Stem elongation

Diagrams and pictures


Wheat in the field starts to extend its stems in the spring. The stem is still hidden inside the upper leaves which continue to grow before they sequentially unfurl. The lower leaves completed their growth during the winter. The flag leaf is the last to unfurl. Wheat relies on its two topmost broad leaves to produce sucrose to fill the grains.
The stem node, which was laid down at the same time as the adjacent leaf was first formed, retains the capacity to grow. But it does not do this until it is time to lift the maturing ear up into the crop canopy in preparation for flowering and fertilization. Meristematic activity in the node produces growth of the internode and, by succession, the internodes extend the stem. Here the lower leaves have been stripped away. You can feel the nodes as hard lumps along the stem.
The stages of stem extension. Once the last spikelet, the terminal spikelet, has been produced the plant stem raises the immature ear up through the unfolding leaves of the upper stem. Seven remaining unfolded leaves have been stripped away from these stems to show the immature ear and the extending internodes. The bottom nodes of the shoot will not extend but the upper ones will.
Stem at 1cm (as shown on the left) is a key development stage for the timing of agricultural husbandry events. This is a time of intense competition for nutrients and reserves. The secondary and the weak primary tillers of the plant will abort as the mainstem and the strong primary tillers grow away. The later formed florets, at the tip of each spikelet, will abort before the ear emerges from the flag leaf sheath.
The wheat stem is a continuous tissue made up of several sections. Each section comprises a leaf, which is joined to the stem at a node, and an internode, which, as its name implies, is the tissue between the nodes. Each node retains the capacity to grow although not all of them do. Growth of a node increases the length of the internode at that point. The internodes at the base of the stem, where the tillers are inserted, do not extend. The internodes higher up extend in sequence, each one growing a little longer than the previous one. In winter wheat, which has a total of 14 leaves on the main stem, the bottom 7 nodes, where the tillers are inserted do not extend but the top 6 or 7 nodes do. By this means the ear is raised up through the upper leaf sheaths until it emerges at the top, above the crop canopy. Stem elongation is closely correlated with the stage of ear development; it starts after the terminal spikelet has formed and when the florets of the mainstem ear start to produce the stamen primordia.
The stem is fully elongated at anthesis. The complete wheat plant can be seen in diagrammatic form on the right. This hypothetical plant has three emerged tillers, two un-emerged tiller buds and a final count of nine mainstem leaves. Internodes above nodes bearing tillers do not extended, except for the coleoptile internode which may have been below the soil surface at sowing time. Growth at the nodes subtending the upper leaves has raised the ear to the top of the canopy. The stripped stems show the final length of the stem and the ear in flower.
At the same time as the stem is rapidly growing the immature ear is completing the development of the male and female parts of the flower. The ear is hidden in the flag leaf sheath during the booting stage.
On the left the spikelet is at 'white anther', on the right a dissected spikelet at the 'green anther' stage. The excess florets, at the tip of the spikelet on the right, are starting to abort at the green anther stage.
Notice the scale bars. meiosis in the carpel and the anthers takes place at the green anther stage.
For more pictures of the developing flower parts go to the section on Development of the Wheat apex.