VERNALIZATION AND PHOTOPERIODISM 

 IN THE TROPICS 



by 



S. M. Sircar 



Department of Botany, University of Calcutta 



Work on vernalization and photoperiodism in the tropics is comparatively recent. 

 During the last decade numerous experiments on these phenomena were performed at 

 several institutions in India and encouraging results obtained. The investigations car- 

 ried on in the tropics up to the year 1935 failed to give any positive result and vernaliza- 

 tion as an agronomic measure was considered to be of very little practical importance 

 in Indian agriculture. This was largely due to the fact that systematic and suffi- 

 ciently extensive work on developmental phases of tropical plants was not done. Later 

 researches have shown a much wider scope for work on these phenomena in the tropics. 

 Investigations done so far in India have mainly dealt with vernalization and photo- 

 periodic response of crop plants under tropical conditions, while the physiological causes 

 of these phenomena and the associated assimilation changes have been little studied. In 

 a short discussion it is impossible to do more than give an outline to illustrate the 

 main points in the researches on various Indian crops. 



Rice (Orysa sativa L.) : — Following the announcement of the princi- 

 ples of phasic development of plants by Lysenko, experiments on vernali- 

 zation by pre-sowing low temperature treatment of rice seeds were 

 performed at several agricultural research stations in India with the object 

 of escaping flood and other adverse conditions for growth and development 

 of this plant. These experiments failed to show any change in the time and 

 period of flowering; on the contrary in some varieties of rice a delaying 

 effect of ear emergence was noted by S. M. Sircar (1945), and B. K. 

 Kar and A. K. Adhikary (1945). The negative effect of low temperature 

 on rice is in accordance with the natural condition of high temperature under 

 which this crop grows in India. Throughout the life cycle of this plant, 

 the average temperature requirement is from 20° C. to 37° C. and at no 

 stage of its development does it benefit from a temperature below 15° C. 

 P. Parija and K. P. Pillay (1944) have however noted that pre-sowing 

 low temperature and anaerobiosis treatments induce f^ood resistance in some 

 varieties of rice. The influence of high temperature on acceleration of flow- 

 ering has been shown in a number of varieties of rice by P. Parija (1943), 

 S. Hedayetullah and N. K. Sen (1941), B. K. Kar and A. K. Adhikary 

 (1945). and S. Hedayetullah and B. N. Ghosh (1946). P. Parija has 

 observed that pre-sowing high temperature treatment, in addition to accelera- 

 tion of flowering, results in better resistance to drought conditions. The 

 results of their work indicate that rice has a vernalization stage of high 

 temperature. The response is of varietal character, but it remains to be seen 

 how far these variations are due to different reactions of varieties to day 

 length. 



According to the time of harvest there are two main crops of rice in 

 India, viz., summer and winter crops. Flowering of summer varieties 



