58 • Temperature and Flowering 



to vernalization appears to be very brief. If the germinating seeds 

 are kept at 20° for 3 clays or at 26° for 1 or 2 days, they can 

 no longer be vernalized, even though no new nodes have developed 

 during the short time involved (Highkin, 1956). 



The term vernalization has been extended to cover similar 

 effects of low temperature given not to germinating seeds but to 

 already developed plants. Such effects are typically found in bien- 

 nials and many perennials, and are at least formally similar to 

 those obtained with the very young plants used in "true" vernal- 

 ization. One plant frequently studied is the biennial strain of 

 Hyoscyamus niger, previously introduced as an LDP. The strain 

 discussed in Chapters Two and Three was the annual, from which 

 the biennial appears to differ only in having a cold (vernalization) 

 requirement. After this requirement is satisfied, it responds to 

 davlength in the same way as the annual strain, but it cannot 

 flower otherwise. It thus shows a qualitative vernalization require- 

 ment, unlike the plants so far discussed. 



Some of Lang's (1951) results with biennial Hyoscyamus 

 illustrate how vernalization depends on both the temperature and 

 duration of exposure. Plants were exposed to temperatures from 

 3° to 17° C under 8-hour day conditions for varying periods of 

 time, after which they were placed in 16-hour days at 23° C. The 

 vernalizing effectiveness of the various temperature treatments was 

 then expressed by the time required under long days before flower 

 initiation was detectable; the shorter the time, the more effective 

 the vernalization. With a vernalizing time of 105 days, all tempera- 

 tures from 3° to 14° were highly effective: flower initiation was 

 detected after 8 days under the long-day conditions. With only 

 15 days of vernalization, 10° was the most effective temperature, 

 giving 23 days to initiation as compared to the 35 days given by 3° 

 and the 28 days given by 14°. With an intermediate vernalizing 

 time of 42 days, both 3° and 6° allowed initiation alter 10 long 

 days; 17° gave initiation after 20, and the values for the other 

 temperatures lay in between these. Thus die temperature optimum 

 for vernalization shifts considerably depending on the length of 

 exposure (10° for 15 days, 3 to 6° for 42 days), but ceases to exist 

 if the exposure is long enough. 



As in the rye embryos, cold given to the apex alone is sufficient 

 to vernalize Hyoscyamus and many other biennials. The gcrminat- 



