Vernalization and Photoperiodism — 12 — A Symposium 



in 1934-5, on Jan. 1 in 1935-6, and almost entirely by Nov. 19 in the 1936-7 

 season. 



Such a variation in time of completion of the thermo-phase cannot be 

 attributed to variation in climatic conditions alone. A partial or complete 

 vernalization as far as the thermo-phase may occur while the developing 

 grain are still attached to the mother plant, and is also regarded as possible 

 during prolonged drying on the iield in a cold and wet season, when the seeds 

 may be sufficiently imbibed to begin the slow growth necessary before de- 

 velopment through the thermo-phase can begin. 



TuMANOV recognizes two possibilities resulting from the after-sowing 

 vernalization in the open : (1) the seeds may be completely vernalized before 

 the beginning of winter, (2) the vernalization of the thermo-phase may be 

 completed either by the beginning of winter or during the winter. In the 

 first case, especially when the plants have had an opportunity to grow for a 

 considerable period, they would lose their ability to harden, and their resis- 

 tance to frost would thus be much reduced. Some investigators believe, 

 however, that frost resistance begins to fall in vernalized plants with the 

 beginning of the photo-phase. Some investigations have indeed shown that 

 the capacity to resist frost falls rapidly when the plants are grown after 

 completion of the thermo-phase under conditions which favour the progress 

 of the photo-phase, namely, high temperatures and long day. On the other 

 hand, Tumanov states that a fall in frost resistance may also be noted when 

 plants are grown under conditions not favouring the photo-phase, as may be 

 the case with late autumn sowings of vernalized seeds. 



Tumanov refers to his own investigations in 1936/7. When the plants 

 were tested on Nov. 2, and the thermo-phase was not completed until Nov. 

 19, frost resistance was found to be reasonably high ; the percentage of sur- 

 vival in Lutescens 0329 at -12 to -18°C. varied from 98 to 84. When the 

 plants were tested on Dec. 14, that is, 25 days after the end of the thermo- 

 phase, frost resistance had fallen, percentage survival varying from 85 to 27 

 at temperatures of -13 to -22°C. Before the tests were made, all plants had 

 been kept under conditions which would ensure their hardening. In these 

 investigations, the loss of the capacity to resist frost was observed only when 

 plants were grown for some time after the completion of the thermo-phase ; 

 this was the case with winter rye, Vjatka, the percentage survival being 78 

 to 44 in the first test, and only 25 at -13°C. in the second test, while no plant 

 survived temperatures lower than -15°C. 



The situation is different when the thermo-phase is completed so late 

 that no growth is possible before the plants are frozen. In Tumanov's in- 

 vestigations in 1935/6, when vernalization was completed by Jan. 1 in winter 

 wheats and by Dec. 1 in winter rye, the percentage survival at -14 to -18°C. 

 varied from 96 to 74 in Lutescens and from 81 to 70 in Vjatka. Similar re- 

 sults were obtained in another experiment where the percentage survival 

 was much higher after later sowings. 



It may be noted at this point that Oleinikova has shown that the 

 thermo-phase can be completed in the open during the winter under the 

 snow cover in the winter conditions at Leningrad. Winter wheats were 

 sown on Nov. 17, Jan. 1, Feb. 1, and March 1 and 31 in the open; from 

 the beginning of April, the various series were transferred to greenhouses 



