PHOTOPERIODISM 491 



THE TIME-MEASURING REACTION 



Discoveries based on action spectra for seed germination (Borthwick, 

 Hendricks, Parker, et al, 1952) have clarified the nature of the photo- 

 reactions and the time-measuring dark reaction in photoperiodism and 

 related phenomena (Borthwick, Hendricks, and Parker, 1952). The 

 characteristics of these action spectra are given in the discussion on seed 

 germination by Evenari (Chap. 11). In brief, imbibed seed of Lactuca 

 saliva var. Grand Rapids require visible radiation for germination with a 

 minimum energy for response near 6600 A. Germination is suppressed 

 by radiation in the region 7000-8000 A (Borthwick, Hendricks, Parker, 

 et al., 1952; Flint and McAlister, 1935), with a minimum energy require- 

 ment near 7350 A (Borthwick, Hendricks, Parker, et al., 1952). Some 

 other lettuce varieties, as well as seed of Amaranthus caudatus (Reslihr, 

 1939), Phacelia tanacetifolia {ibid.), and some varieties of tobacco, are 

 similar in this response. The action spectrum for promotion of germi- 

 nation of these seed is closely similar to that described in the previous 

 section for photoperiodism of soybean, cocklebur, and other plants. 



The promotion and suppression of seed germination are controlled by 

 a reversible photoreaction in which the pigment with maximum absorp- 

 tion near 6600 A is changed by radiation in this region to an isomeric 

 form with maximum absorption near 7350 A. The latter form is returned 

 to the former by irradiation in the region 7000-8000 A. The intercon- 

 version can be made repeatedly (Borthwick, Hendricks, Parker, et at., 

 1952). 



These findings led to a search for a promotive action of radiation 

 on floral initiation in the cocklebur (Borthwick, Hendricks, and Parker, 

 1952), in addition to the inhibitory one described in the preceding section. 

 Such an effect was found for radiation in the near infrared, with a mini- 

 mum energy required for response in the region of 7350 A. The equiva- 

 lence of the seed-germination action spectrum with that for photoperiodic 

 response is thus clearly established. This equivalence permits the use of 

 seed germination, which has many experimental merits, for further exami- 

 nation of the phenomenon. 



That the pigment with an absorption maximum at 7350 A changes 

 thermally to the pigment with absorption maximum at 6600 A is shown 

 by the behavior of imbibed lettuce seed held several days in darkness at 

 temperatures of 30°C or higher. Such seed fails to germinate upon return 

 in darkness to lower temperatures irrespective of whether or not it was 

 irradiated in the region 6000-7000 A at the start of the treatment. If 

 the seed is irradiated in this region at the end of the period at high tem- 

 perature, it germinates at the lower temperature. Thus, although germi- 

 nation is blocked by the higher temperature, the pigment in the form to 



