Sircar — 127 — In the Tropics 



that short day treatment increases the dry weight of the plant, and the earher 

 leaves at the end of the daily photoperiod show a greater accumulation of to- 

 tal sugars than the control while there is a decreased sugar content in the 

 later leaves. The accumulation of sugar in the earlier leaves was found to 

 be associated with more production of tillers at these stages, while a fall of 

 sugar content in the leaves of later stages was related to the translocation 

 of sugars to the developing ears. It thus appears that the reduction in day 

 length from normal daylight of about 13 to 10 or 8 hours has not reduced 

 the photosynthate formed; on the contrary, suitable photoperiods stimu- 

 late the plants to the production of increased dry matter. It was observed 

 by S. M. Sircar and B. N. De (1947) that the nitrogen metabolism of rice 

 plants is greatly influenced by 10 hours photoperiod in the seed bed and 

 these effects are noticeable throughout the life history of the plant. Absorp- 

 tion of nitrogen is greatly increased in these plants and a part of it is 

 metabolised to protein. The increase in nitrogen content occurs in conjunc- 

 tion with increase in growth rate. It is interesting to note that at the grow- 

 ing apex of the short day plant, where flower initials have been laid down, a 

 large accumulation of amino acids takes place. How far the accumulation 

 of amino acids and their nature are related to the initiation of flowering is 

 an interesting problem for further investigation. 



General Considerations : — The results presented in the foregoing 

 pages clearly indicate the influence of day length on the flowering of tropical 

 plants and refute the view expressed by W. W. Garner and H. A. Allard 

 in their first communication (1920) that small changes in natural day length 

 of the tropics do not produce any photoperiodic effect on the plants. The 

 facts that winter varieties of rice flower only when day length shortens, and 

 that short day length accelerates flowering without adverse effects on growth, 

 are ample evidence for the existence of short day requirements in these va- 

 rieties. In other varieties of rice the influence of day length is less marked ; 

 they are not responsive to short days and flower at day length more than 

 12 hours but without any acceleration after exposure to long days. An in- 

 teraction between short day and high temperature in the phasic development 

 of plants is exhibited in tropical crops, except in the cold weather crops like 

 wheat, mustard and gram where there are indications of relationship be- 

 tween low temperature and long day. High temperature and short days are 

 obligatory factors for winter rice, while the absence of low temperature ef- 

 fect on the flowering of Indian wheat or of mustard in both low and high 

 temperatures and in short and long days shows that such an obligatory re- 

 lationship between low temperature and long day does not exist. It ap- 

 pears that by centuries of adaptation in tropical climates these plants have 

 lost the significance of low temperature in the transition from vegetative to 

 reproductive phase, although its influence on vegetative growth persists. 

 Assimilation changes induced in the rice plant by short day exposure of the 

 seedlings indicate that with the photoperiodic perception in the seedling 

 condition metabolic activities are stimulated, resulting in the formation of an 

 unknown substance or substances which on transmission to the growing point 

 participate in flowering. 



References: — 

 Alam, M., Det. Sci. Rep. Rice Res. Stat., Sabour. Bilhar, India. 1940-41. — Cailahjan, M., 

 H., Compt. Rend. (Doklady) Acad. Sci. 3: 9, 1936. — Garner, W. W. and Allard, H. A., Jour. 



