AGR6422C_2_Phosphorus-(P)_description

Question 1

Where does the plant typically find phosphorus?

Answer 1

In the soil, though it can also be found in organic matter.

Question 2

How does phosphorus naturally get into the soil?

Answer 2

As a result of weathering of primary and secondary minerals.

Question 3

In what form is phosphorus primarily taken up?

Answer 3

Phosphate anion (H2PO4^-), though to a lesser extent as HPO4^2-.

Question 4

What are two methods used by the plant to facilitate P uptake?

Answer 4

They:

1. Form mycorrhizal associations with fungi
2. Release organic chelating acids to increase soil available P

Question 5

Is phosphate concentration in the root cell cytosol higher or lower than phosphate concentration in the soil solution?

Answer 5

Phosphate concentration in the root cell cytosol is about 1000 times greater than in the soil solution.

Question 6

I uptake of phosphate active or passive?

Answer 6

It is active, and mediated by H+ cotransport.

Question 7

Phosphate is found in what three forms inside the plant?

Answer 7

1. Inorganic phosphate (Pi)
2. Esterified (C-O-P) as a simple phosphate like sugar phosphates
3. Attached to another phosphate by the energy rich pyrophosphate bond (i.e., ATP)

Question 8

How is phosphate transported once within the plant from the soil?

Answer 8

It is loaded into the xylem and transported to transpiring tissues in the canopy.
It is also readily mobile in the phloem, so it can be translocated from older leaves to newer leaves and from vegetative plant tissues to seeds.

Question 9

Is P mobile or immobile within the phloem?

Answer 9

It is mobile in the phloem and can be translocated from older leaves to newer leaves and from vegetative plant tissues to seeds.

Question 10

What are the three general functions of phosphorus in the plant?

Answer 10

1. Structural elements
2. Energy transfer
3. Metabolic regulatory role

Question 11

In what two cell structures is phosphorus a structural constituent?

Answer 11

1. Nucleic acids

2. Cell membranes

Question 12

What is the role of phosphoric nucleic acids in DNA and RNA?

Answer 12

It forms a bridge between the ribose units, creating the phosphate deoxyribose backbone.

Question 13

What is the role of phosphorus in cell membranes?

Answer 13

It is found as a constituent of the phospholipid bilayer.

Question 14

What is the role of phosphorus in energy transfer?

Answer 14

Phosphorus is found in phosphate esters (C-P) and energy rich phosphates (P~P), which are responsible for the metabolic energy of cells, i.e., adenosine triphosphate (ATP).

Question 15

What percentage of the total P in the cell is held as non-metabolic P in storage vacuoles in the cytosol and chloroplasts under optimal P nutrition?

Answer 15

95%.

Question 16

In P-deficient plants, where can most of the P be found?

Answer 16

In the cytosol and the chloroplasts.

Question 17

What are symptoms of P deficiency?

Answer 17

1. Older leaves first
2. Purpling on leaf (sometimes)
3. Reduced leaf expansion (small leaves)
4. Reduced leaf number
5. Decrease in shoot:root ratio
6. Premature leaf senescence
7. Delayed flowering/reduced flower number/reduced seed number

Question 18

What are some soil conditions which might lead to P deficiency?

Answer 18

1. Highly weathered soils
2. Cold soils
3. Dry soils
4. Compacted soils

Question 19

Where are the largest pools of soil P generally found?

Answer 19

In:

1. Primary P minerals (apatites)
2. Secondary P minerals (Ca/Fe/Al phosphates)

Question 20

As far as weathering goes, which soils have the most soil available and organic P?

Answer 20

Moderately weathered soils.

Question 21

As far as weathering goes, which soils have the least soil available and organic P?

Answer 21

Unweathered soils and highly weathered soils.

Question 22

What factor greatly contributes to the concentration of available phosphate in the soil solution?

Answer 22

pH

Question 23

Why are unweathered soils low in available phosphate in the soil solution?

Answer 23

1. Phosphate may be locked up still as primary P minerals (apatites)
2. They may have a high soil pH where insoluble calcium secondary phosphates can form reducing available P

Question 24

Why are highly weathered soils low in available phosphate in soil solution?

Answer 24

They may have a low soil pH were insoluble iron and aluminum secondary phosphates can form and reduce available P.

Question 25

What is the pH of a soil which is considered moderately weathered?

Answer 25

Around 6.5 pH.

Question 26

Is phosphate mobile or immobile in the soil?

Answer 26

It is relatively immobile due to its ability to form secondary phosphates which bond with soil elements.

Question 27

What is sufficient P concentration in most plant species?

Answer 27

2-5 mg/g (0.2-0.5%)

Question 28

Do leaf tissue concentrations of P increase or decline over time?

Answer 28

Since P is readily mobile in phloem, leaf tissue concentrations tend to decline with age.

Question 29

What soil test is now used in the UF soil testing lab for sandy soils?

Answer 29

Melich-3 phosphorus is now used by the UF soil testing lab for sandy soils, but historically Melich-1 P had been used.

Question 30

What method of P fertilization is recommended at or after planting?

Answer 30

Banded applications, because they increase the amount of applied P taken up by the plant and reduce the amount tied of up by the soil.

Question 31

Under what conditions can P leach from sandy soils?

Answer 31

When sandy soils are not coated with Fe or Al compounds, P is not readily sorped to the soil, and fertilizer P can leach.

Question 32

From where are phosphate fertilizers largely derived?

Answer 32

From rock phosphate, predominantly (3/4 US) mined in Florida and North Carolina.