THERMOPERIODIGITY 



by 

 F. W. Went 



IVilUatn G. Kerckhoff Laboratories of the Biological Sciences, 

 California Institute of Technology, Pasadena, Calif, 



In the previous discussions the effects both of temperature and of periodic 

 light-changes on plant development have been described. In each case 

 direct effects and delayed or after-effects were observed. 



In discussing vernalization of seeds it was shown that during the first 

 thermophase the future development of the plant is strongly influenced by 

 temperature, and this effect may be enhanced or counteracted to some ex- 

 tent by the second photophase. To obtain optimal development a marked 

 temperature change has to occur between early stages of germination and 

 later growth. This temperature change is a long-term process, measured 

 in terms of months. 



In photoperiodism the daily change from light to dark is essential to 

 bring about developmental processes, and the effects of photoperiod are in- 

 fluenced or even determined by temperature. Theoretically this is inter- 

 preted by assuming that during the light and the dark periods different 

 processes take place, which have to be balanced to obtain specific responses. 

 Whereas in photoperiodism stress is laid on the direct effects of light and 

 darkness, in thermoperiodicity the daily light cycle is given, and the effects 

 of temperature during the light and dark periods are considered. Since de- 

 velopment can be completely changed by varying temperatures during the 

 dark period, and since optimal growth in most plants only occurs when the 

 temperature is lower during night, stress should be laid on this daily cycle 

 of optimal temperatures, and this is done by referring to this cycle as thermo- 

 periodicity. Before discussing this latter phenomenon in greater detail, 

 other cyclic temperature effects have to be mentioned, beginning with those 

 having approximately a yearly cycle. 



For a good understanding of temperature effects on the development of 

 plants the fundamental work of Blaauw and co-workers at the Laboratory 

 for Plant Physiology at the Agricultural College in Wageningen, Nether- 

 lands, is essential. This work has mainly been carried out with bulbs. The 

 latter have the great advantage, that during the greater part of their develop- 

 ment they do not need any light, since the initiation and early growth of 

 shoots, leaves and flowers occurs inside the apparently dormant bulb, while 

 they are being stored at any desired temperature. Therefore the investiga- 

 tion of the temperature requirements during this apparent dormancy does 

 not necessitate air-conditioned greenhouses, which were not available until 

 recently. For this reason Blaauw's data are the most complete available 

 at present on the optimal temperatures of the various stages in plant de- 

 velopment. 



