer 11

1. Plants are small and stunted
2. Young plants have a bluish green color in the early stages of growth
3. Generally the symptoms of P deficiency appear in the older leaves which are often of a darkish green color
4. The stems of many annual plant species suffering from P deficiency are characterized by a reddish coloration originating from an enhanced formation of anthocyanins
5. P is mobile in the plant and moves to the growing points; therefore, leaf deficiency symptoms will start on older leaves and move up the shoot as deficiency intensifies
6. In cereals tillering is decreased
7. Fruit trees show reduced growth rates of new shoots and flower initiation is impaired
8. Fruit and seed formation is especially depressed in plants suffering from deficiency

Question 12

What are the major problems for the plant growth related to P?

Answer 12

1. most of the phosphate present in soil is not accessible for the plants
2. in ¹/₄ of agricultural grounds, phosphate is the limiting growth factor
3. ³/₄ of the phosphate used in the fertilizer is not accessible for the plants
4. reserves of “cheap” phosphate fertilizer will be exhausted in 50-80 years

Question 13

What is the plant´s response to limited P availability in the soil?

Answer 13

Morphological changes:

1. Root architecture and root hair growth
2. Mycorrhizal association

Physiological changes:

1. Rhizospheric pH decrease
2. Release of organic acids and flavonoids
3. Release of P-hydrolytic enzymes (phytase, phosphatase, RNase)
4. Synthesis of Pi transporters

Question 14

How does the root architecture change as a reaction to P deficiency?

Answer 14

1. more aerenchyma
2. more adventitious roots
3. shallower basal roots
4. more dispersed laterals
5. more taproots laterals
6. longer, denser hairs

Question 15

What do we have to take in cosideration when deciding on Phosphate application?

Answer 15

1. Up to 90% of applied phosphate (P) fertilizer goes unused in the year of application as it gets tied up or bound to soil particles and other elements,
2. Some of this is used over subsequent years, but at least 25% never becomes available

→It is crucial to make the most efficient use of fertilizer phosphate to maximize yields

Question 16

When should Phosphate fertilization be best carried out?

Answer 16

1. because phosphate is relatively immobile in the soil and rapidly becomes unavailable, phosphate applications are often best timed at seeding, with placement close to or with the seed

Question 17

What should be taken into consideration when applying seed placed phosphate?

Answer 17

1. Seed-placed phosphate Phosphate fertilizer placed with the seed provides early season access to plant available phosphate.
2. However, seed safety can be a concern (if phosphate fertilizer levels are too high, they can reduce germination and emergence)

Question 18

What are different application methods for phosphate?

Answer 18

1. Side banded
2. banded
3. Broadcast application ( poorest methods)

• a) broadcast unicorporated
• b) broadcast incorporated

Question 19

Is foliar application of P an option?

Answer 19

1. Only a limited amount of phosphate can be absorbed by plant foliage and there is an increased risk of burning
2. so foliar application of phosphate should only occur if the plant is severely deficient,