PHOTOREACTION AND ASSOCIATED CHANGES 



427 



ture coefficient of 1.0, the reaction rate is not limited by collision and 

 is at least pseudo first order. This is compatible either with reaction 

 (1) or (2), but if (2) holds, then A and AH2 must be present in high 



concentrations relative to Pigment and Pigment FR 



DARK REACTION 



The photoreaction is reversible in darkness — the change being from 

 the far-red-absorbing form of the pigment to the red-absorbing form. 

 This was shown for lettuce seed germination by Borthwick et al. 

 (1954) as indicated in Fig. 2. Imbibed seed with the pigment shifted 

 to the far-red-absorbing form by exposure to red radiation are held in 

 darkness above 30 °C which prevents germination otherwise promoted 



30°C 



(Germinotion Corrected) 



24 48 



Hours ot Indicated Temperoture 



Fig. 2. Variation in germination of Grand Rapids lettuce seed at 20°C 

 with time of holding imbibed seed in darkness at 30° and 35°. Seed 

 initially irradiated with red radiation adequate for complete germination. 



