Introduction to the Botany of Field Crops: Non-cereals pp. 703-705 (1936)

James Masson Hector

REACTION OF SOY BEAN TO PHOTOPERIODISM

The early experiments on the soy bean plant in relation to photoperiodism indicated that it was a "short-day" plant. Nevertheless, some varieties appear to be intermediate in reaction, and a few show reactions approximating to "long-day".

On the basis of his experiments, Lysenko considers that a short-day plant such as the soy bean can pass through all its developmental stages, with one exception, in almost any light conditions. This stage which is influenced by light in reality requires continuous darkness, not an alternation of photoperiods. In other words, after the first or thermo-stage has been completed, the plant requires 10 to 15 days of darkness (or short-day) and thereafter its development is uninfluenced by the length of day. As already noted, Lysenko regards these so-called short-day plants as "plants which in one of their developmental stages require darkness or low light intensity".

On this basis, Lysenko has devised a method of vernalizing the soy bean (and other short-day plants). The procedure is as follows: The seeds are allowed to germinate (for approximately 24 hours) until cracks appear in their coats. This is done by allowing the seeds to absorb water for the period mentioned, the amount of water required being 75 parts to 100 parts of soy bean seed (by weight). Thereafter, the seeds are held for 10 to 15 days in darkness at a temperature of 20º to 25ºC. During this period no light should be admitted and the temperature should be strictly controlled.

The following table summarizes the conditions required for the vernalization of the soy bean and certain other short-day plants.

Table XLIX
CONDITIONS FOR VERNALIZATION OF SHORT-DAY PLANTS
(Data from Imp. Agr. Bur. Bull. 9.)

CROPS Quantity of water per 100 parts of seed (by weight) Number of days (24h.) required for complete vernalization Temperature inside seed layers or in sacs during vernalization
Maize 30 10-15 20-30°C
Millet 26 5 25-30°C
Sudan Grass 26 8-10 25-30°C
Sorghum 26 8-10 25-30°C
Soy Bean 75 10-15 20-52°C

Lysenko regards the process of vernalization in the thermo-stage as a succession of gradual quantitative changes up to a certain definite limit. Until this limit is reached, no flowering can take place. Further, the change is at first local, being confined to the cells of the growing point. "Once the changes have occurred in the growing point, however, they are transmitted to all later-formed cells from that growing point, so that a plant may be formed in which its different parts are at different stages of preparation for reproduction." (I.A.B. Bulletin, No. 9.) Thus in the soy bean cuttings taken from above the point of insertion of the first flowering shoot flowered almost immediately: those from below flowered at later periods, the period being longer the greater the distance from the first flowering shoot.

Other investigators have obtained more or less similar results, though the interpretations tend to differ. The student should consult Bulletins No. 9 and No. 17 of the Imp. Bur. of Plant Genetics for fuller details and for complete literature. (See also pages 138-43.)