Investigating Vernalisation
Technical & Teaching Notes
Background information
Turnips, Chinese cabbages and rapid-cycling brassicas look very different but they are
all varieties of the same species [Brassica rapa (syn. campestris) L.] The three plants
illustrate very clearly how selective breeding can lead to the production of different
varieties. You can grow the plants and then, if they are the same species, it should be
possible to make crosses between them. You would expect the progeny of these
crosses to be fertile, for this is the definition of a species.
If you are going to make genetic crosses between these plants, obviously you must have
the three variants in flower at the same time. Rapid-cycling brassicas (fast plants) can
easily be induced to flower, but it is sometimes more difficult to get turnips and Chinese
cabbages to flower. This sheet describes the process of vernalisation and how it can be
done to help induce flowering in Chinese cabbages. The same procedure can be used
with turnips.
Vernalisation in the life cycle of a plant
Certain plants require a cold spell in their life cycle to initiate the sequence of events that
leads to flowering. The term vernalisation is used to describe the promotion of flowering
by exposure of young plants to cold temperatures. It is effective on seeds that have
taken in water or on a growing plant, but not on dry seeds. The necessary 'chilling'
treatment may be for only a few days but often several weeks are required. The
vernalisation requirement is associated with species found growing in regions with winter
temperatures that are unsuitable for plant growth. You can see the disadvantage to the
plant of premature germination and growth (at the end of the summer season) before the
cold winter season.
Vernalisation can be illustrated by the well known example of 'winter' wheats and
'spring' wheats. In temperate climates, winter wheats are sown during the autumn and
must have a cold period if they are to flower in the following summer. Spring wheats,
however, do not require chilling and so can flower later in the same season, but the yield
is likely to be lower than that of the winter wheats. Winter wheats can be sown in the
spring but would require an artificial cold treatment to ensure flowering in the summer
season of the same year. This artificial treatment is done in some northern regions, such
as Russia.
The mechanism of vernalisation at the cellular level is not understood. There is evidence
that the requirement is for chilling of the shoot apex and there have been suggestions
that gibberellins (GA) are linked with the response. Involvement of a hypothetical plant
growth substance, given the name vernalin, has been proposed and clearly there are
interactions between temperature requirements and daylength control (photoperiodism).
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Vernalisation offers an interesting example of the way that plants have evolved to exploit
the predictability of seasonal change and, in this case, to ensure that flowering occurs at
an appropriate time. It also gives an example of how environmental factors control and
interact with different stages in the life cycle.
Further Investigations
Carrying out genetic crosses between the different plants
If vernalisation has been successful, after 2 to 3 weeks the Chinese cabbage plant 'bolts'
producing a rapidly growing flowering shoot. Turnips are treated in the same way but a
longer time should be allowed for vernalisation, perhaps as much as six weeks. The
vernalisation period required to induce flowering in these vegetables may vary at
different times of year. It also depends on how the vegetables have been stored before
you buy them. Imported Chinese cabbage almost certainly will have been stored at a low
temperature for some time and may, as a result, already have become vernalised.
The Chinese cabbage (or turnip) can be crossed either with rapid-cycling brassicas or
with each other. As soon as flower buds are visible, on either Chinese cabbage or turnip,
sow the rapid-cycling brassica seeds. Two weeks later both will be in flower and crosses
can be made. The resulting seed can be grown and tested to see if it produces plants
that are capable of reproducing and producing viable seed. Remember that this is the
point that emphasises that the three varieties, which looked so different, are in fact
members of the same species. If the rapid-cycling brassica is chosen as the female
parent, seed can be harvested only three weeks later. If Chinese cabbage or turnip is
the female parent it obviously takes longer before seed can be harvested.
Seed collected from these crosses provide many opportunities for further investigations.
As an example, interesting results can be obtained by carrying out crosses between the
offspring (1) progeny (1st generation) and either or both parents, or with crosses
between the offspring (1) progeny. You need to sow the offspring (2) progeny (2nd
generation) outside or in large pots with light of full spectrum.
References
Ridge I (ed.) (1991) Plant Physiology Hodder & Stoughton
Salisbury F B & Ross C W (1992) Plant Physiology Wadsworth
Taiz L & Zeiger E (1999) Plant Physiology Benjamin Cummings
Acknowledgements
Artwork by Christine Grey-Wilson
Science & Plants for Schools: www.saps.org.uk
Investigating vernalisation: p. 2
This document may be photocopied for educational use in any institution taking part in the SAPS programme.
It may not be photocopied for any other purpose. Revised 2012.