J Hered 1934 25: 261-263.
TWISTED WHEAT AND TWISTED TREES
Twisting of the Rachis of Wheat Heads by Short Daily Photoperiods Suggests a New Approach to Studies of Twisted Trees
H. H. MCKINNEY AND W. J. SANDO
Division, of Cereal Crops and Diseases, U. S. Department of Agriculture
DURING the past four years several observers have published on the twisting of the trunks and branches of trees. These papers have dealt with observations, and in some instances there have been discussions as to the possible cause of twisting. The writers are presenting the results of tests with an abnormal twisting of the rachis which occurs in certain varieties of wheat when grown unsuitable conditions.
During the spring of 1930 it was found that many plants of Harvest Queen wheat developed very long twisted internodes at the lower portion of the rachis of the heads when the plants received light for eight hours daily. Other tests have shown that this twisting occurs in this variety when the daily photoperiod is twelve and one half hours in length during the autumn. Twisting has occurred in Siberian spring wheat when the summer day was reduced to thirteen hours of light. Examples of twisting are illustrated in Figure 1. Twisting has not occurred when the daily photoperiod was fourteen hours or more in length.
Twisting has been associated with the elongation of the internodes at the base of the rachis. In some instances this elongation is slight, but in others the lowermost internode has measured as much as 6 inches. A few specimens have been observed in which elongated internodes were not twisted. These internodes have been twisted as many as two and one half revolutions. Frequently the entire rachis is twisted, and sometimes the complete stem is twisted, but the latter condition is not frequent.
The direction of the twist of the rachis seems to be influenced by some condition arising in the separate stalks as single plants have been found to produce heads with dextrorse (left to right) twists as well as heads with sinistrorse (right to left) twists. Counts on 15 heads from 4 plants of Marquis wheat showed 9 heads with no twists, 3 with dextrorse, and 3 with sinistrorse twists. Counts on 66 heads from 17 plants of Harvest Queen wheat showed 15 heads with no twists, 25 with dextrorse, and 26 with sinistrorse twists. In 7 of the Harvest Queen plants both directions of twisting were present on the same plant, and in 2 of these plants some of the stalks produced heads which had no twists.
In Table I it will be observed that 4 of the varieties tested did not develop twisted heads when grown under conditions favoring twisting in the remaining 9 varieties.
|TABLE I. Characteristics of wheat varieties with respect to twisting of the lnternodes of the rachls when the plants were cultured with a daily photoperiod of eight hours until after heading. Temperatures during the night were 70 to 75°F. During the day they went to 80°F. or slightly above when the sun was bright.|
elongated and twisted*
|*Plus signs in the column signify twisting and the minus signs signify no twisting.|
TWISTED HEADS OF WHEAT
|Twisting of the rachis and clavateness in Harvest Queen wheat (right and center) compared with a normal head showing considerable portion of the peduncle (left) The normal head has short rachis internodes but the abnormal heads show elongation and twisting of the lower rachis internodes.|
Usually the spikelets on the nodes of the very long twisted internodes are abortive, and in many instances two leaflike structures developed from the lowermost node of the rachis as shown in the center specimen in Figure 1. This specimen illustrates bending at the base of the twisted rachis, a condition which occurs frequently. Sometimes stem branches develop from the nodes of the rachis in place of the spikelets, and in the case of Marquis such a branch produced several leaves and nodes and a head with abortive flowers.
On the basis of the behavior of the varieties listed in Table I it seems reasonable to assume for wheat, as Knorr3 does for timber trees, that the twisting described is controlled by heritable factors. However, in the writers' tests twisting was not expressed in varieties possessing the factors except when short days obtain, or when artificial illumination was of very low intensity during winter nights, as cited for Romanov wheat.
In the case of trees it appears to the writers that environmental relationships are still open for rather careful consideration. Granting that the character of twisting is controlled by heritable factors the expression of the character may be due to the environment in which the twisted specimens are growing.
Butler1 believes that sunlight influences twisting, and he presents the proposition that right-hand twisting prevails in northern latitudes, whereas left-hand twisting prevails in southern latitudes. On this basis twisting should he at the minimum on the equator. As stated above, it appears that the direction of the twist is incidental in wheat.
Tests on the length of the daily light period and on light intensity could be carried out with tree seedlings from seed obtained from twisted and straight-grained trees of the same species. Such tests may be conducted with controlled light conditions at one station, and they may be conducted at various stations along a north and south line in order to determine the influence of climatic differences, especially light conditions, due to differences in latitude. It would be of interest to make a survey from the north and south extremes to the center of the natural habitats of several species to determine if the percentage of twisted individuals is correlated with latitude.
Copisarow2 infers from the spiral tree forms produced in the Liesegang phenomenon by forced irregular diffusion in a mixed solution of pure sodium carbonate and pure calcium chloride in vitro, that certain salts when present in the soil may cause spiral or twisted trunks in forest trees. While such may be the case, the inference seems to be rather far-fetched when it is considered that no experimental work was carried out with living plants and that the so-called "trees" of the Liesegang phenomenon in reality are only pseudo trees of almost microscopic dimensions.