Lecture #6

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Discussion the two proposed mechanisms by which water is

released into the xylem? Plant physiology

Lecture #5

2/5 – 2/6

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• Osmosis– The diffusion across a semi-permeable

membrane from a region of high to low concentration

• Exudation – Water leaks out to the xylem parenchyma

cells thru pits– Evidence for this : K+ in xylem tissue (related

to roots pressure) have been discovered via analysis of SAP that exudes form stem plant.

3

Define guttation

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• The water at the tip of the plant leaf.– Also related to root

pressure– High in Boron (mass

flow)

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What is the composition of xylem sap and explain how K increases

root pressure.

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K levels Sap vol K Na Ca Mg totals

1 46.5 32.3

2 50.5 36.5

3 55.9 37.6

Three levels of sap volume are presented in the above chart. The data shows that as the K levels increases, there is a corresponding increase in the levels of sap and nutrients.

Conclusions:

1. all water movement in xylem can affect nutrient uptake.

there are 2 separate processes involved (different from adsorption-osmosis and exudation in roots).

higher water uptake does not necessarily mean high ion uptake.

2. Where water movement is unimpeded, as in transpiration stream, plant nutrients may be translocated along with water by mass flow.

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In labeled diagram explain what happens during phloem loading.

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• The movement of sugars in the phloem begins at the source, where (a) sugars are loaded (actively transported) into a sieve tube. Loading of the phloem sets up a water potential gradient that facilitates the movement of water into the dense phloem sap from the neighboring xylem (b). As hydrostatic pressure in the phloem sieve tube increases, pressure flow begins (c), and the sap moves through the phloem. Meanwhile, at the sink (d), incoming sugars are actively transported out of the phloem and removed as complex carbohydrates. The loss of solute produces a high water potential in the phloem, and water passes out (e), returning eventually to the xylem.

PHLOEM

PHLOEM

ChloroplastCytoplasm

Fre

e sp

ace

Amino acids

(Glycol sis)

TCA cycle

CO2

HNO2

NH3

Inorganic P

SucroseTri

phosphates

Sie

ve t

ub

e c

om

pan

ion

cel

l c

om

ple

x

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What type of cells are in phloem and what are their functions?

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• Sieve tube– Campion cell complex – Small holes that allow movement in phloem– Essential in phloem loading– elongated parenchyma cells in phloem tissue

– The main function of the sieve tube is transport of carbohydrates in the plant (e.g., from the leaves to the fruits and roots).

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What are the factors affecting phloem loading?

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• Energy supply (ATP)

• K nutritional status

• P nutritional status

• Turgor pressure– The higher the turgor pressure the more

phloem loading

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Explain the mechanism of phloem transport

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• Munch (1930) proposed the – “pressure flow hypothesis”– Loading is by mass flow created by pressure in sieve

tube

• Phloem loading increases pressure in sieve tubes. This pressure is responsible for mass flow action.– Can be back-up because of feed back mechanism

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List 5 characteristics of components of phloem sap

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• Sucrose

• Amino acid

• K and Mg

• Phytohormones– Auxin (IAA), GA, cytokinins

• High pH due to HCO3-

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List seven the roles of auxin?

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• Responsible for growth at the tip (snipping off apex will increase yield.)

• Rooting• Parthenocarpy• Flowering and fruit thinning• Dormancy• Control preharvest (keeping fruit longer on tree)• herbicide

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What is parthenocarpy?

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• the natural or artificially induced production of fruit without fertilization of ovules.

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Transportation of nutrients thru the phloem depends on what

factor?

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• The speed of loading in the sieve tubes.– Once loaded, moves by gradients

• K & Mg ie are very mobile because they are at high concentration.

• Movement is back and forth

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What is the main way Ca++ and Boron get into the plant?

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• Via mass flow through transpiration stream.

• Which means that it is not moving (immobile).

• It is retranslocated in the phloem

• Deficient in newer leaves

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Where would you expect Ca++ and Boron deficiency in plants,

why?

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• Ca++ and Boron deficiency would be observed in newer leaves because – they occur in lower concentrations– They are immobile

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In what part of the plant would Ca++ and Boron be in high

concentration?

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• In older leaves

• Because they are not mobile

• Once they get their by mass flow, they remain.

• For example cause Blossom end Rot– A whole in the leaves due to high con of Ca++

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What is a sink?

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• Sinks are areas in the plant where nutrients and sugars are stored:

• Fruits

• Leaves

• Stems

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What is a source?

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• A source is the areas of productivity or where food is manufactured

• They include:– Roots– leaves

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What is natural thinning, what are the consequences for not

thinning?

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• When a tree is very productive, the plant removes the fruit by a process called natural thinning.

• The tree cannot handle all of the storage

• Branches may break

• Also, if this does not occur, a lot of tiny fruits are produced.

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Result of thinning?

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• Small number fruits

• large size fruits

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Describe transportation in the phloem.

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• Bidirectional– Photosynthates are synthesized in older

leaves.– This is why

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In terms of pn, where is most of the activities taking place?

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• At the top (older leaves)

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What is the sink for photosynthates of new (younger)

leaves?

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• Younger leaves supply photosynthates to– fruits– Apex of plant

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What is the sink for photosynthates produced by

older leaves?

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• The roots and other lower parts of the plant.

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List sinks and their sources in order of priority (most impo to least impo) and the direction

movement.

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# Part of plant Source Movement or mobilization

1st Sink

Younger leaves(eventually becomes a source)

Older leaves upward

2nd sink

Developing organs

Bidirectional phloem

downward

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What is mobilization

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• Them movement of nutrients from storage to growing organs (or points).

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What is the mobilization sink?

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• Active deposition of nutrients to storage.

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List 4 examples of mobilization sinks

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• Tubers

• Bulbs

• Flowers

• fruits

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What is the mobilization sink of a germinating seed?

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• The endosperm– They store:

• fats• oil

• The nutrients are mobilized to the embryo

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List six factors affecting mobilization

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1. Topping (or top working)– Pruning or renewing branches

and leaves– Chop off apical dominance

• Produce lateral growth to produce new leaves

– N is mobilized

2. Effect of ovary and stamens• Removing the stamen and ovary

produce 30% increase, a sink• Starch in the peduncle• Removing the peduncle, the starch

disappears

3. Age of the fruit• Young fruits = stronger sinks• Older leaves = less effective

1. Light• Pn increases • Dark most translocation &

movement takes place in darkness

2. Temperature1. Yields a demobilization effect

• The respiration, the more plants looses the stored material

• Higher temp yields high transpiration rate

• After harvest, plants must be cooled down, or sugars will be converted to starch

3. Nutrient deficiency

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What is GA?

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• Gibberellins• plant hormones that regulate growth and

influence various developmental processes, including – "stem elongation, – germination, dormancy, – flowering, – sex expression, – enzyme induction – leaf and fruit senescence.

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List some differences between Auxin and Gebberellins

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AuX GA

Transport polar Yes No

Promote root initiation Yes No

Inhibit root growth Yes No

Delay leaf abscission Yes No

Inhibit laeral buds Yes No

Induce callus formation Yes No

Promote growth of dwarf plants

No Yes

Promote seed germination and dormancy

No Yes

Promote bolting or flowering No Yes

61

List four major physiological roles of Abscisic acid.

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• Stomata regulation

• Bud dormancy

• Seed dormancy

• abscission

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What is abscission? What hormone causes this ?

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• Abscission is the shedding of a body part.

• This is caused by Abscissic acid.

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