Vernalization and Photoperiodism — 122 — A Symposium 



takes place when seasonal day length is above 12 hours and winter varieties 

 flower at day length less than 12 hours. Thus the time of flowering due to 

 variation in day length as the basis of classification of paddy varieties was 

 known before the principles of photoperiodism were announced by Garner 

 and Allard. The influence of different day length on flowering has been 

 studied by M. Alam (1940-41), S. M. Sircak (1942, 1944, 1946), S. M. 

 Sircar and B. Parija (1945), B. K. Kar and A. K. Adhikary (1945), 



A. B. Saran (1945) and J. C. Sengupta and N. K. Sen (1944a, 1945). 



B. K. Kar and A. K. Adhikary (1945) have reported that winter varieties 

 which do not respond to high temperature treatment show earliness when 

 given short-day treatment. These results indicate that short days are essen- 

 tial for flowering of winter rice. But the delaying effect of pre-sowing low 

 temperature on ear emergence persists even after short day exposure of 

 seedlings, for S. M. Sircar (1945) and J. C. Sengupta and N. K. Sen 

 (1944a) have shown delay in the onset of flowering of some varieties of 

 paddy by pre-sowing low temperature followed by short day treatment of 

 seedlings. J. C. Sengupta and N. K. Sen (1945) have observed that 

 when winter varieties are sown late in September (when the plants are 

 normally exposed to photoperiods of less than 12 hours) 16 hours day 

 length in the seedling stage for one month tend to induce early flowering. 

 On the other hand the work of A. B. Saran (1945) shows that long days 

 are detrimental for growth and flowering of winter varieties. It is interest- 

 ing to note that in summer varieties short day treatment delays flowering 

 and also annuls the earliness by high temperature treatment (S. M. Sircar 

 and B. N. Ghosh, 1947). 



Vernalization by high temperature as an agronomic method has obvious 

 difficulties. The germinating seeds when exposed to high temperature 

 vernalization rapidly grow into seedlings, involving difficulties of transporta- 

 tion, and these seedlings are often attacked by fungus. On the basis of short 

 day requirement of paddy varieties a method of vernalization by the appli- 

 cation of short days to seedUngs for varying periods has been suggested by 

 S. M. Sircar (1944). The method has been found to be effective in trans- 

 planting varieties of rice, since the process of supplying short days by cutting 

 off the excess sunlight in seed beds does not entail practical difficulty to 

 rice growers. Vernalization response varied according to the nature of a 

 variety; in a winter variety, Bhasamanik, earliness with increased grain 

 yield has been noted. The increase in grain yield is associated with an in- 

 crease in the number of ear-bearing tillers. Under normal conditions all 

 the tillers of a rice plant do not survive to maturity, while later formed ones 

 die away without forming ears, but an exposure of seedlings to effective 

 photoperiods resulted in almost all the tillers bearing ears (S. M. Sircar, 

 1946). The vernalizing effect of short days is of a quantitative nature as 

 the degree of earliness increases with the duration of the treatment. A 

 remarkable influence of short days on the reduction of flowering duration 

 from 133 to 47 days has been noted by S. M. Sircar and B. Parija (1945) 

 in a winter variety, Rupsail ( Plate 11). The flowering of rice within such a 

 short period has not been reported previously as J. M. Hector (1936) and 

 M. Alam (1940-41) have noted that the minimum flowering duration of 

 rice is 60 days. The effect of short days on the acceleration of flowering in 



