Comptes Rendus (Doklady) de l'Académie des Sciences de l'URSS.,
30(8): 754-765 (1941)
Formation of Organs in Wheat and Barley while Entering into the Light Phase
A. A. Sapehin,
Member of the Ukrainian Academy
In one of my previous papers (1) I pointed out briefly the presence, at the basis of the wheat spike, of two scale-like formations set on passing from the vernalization to the light phase. But their actual number ranges from 3 to 5, and now that this period of development has been subjected by me to a closer investigation in barley and wheat, I am able to report some results which will clearly bring out the relation between the phasic contents of a growing stem top and the shape of the organs formed in this process.
|Fig. 1. Wheat; a—bead of the last stem leaf; 1, 2, 3, 4, 5—bead-like tubercles of the scales; 110x.|
|Fig. 2. Wheat; k—spikelet tubercles; the rest as in Fig. 1; 110x/|
|Fig. 3. Barley: g—glumae: the rest as in Fig. 1; 55x.|
Until the top of the stem has passed the vernalization phase, it forms leaf ridges which are beads circular in section (1) (Fig. 1) and will later on be transformed into leaves. And after the light phase sets in, the morphology of forming organs retains its outward appearance for several days (from 3 to 10, according to the variety and external conditions); the bead-like ridges reminding of leaf primordia, continue their growth. The new contents initiated by the light phase begins to accumulate under the old vegetative form, and an acting minimum is eventually reached, whereupon the formation of organs proceeds along new lines peculiar to the new phase: the top of the stem stretches up and widens, and spikelet ridges in the form of beads semicircular in section appear on it arranged in a definite order. This period as measured from the outset of light phase to the start of stem top elongation, was described by me as a preliminary phase to the formation of spikelets.
Fig. 1 shows a stem top on the 10th day after entering the light phase. The preliminary phase has come to an end and the elongation of the top is already well expressed. In 9 days that have passed several new ridges have made their appearance. The bead that is growing out to form the last leaf of the culm (n) began buldging at the outside of the light phase. Later on there appeared the beads 1, 2, 3, 4 and 5 (the latter appears rather often but not always). These will never turn into stem leaves under normal conditions.
3 or 4 days later we find the same top (Fig. 2) to have made further progress. It is now still longer and broader. The last leaf of the stem has markedly increased in size and so have the heads of the spike-like formations situated just above them. Spikelet ridges have begun swelling out of the 3rd, 4th and 5th beads (k).
A week or a fortnight later (according to the variety and the conditions of growth) young stamens can already be easily outlined and the bead-like ridges set during the preliminary phase will assume their ultimate scale-like shape (Fig. 3). The largest of them will be the lowest, which was set first. The second scale from below is usually also a rather large size, and it is also an annular shape. These two scales have been known for a long time, but no correct interpretation has been given as to when and how they come into being (3,4). The 3rd and the 4th scales are inconspicuous, as is the 5th in case its formation has taken place. They are carried upwards with the base of the spike and sitting in it form small light-coloured processes.
The external spikelet scales (glumae) are not connected in their development with the above scales. They develop independently as a part of an organ forming out of the tissue whose contents is of the light phase nature. As a form corresponding to this particular contents the spikelet growing above the scale makes its development out of the tissue that is not yet determined finally to become a stem ridge. But the primordia of scale-like ridges belong to the vernalized parts of the tissue and so they are to a great extent determined for leaf formation. However, they become neither spikelets nor leaves and continue their existence as scales. This is probably because the physiology and biochemistry of the plant is essentially modified by its entering the light phase. Now the initial substances are used (partly and perhaps in tiniest amounts) to build up new materials, out of which the spikelets are made, while the materials required for the transformation of our bead-like ridges into normal stem leaves are no longer available in sufficient quantities.
The cells of the scales are soon vacuolized; they decrease in plasma and become transparent. It would seem as if they were not only deprived by the developing spikelets of new building materials, but even gave over part of the old. The case is similar to what later on occurs to the stem top, which having entered the third phase of development ceases forming spikelet ridges and, not in a very long while, grows transparent, probably on account of the loss of a part of its plasma.
So we arrive at the conclusion that it is not their age that accounts for the presence of scale-like leaf primordia under the spikelets, as has been supposed by Krenke (2). They should rather be interpreted in terms of phasic development and on the basis of their position in the system.
The character of organ formation in the first days of the light phase may often assist in determining the day when this phase started (with an accuracy of 1 to 3 days). In my previous note (1) I mentioned that in the case of wheat the new bud ridge appeared in the axil of a definite leaf. Just in which leaf did it appear, depended upon the variety. For the majority of wheat varieties this is usually the 5th leaf from above, if growth is made under normal conditions. By determining every day the place where the youngest axillary ridge is set it is possible, as soon as one will find the number of leaf ridges above this place to exceed that typical of the variety concerned by 1 or 2, to state that the-light phase has made its start 2 or 3 days before, that the top of the stem is about to begin elongation and that in a week or so the first spikelet ridges will make their appearance. But it is also necessary to take into account the environmental conditions, especially the fluctuations of temperature. In the case of cold spring and a slowly developing variety the initiation of a ridge will take up about 3 days. In warm weather and for an early variety this term is only 1 or 2 days.
Received 17. XII. 1940.
Translated by D. Rakhmanov