Stem elongation

High yielding winter wheat varieties have a long vegetative phase and are induced to flower by a period of cold. Floral initiation is defined as the time when the growing point, the shoot apex, stops producing leaves and starts producing spikelets. Usually about 20 spikelets are initiated with the last one, the terminal spikelet, being set at right angles to the others. Every spikelet will differentiate 8-12 floret primordia each containing a single carpel and three anthers; each floret is a potential grain site. (There are photographs of the changes in the growing point as it matures in the section 'Development of the Wheat Apex').

Once the terminal spikelet has been initiated the stems of the plant can start to elongate. Each of the last 5-6 leaves joins the stem at a node. Each node, which can later be felt as a hard lump on the stem, has the ability to make the stem grow (the growth between the nodes is called the internode). This happens in an ordered sequence, from the bottom up, and the pattern of internode growth resembles the opening of a telescope with each new section being longer than the previous one. As the sections of the stem expand, the young developing ear, which has been at or below soil level all winter, is pushed up through the surrounding leaves. The ear itself grows rapidly as the last three stem internodes expand. With the expansion of the final stem internode (the peduncle), the fully developed ear emerges through the ligule of the last-formed (flag) leaf and soon afterwards anthesis (shedding of pollen) and fertilisation occurs.

A consequence of all this rapid growth is that there is intense competition for resources and this causes die-back in some parts of the plant. First, as the stem starts to grow, the last formed tillers die back and second, as the ear is pushed above the canopy, the last formed florets within each spikelet are aborted.