METABOLIC ASPECTS OF PHOTOPERIODISM 



IN PLANTS 



IRWIN SPEAR 

 University of Texas, Austin 



It is well known that the photoperiod controls a great many physio- 

 logical, morphological, and anatomical processes of plants and ani- 

 mals. However, the metabolic changes that bring about these responses 

 are largely unknown. Two lines of evidence suggested that a study of 

 the €02 metabolism of short-day plants would be fruitful. The first of 

 these was the need for a supply of CO2 during the light phase for the 

 flowering response of short-day plants (Harder and von Witsch, 1941 ; 

 Parker and Borthwick, 1940). The second came from other experi- 

 ments (Bode, 1942; Schmitz, 1951) with isolated leaves and shoots 

 of Kalanchoe plants exposed to different day lengths, which demon- 

 strated that both the photosynthetic rate in the light and the pattern of 

 CO2 production in the dark are affected by day length. 



In our experiments (Gregory et al, 1954; Spear, 1953; Spear and 

 Thimann, 1954), the diurnal CO2 metaboHsm of the short-day plant 

 Kalanchoe blossfeldiana was studied as a function of day length. The 

 plants for these experiments were grown on long days ( 16 hr light and 

 8 hr dark) in air-conditioned Hght rooms maintained at 19°C and 

 75% relative humidity with a light intensity of 1500 ft-c from incan- 

 descent and fluorescent light sources. The plants were then enclosed in 

 gastight chambers as shown in Fig. 1, the seal being effected by affixing 

 a slit piece of gum rubber tubing around the lower portion of the stem 

 and filling the space with latex. The base of the chamber is assembled 

 and bolted together around the rubber tubing, thus forming a gastight 

 seal. Air was passed over the plants at a rate of 27.5 ± 0.5 liters per 

 hour, and the CO2 content of the air entering and leaving the chamber 

 was measured with an infrared gas analyzer. 



The CO2 exchange of plants which had always been grown on non- 

 flower-inducing long days was studied for several long-day cycles, after 



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