Peter Griffith

There have been a lot of winter wheat crops sown late this year and the last week of February was the first opportunity for many to catch up on sowing.  This has meant a lot of questions about vernalisation what it means and how does it work.

Vernalisation refers to a requirement in wheat to undergo a period of low temperatures in order to trigger flowering. It is an inherited characteristic that prevents winter wheat developing a flowering meristem too early and consequently being damaged by cold weather.  The plants response to vernalisation is dependent on two factors, the temperature during and the duration of the vernalisation period.

Temperature requirements

There are three principle temperatures involved in the process; the minimum below which no vernalisation takes place usually set at between –1.3°C to  -4°C, the optimum when vernalisation is fully effective usually between 3 and 10°C with a peak at 4.9 and a maximum beyond which vernalisation ceases to take place, set at 15.7°C. Recent research from New Zealand suggests that there is a clear linear response to vernalisation between 0 and 8°C. True winter wheats are said to require exposure to the optimum vernalisation temperatures for 50 days. This is known as the effective days to saturate the response to vernalisation.

Role of VRN1 Gene

Genetic analyses have identified three main genes associated with the vernalisation response in wheat: VRN1, VRN2, and VRN3 . The key temperature response gene is VRN1. Most of the variation in wheat varieties vernalisation response is thought to be determined by allelic differences in this gene. Extended exposures to low temperatures during the winter induce VRN1 expression which in turn causes the apical meristem to start the reproductive phase. Vernalisation is complete once the meristem of the plant has reached the double-ridge stage.

The VRN1 gene carried by winter wheat only activates to trigger flowering after vernalisation. This helps to delay flowering until spring, when conditions are more suitable for grain production. In contrast, spring wheat does not require cold temperatures to trigger flowering. They carry a version of VRN1 that is always active. The version of VRN1 carried by a particular variety of winter wheat will largely determine when flowering will occur. This becomes increasingly important if sowing is delayed to the late winter and early spring when temperatures are rising and day length is increasing and forms the bases of the latest safe sowing date for winter wheat varieties.

 

Influence of Copy number of Vrn-A1 allels

A recent piece of research has examined the genetic response to vernalisation for three UK winter wheat varieties. These were picked for their different temperature treatments needed to saturate the response to vernalisation.

The winter wheat varieties 'Claire', 'Malacca' and 'Hereward' require short, medium or long low temperature treatments, respectively, to achieve full vernalisation. To study this quantitatively, plants were vernalised at 7°C in 8 h days in increments of 1 week for 0 to 10 weeks ('Hereward') or 0 to 6 weeks ('Claire' and 'Malacca') after which they were transferred to a heated lit glasshouse providing long days (18 h light)." Day length was sufficient to satisfy photoperiod response.

Plants with an increased copy number of the Vrn-A1 alleles have an increased requirement for vernalisation. The researches show that Claire has one copy of Vrn-A1, Malacca 2 and Hereward 3 copies.
Although the researchers did not state the growth stage of the plants when vernalisation ceased, it is interesting to note that for all three varieties the shortest period to ear emergence was 50 days (diag).

In this experiment unvernalised 'Claire' plants flowered about 40 days before 'Malacca' but the acceleration of flowering per week of vernalisation was greater in 'Malacca' so that flowering times were similar after 4 or more weeks of cold. Unvernalised plants of 'Hereward' and 'Malacca' flowered at a similar time but the 1 to 4 week treatments in 'Hereward' had less effect on flowering, after which the response was similar to 'Malacca'.

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Vernalisation starts once seed is imbibed

Plants become receptive to vernalisation from imbibition onwards, in other words as soon as the seed has absorbed enough water for the seed coat (testa) to burst. They do not have to emerge for the process to take place. So pre-emergence it is soil temperatures that are operative in the vernalisation process and air temperatures once emergence has occurred.

Extrapolating from the data above and using the UK mean average daily temperatures for March (7°C) and April (8°C) an estimate can be made of when a particular variety sown at the end of February will be fully vernalised. Each week of this experiment equates to 56 vernalisation days. Varieties like Claire and Malacca are unlikely to reach double-ridge until mid April. Varieties requiring a long period of vernalisation like Hereward probably would not reach double-ridge stage until mid May. We can only hope that the cold spell forecast for next week doesn't put a spanner in the works.

It's not just vernalisation

However, vernalisation is not the only factor to determine if a winter wheat crop will flower. There is also a photoperiod response and it is thought that crops that have not received enough vernalisation will eventually flower, it is just a question of when.

Sources.

European Journal of Agronomy 11 (1999) 63-82

Field Crops Research 105 (2008) 193-201

Field Crops Research 57 (1998) 289-300

Agron J 95 (2003) 155-159

Annals of Botany 78 (1996) 507-512

http://www.plosone.org/article/info:doi/10.1371/journal.pone.0033234#s4

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3521655/

http://www.readcube.com/articles/10.1371/journal.pone.0033234

HGCA Project Report No. 496 (2012)