This section covers the development of the leaves and shoots of the wheat plant. The leaves capture light energy from the sun, and transform it into photosynthates, which support the growth of the shoot and will, eventually, be transported to and stored in the harvested seed.
In the temperate zones of the world, where summer days are long and winter days are short, two distinct types of wheat have been developed. 'Spring' wheat is similar to wild wheat in that it grows quickly, moving through all the leafy development stages without check. It is sown in the spring and is induced to flower by increasing daylength only, so that mature grains can be harvested within 20-25 weeks. In contrast, 'winter' wheat has been selected to take advantage of a longer leafy stage when plant development is checked by the cooler temperatures and shorter day length of winter. During this time the extra leaf area can build up greater reserves which support higher grain yields when the crop matures. Winter wheat is sown in the autumn, grows slowly through the winter, is induced to flower by a period of cold and then flowers in the increasing daylength of spring, maturing the following summer.
The long vegetative phase produces a plant with more side shoots, called tillers. The tillers form at the base, in the axil, of the first formed leaves of the mainstem and of the coleoptile. In fact a coleoptile tiller bud is present in the dormant seed but the tiller emerges only if the seed is sown deeply. Tillers in the axils of Leaf 1, Leaf 2 and Leaf 3 emerge first and are usually strong enough to grow to full canopy height and to set grain. These tillers produce fewer leaves than the mainstem which has the effect of synchronising ear emergence, pollen release, grain growth and ripening for all the shoots in the canopy. Tillers in the axil of Leaf 4 and above and any secondary tillers, which form in the axils of the leaves of the tillers themselves, will die back during the phase of rapid shoot growth in the spring. These late tillers contribute to the stored reserves of the plant.
The photographs associated with this section are of plants of the winter wheat cv Mercia that were grown in the field in southern England. The photographs illustrate the stages of growth that can be seen above ground; that is, vegetative (leafy) growth. There are also some micrographs of the growing point (apex) of the plant (this is buried beneath the soil surface during the winter). There are additional photographs of the apex in the section 'Development of the Wheat Apex'.
Compare the field grown plants with the plants we grew in a Controlled Environment, which had an artificial winter and spring, in the section 'How We Grew the Plants'.