298 CONTROL OF REPRODUCTION 



2. The peak of CO2 fixation occurs 11 to 12 hr after the onset of 

 darkness, and this is the critical dark period for the onset of flowering 

 in Kalanchoe. 



3. CO2 fixation ceases after 16 hr of darkness, and this is the opti- 

 mal dark period for flowering as determined by Harder (1948). 



4. CO2 fixation does not occur unless preceded by a light phase 

 (Fig. 5) and this is also true of the initiation of flowering by long 

 dark periods (Hamner, 1940; Harder and Gummer, 1947). 



5. Both are inhibited by interrupting the long dark period. 



6. Both are induced after sufficient short-day cycles have been re- 

 ceived. 



7. In other experiments (Spear, unpublished) we have found that 

 in using 96-hr cycles with a long (84-hr) dark period interrupted at 

 different intervals, treatments that inhibit flowering also inhibit the 

 characteristic CO2 metabolism. 



On the other hand, it should be noted that we have been unable to 

 inhibit flowering by withholding CO2 during the dark period in 

 Kalanchoe (Spear, 1953). However, a succulent such as Kalanchoe 

 undoubtedly has a tremendous store of metabolic CO- available in the 

 form of carboxyl groups of organic acids. Inhibition of floral initiation 

 in soybeans and Xanthium by C02-free dark atmospheres has been re- 

 ported by Langston and Leopold (1954). These authors have also 

 noted the promotion of C^^O- fixation in the dark by these plants after 

 photoinduction. 



In studying the organic acid content of Kalanchoe leaves on differ- 

 ent photoperiods, Neyland and Thimann (1956) found that the total 

 acidity rose slowly in the first part of the dark phase, then at an in- 

 creasing rate, and reached the highest measured level at the end of the 

 dark period. In the subsequent light period, the acidity dropped 

 rapidly at first, 2 hr of light being sufficient to reduce the acid gained 

 in the dark by one half. In all other respects the organic acid pattern 

 followed what would be predicted from the C02-fixation studies above. 

 The principal organic acids formed were malic, citric (isocitric), and 

 glycolic, with lesser amounts of succinic. 



Kunitake et al. (1957), using C^^02, have confirmed the effect of 

 short days in inducing an increase in dark CO2 fixation by Kalanchoe 

 blossfeldiana. However, they found that in the products of fixation, the 

 difference was only quantitative rather than qualitative. Neither the 



