Vernalisation is the process where some plants require a prolonged period of cold to either initiate or speed up their flowering. This mechanism is an adaptation for plants in temperate climates, ensuring they transition to their reproductive phase at the most opportune time, typically in spring or early summer. This timing helps the plants avoid the harsh conditions of winter for their delicate flowers and subsequent seed development.
The Process of Vernalisation
At its core, vernalisation is a plant’s way of measuring the duration of winter to ensure it flowers at the correct time. This process allows the plant to become competent to flower, but flowering itself often requires additional environmental signals, such as the right day length. The plant “remembers” that it has experienced a sufficient cold period through stable changes in how its genes are expressed.
This “memory of winter” is a form of epigenetic modification. In plants like Arabidopsis, a model organism for plant biology, a specific gene called FLOWERING LOCUS C (FLC) acts as a brake on flowering. Before winter, this gene is active, preventing the plant from producing flowers. During a prolonged cold spell, the FLC gene is gradually silenced through changes to its associated chromatin structure, lifting the brake on flowering. This silenced state is then maintained after temperatures rise, allowing the plant to bloom.
The effectiveness of vernalisation depends on specific factors. The temperature range is generally just above freezing, between 0°C and 10°C (32°F to 50°F) for many species. The duration of this cold exposure is also important and can range from a few weeks to several months. If a plant is exposed to high temperatures immediately after the cold treatment, a process called devernalisation can occur, erasing the memory of winter.
Plants Requiring a Winter Chill
A wide variety of plants rely on vernalisation and can be categorized based on their dependency on the cold treatment. For some, vernalisation is an absolute requirement for flowering, a condition known as obligate vernalisation. Without a cold period, these plants will remain in a vegetative state. Winter cereals, such as winter wheat and barley, are examples; they are planted in the autumn to experience the winter cold needed to produce grain.
Other plants exhibit facultative vernalisation, where cold exposure promotes or accelerates flowering but is not strictly necessary. These plants will eventually flower without a cold period, but the process will be significantly delayed. This category includes many biennial vegetables like carrots and cabbage, and ornamental flowering bulbs like tulips and hyacinths, which require a winter chill.
The specific requirements for temperature and duration vary greatly among species and even between varieties of the same species. For instance, some apple and cherry trees need a certain number of “chill hours” below a specific temperature to break dormancy and flower properly. This diversity is a result of plants adapting to the winter conditions of their native habitats.
Significance for Plant Life and Agriculture
Vernalisation helps to synchronize the flowering of an entire population of plants in a specific area. When all the individuals of a species bloom at the same time, it greatly increases the chances of successful cross-pollination. This genetic exchange is important for maintaining diversity and resilience. The process also prevents premature flowering during brief warm spells in autumn or winter, safeguarding the plant’s reproductive investment.
In agriculture, an understanding of vernalisation is fundamental to crop management and food production. The distinction between winter and spring varieties of cereals like wheat and barley is based on their vernalisation requirements. Farmers choose which variety to plant based on their climate and schedule, impacting crop yields. The geographical distribution of many fruit and vegetable crops is also limited by their need for a specific duration of winter cold.
Human Applications of Vernalisation
Humans have learned to manipulate vernalisation for agricultural and horticultural benefit. This understanding allows for the control of flowering times to improve crop yields, breed new plant varieties, and produce ornamental plants on demand. Growers can simulate winter conditions with artificial cold treatments to trigger flowering at a chosen time.
A common application is the forcing of bulbs like tulips and hyacinths to flower out of season, such as for holidays. This is achieved by storing the bulbs in refrigerated conditions for a specific period before planting them in a warmer environment. Similarly, some vegetable and wildflower seeds are pre-chilled to ensure uniform germination and subsequent flowering for seed production.
Plant breeders also utilize knowledge of vernalisation to develop new crop varieties. By selecting for plants with different cold requirements, they can create cultivars adapted to different climates or growing seasons. For example, breeders can develop wheat varieties with a reduced need for vernalisation, allowing them to be grown in regions with milder winters.