THE PERIODIC ASPECT 555 



chlorotic, and ultimately the plants may die. A more quantitative 

 measure of this effect of continuous illumination is the wet and dry 

 weight production, which is strongly reduced even when no serious 

 chlorosis develops. A few hours of darkness in a 24-hr cycle greatly 

 increases dry matter production, with an optimum at about 6 hr dark- 

 ness. 



When the darkness is given at any cycle differing very much from 

 24 hr, the effect is just as severe or even more severe than in con- 

 tinuous hght (Hanson and Highkin, 1954; Hillman, 1956). Therefore 

 darkness is effective only if it comes at exactly 24-hr intervals, or, to 

 a lesser extent at 48- and 72-hr intervals. There is also a temperature 

 effect, inasmuch as a high temperature causes much more severe 

 injury in continuous illumination than a low temperature (23° against 

 14°; at 10° constant there is very little growth at all). Kristoffersen 

 has found that injury will occur only under conditions of maximal 

 growth with daily applications of nutrient solution. 



It is not necessary to fluctuate light over a 24-hr cycle to prevent 

 injury. Hillman found that a 24-hr temperature cycle in continuous 

 light produces perfectly normal tomato plants. Kristoffersen has now 

 studied the quantitative aspects of the effect of temperature fluctuation, 

 and has found that temperature and darkness are practically equiva- 

 lent: in both cases a 6-8 hr interval of darkness or lower temperature 

 is required to get normal plants, and in both cases this causes about 

 the same absolute increase in weight compared with continuous light 

 at constant temperature. There is one additional fact: to prevent 

 injury the temperature fluctuation must occur above and below 

 14°C; a 23-17° change is hardly effective, but a 17-10°, or 23-10° 

 sequence is fully effective. The greatly increased weight due to a daily 

 exposure to 10° (i.e., during the photoperiod) indicates that at that 

 temperature photosynthesis must take place, and also when the 10° is 

 given for 1 6 hr out of every 24 hr. 



We have seen that for normal development a tomato plant must 

 have periodic changes in its environment, and that this periodicity 

 must have a 24-hr rhythm. The most remarkable aspect of this 

 essential periodicity is that temperature fluctuations are just as effective 

 as light-dark sequences; in this case, periods of lower or higher 

 temperature can fully substitute for periods of darkness. 



