Vernalization and Photoperiodism — 152 — A Symposium 



tathione of yeast extract was the active agent breaking dormancy, but Ben- 

 nett, OsERKOWSKY and Jacobson (1940) showed that the effect of yeast 

 extract was due to a compound differing from glutathione. 



Interpretation of these facts is hardly possible as yet. On the one hand 

 it would appear that the dormancy induced in buds in the course of the sum- 

 mer is due to a growth inhibiting substance which accumulates in the fully 

 developed buds. This substance is probably not identical with auxin, since 

 auxin extractions at the time of deepest dormancy show the lowest auxin 

 content. Yet it is possible to keep buds dormant beyond their normal sea- 

 sonal loss of dormancy by spraying with auxin-like substances (Guthrie 

 1938). This auxin-induced dormancy therefore is different from normal 

 dormancy. 



The effectiveness of ethylene, ethylenechlorohydrin and similar sub- 

 stances in breaking bud dormancy could indicate destruction of inhibitors, as 

 has been shown to be the case in the breaking of dormancy of potato tubers 

 (MiCHENER 1942). On the other hand the experiments of Bennett and 

 Skoog (1938) and Guthrie (1940) indicate that application of materials 

 which can be considered to contain growth promoting substances can break 

 bud dormancy. From this one would be led to assume that dormancy is not 

 due to accumulation of inhibitors, but to a lack of growth substances. It is 

 conceivable that both mechanisms occur, but much work remains to be car- 

 ried out before binding conclusions are possible. 



Finally these findings will have to be correlated with the temperature 

 effects. Not enough facts are at hand to even suggest a hypothesis how low 

 temperatures could remove inhibitors or cause the production of growth 

 promoting substances. 



Already very early in the periodic development of plants with chilling 

 requirements, periods of low temperature are necessary. In peach seedlings 

 (Lammerts 1942) embryo-cultured seeds did grow immediately, but they 

 soon became dormant. When these dormant seedlings were placed in cold 

 storage at 5°C., 20-40 days sufficed to break their dormancy. The seeds of 

 Convallaria majalis will start to germinate at medium temperature, but the 

 epicotyl soon becomes dormant and will not continue development until ex- 

 posed to low temperatures (Barton and Schroeder 1942). 



For a fairly complete discussion of all these facts we can go back to 

 Sachs (1860). He was the first to study in detail (with a hardly adequate 

 technique) the temperature relationships in the growth and development of 

 plants. If we consider how primitive the methods were with which he had 

 to obtain and maintain the various temperatures, his results and conclusions 

 are truly remarkable. By using many different plants, he initiated a com- 

 parative growth physiology. The greater part of his work was carried out 

 with seeds and seedlings, but in some further observations and considerations 

 he extended his treatment of the temperature relationships of the growing 

 plant until its maturity. Sachs realized, that each growth stage of the 

 plant has its own temperature characteristics, which he considered to be ade- 

 quately qualified by: minimal (z), optimal (y) and maximal (x) tempera- 

 tures. As stages he considered seedling growth ( S ) , vegetative growth ( V ) , 

 blossoming (B) and fruiting (F). He even distinguished a sub-stage: 

 germination (G) which was somewhat different from seedling growth. 

 Each plant could then be characterized as follows : 



