Core practical 4

Effects of Sucrose Concentrations on Pollen Tube Growth

Background

·       Pollen is the male gametophyte of flowering plants

·       Gametophyte – structured of plants that produce gametes: sperm and eggs

·       Pollen grain lands on stigma

·       Nucleus g of pollen is generative and a vegetative nucleus. The generative nucleus divides to produce 2 sperm cells

·       The female gametophyte, is an embryo sac and it produces an egg cell

·       Consists of egg cell, synergids, polar nuclei and antipodals

·       For fertilisation to occur, sperm in pollen tube must reach the egg in embryo sac.

·       In most flowering plants, embryo sac is buried within the flower

·       Advantages – plant can protect the ovule from drying out and supply it with nutrients

·       Means that the pollen tube must grow through the style to reach the egg

·       Pollen tube growth is often remarkably fast as pollen tubes are often in competition to reach the ovule first.

·       Pollen tube grows from pollen grain by elongating at the tip of the tube = germination

Planning

·       Pollen culture medium is made up of a range of minerals

·       Make a range of sucrose solutions from a stock sucrose solution – dilutions

·       Mix pollen culture medium with sucrose solutions this causes sucrose conc to change

·       Observe pollen tubes growing on microscope slides but need to stop them from drying out

·       Put moist filter paper in a Petri dish = moist environment for the pollen

·       Take a clean slide + place a single drop of your mixture onto the centre.

·       Add the pollen to your plant use a low power microscope + knock pollen off the anthers of a flower using a mounted needle

·       Note the time pollen is added to the slide

·       Repeat these steps for each sucrose concentration you want to test so that you have a set of slides each with pollen in it’s culture medium

·       Pollen tubes should start to grow after about 15-30 mins depending on species

·       Start recording the tube length under the microscope every 3 mins for around half and hour

·       Record the length using a graticule of the microscope to measure the amount of growth in each time interval after germination

Implementation

·       Some may suffer from hay fever and asthma

·       Every sucrose conc you make will have

·       E.g. 30% sucrose conc you need to start with a 60% conc

·       Best flowers need to be mature already shedding pollen germination is more likely in these

·       Huge variety of pollen in different species finding a species that germinates quickly will aid the exp

·        Need to calibrate the graticule before measuring the lengths of the pollen tubes

Analysis

·       Compare difference sucrose concs affected by the pollen growth over time

·       Plot a graph length on y axis and time

·       Which each line showing the sucrose conc

·       You can calculate the growth per unit time for each sucrose concentration

·       To find this

·       Growth rate = total growth distance /total growth time

·       Make it easier to see the effects of sucrose on the rate of growth of the pollen tube.

Problems and limitations

·       Don’t just from a range of flowers as each flower has a different germination rate

·       Species of pollen don’t grow straight

·       Measuring the growth length might be inaccurate

Interpreting results

·       Pollen tubes grow by extending their cell membrane through changes in their cytoskeleton and changes in water potential

·       Pollen grain is released from anther it has a lower water potential + is partially dehydrated

·       When the pollen reaches the stigma pollen is hydrated either by water in stigma or surrounding humidity

·       Influx of water directs pollen growth

·       At the same time pollen is stimulated to germinate by sucrose on the surface of the stigma

·       2 opposing forces at play the sucrose aids germination but presence of sucrose lowers water potential

·       2 opposing forces act on a factor we are likely to get an optimum level extremely low sucrose or extremely high sucrose concs will be non-optimal

·      You can interpret where the optimum of the graph in terms of opposing effects on germination signals and water potential