4. Investigate the effect of sucrose concentrations on pollen tube growth

Question 1

Method:

Answer 1

1. Make up solutions of sucrose solution, making sure to add equal volumes of sucrose solution and mineral salt culture medium when added to the slides.
2. Collect and label 5 Petri dishes with each concentration. Place a piece of filter paper onto the dishes and moisten with water before replacing the lids. This will mean the dishes act as a humid chamber and the pollen tubes will not dry out. This slide will be used without coverslips to prevent anoxic conditions developing that might prevent pollen tube growth. Collect 5 slides and label (do not touch the centre of the slides to ensure they remain clean). Place a few drops of the sucrose solution and the mineral salt in the central cavity of each slide.
3. Collect a flower with several mature anthers that is shedding pollen (riper pollen has a better chance of successful germination). Rub mounted needle over anthers so pollen falls onto medium. Repeat for each concentration (using same flower if possible).
4. Note down the time the pollen was added and place the slides in the Petri dishes. Use the microscope with low power magnification and eye piece graticule that has previously been calibrated with a stage micrometer. Measure the pollen tube growth quickly to prevent samples drying out or overheating.

Question 2

What is pollen?

Answer 2

Pollen is the male gametophyte of flowering plants.

Question 3

What are gametophytes?

Answer 3

Gametophytes are structures of plants that produce gametes:
Male- pollen grain.
Female- embryo sac.

Question 4

Where are the pollen grains made?

Answer 4

In the anther.

Question 5

What does the nucleus of the pollen divide into?

Answer 5

The nucleus of the pollen divides into a generative and vegetative nucleus. The generative nucleus divides to produce two sperm cells.

Question 6

What does the embryo sac (female gametophyte) consist of?

Answer 6

An egg cell
Synergids
Polar nuclei
Antipodals (3)

Question 7

When does fertilisation occur?

Answer 7

The sperm inn the pollen tube reaches the egg in the embryo sac.

Question 8

What is germination?

Answer 8

When the pollen tube grows from the pollen grain by elongating at the tip of the tube.

Question 9

Risk:

Answer 9

Pollen allergies:

-wear breathing mask.

Question 10

What solutions are needed for the experiment?

Answer 10

Two different solutions are needed:

• pollen culture medium, made up of a range of minerals
• a range of sucrose solutions: from a stock solution, serial dilutions.

Question 11

How are the pollen grains prevented from drying out?

Answer 11

Put moist filter paper in the Petri dish.

Question 12

How is the pollen of the plant added?

Answer 12

Use a low power microscope and knock pollen off the anthers of a flower using a mounted needle.

Question 13

How often should tube length be recorded?

Answer 13

eg. ever 3 minutes for half an hour.

after around 15-30 minutes, once growth has started.

Question 14

Why should mature flowers be chosen?

Answer 14

Because mature flowers are already beginning to shed their pollen, therefore germination is more likely to happen.

Question 15

What must be done before each pollen tube length measurement?

Answer 15

The graticule of the microscope should be calibrated.

Question 16

Analysis of results:

Answer 16

Plot a graph of time against length, for each different sucrose concentration.
(pollen growth over time)
(multiple variables/lines)

Or, calculate average growth per unit time (growth rate) for each sucrose concentration.
(only one variable/line)
This makes it easier to see how the effects of sucrose on the rate of growth of the pollen tube.

Question 17

Limitations:

Answer 17

• Germination is a random process.
• Some species of pollen don’t grow straight under certain conditions so measuring the growth length might be inaccurate.

Question 18

Interpreting the results:

Answer 18

• Pollen tubes grow by extending their cytoskeleton, and changes in water potential.
• When a pollen grain is released from the anther, it has a low water potential and is partially dehydrated.

When it reaches the stigma of the flower, the pollen becomes hydrated either by water inn the stigma or surrounding humidity.
-The influx of water into the pollen grain directs pollen growth.

-At the same time, the pollen is stimulated to germinate by sucrose on the surface of the stigma of the flower.

This means there are two opposing forces at play, the sucrose aids germination, but sucrose presence lowers the water potential.

• Whenever there are two opposing forces acting on a factor, we are likely to get an optimum level, extremely low sucrose, or extremely high sucrose concentrations will be non-optimal.
• The optimum of the graph can be interpreted in terms of the opposing effects one germination signals and water potential.

Question 19

Describe the steps required to set up the microscope apparatus to make it ready for measuring length.

Answer 19

The graticule needs to be CALIBRATED. This uses an eyepiece graticule with the stage graticule.

Select the magnification, focus and align the graticules.

Measure the stage graticule distance that corresponds to one unit one the eyepiece graticule. Use this ratio to calculate lengths.

Question 20

Volume*conc equation.

Answer 20

V1C1= V2C2