SEED GERMINATION" 535 



The photorequirement of immature photoblastic seeds is greater than 

 that of mature ones [Shuck (1934), Thompson (1935) for lettuce]. On 

 the basis of this observation Thompson (1935) tries to explain the fact 

 that the heavy-seeding lettuce variety "Hubbard's Market" is much 

 more photorequiring than the light seeder "Iceberg." In his opinion the 

 varieties with many flowers and seeds cannot provide an adequate supply 

 of nutrients to the seeds, which remain immature to a greater degree. 



4-3. PHOTOBLASTISM AND METABOLISM 



Respiration. The germination experiments with photoblastic seeds in 

 different concentrations of oxygen and the importance of the seed coat 

 for the light effect suggested to early investigators a connection between 

 photoblastism and respiration. Detailed studies have been made on 

 Nicotiana. The respiration of Nicotiana seeds when germinated in 

 darkness rises with increasing imbibition. The maximum is reached 

 when the imbibition process is completed. Then the respiration curve 

 falls. When the seeds are then illuminated, the decrease in respiration 

 is stopped almost immediately, and shortly afterward the respiration rate 

 rises steeply (Kipp, 1929; Schroeppel, 1933). 



The respiration quotient remains about constant as long as imbibition 

 continues. Then it falls off, but much more so in the dark than when 

 the seeds are illuminated (Kipp, 1929). After 69 hr in an oxygen-free 

 atmosphere the intramolecular respiration of Nicotiana seeds is no longer 

 measurable. When the seeds are then illuminated and allowed to germi- 

 nate under normal conditions, they germinate as well as if illuminated 

 in air (Wieser, 1927; Schroeppel, 1933). This independence of the light 

 effect from the presence of oxygen for Nicotiana is in good accord with 

 the germination results. In pure oxygen and in darkness, respiration is 

 more intense than in air and reaches its peak earlier in order to drop 

 more quickly than in air. Illumination brings about a rapid rise, fol- 

 lowed by a drop, especially of the carbon dioxide given off, so that the 

 respiration quotient gets smaller and smaller (Schroeppel, 1933). This 

 shows that light counteracts the inhibiting effect of pure oxygen for only 

 a short time. 



We may mention here a strange observation made by Becquerel (1932) 

 which may be of importance for our problem if sustained by other 

 workers. He found that dry isolated seed coats of Ricinus and other 

 leguminous plants show a much stronger gaseous exchange than isolated 

 embryos. When they are illuminated, the gas exchange is strongly 

 stimulated. 



Leggatt (1948) investigated the respiration of lettuce seed in blue light, 

 which produces photodormancy. After 7 hr a strong inhibition of respi- 

 ration sets in. He considers the 7-hr point as a critical stage in the 

 metabohc drift. 



