EFFECT OF LIGHT ON GERMINATION OF SEEDS 



91 



red radiation are immediately and repeatedly reversible has been 

 shown for seeds of both lettuce (Borthwick et al., 1952) and Lepidiwn 

 (Toole et al., 1955a) (Fig. 3). This repeated reversal was carried out 

 at 27° and 7°C with almost identical results. The possibihty of re- 



Time in Hours of Imbibition before Irrodiotion 



Fig. 2. Variation in germination response of seeds of Grand Rapids 

 lettuce with increased time of imbibition at a fixed irradiance in the red 

 and in the far red (near infrared) (Borthwick et al., 1954). 



peated reversal indicates that the pigment is immediately changed 

 without other reactions. 



After promotion by red light, the germination process progresses 

 in 12 hr so far that the promotion effect on most of the seeds cannot 

 be reversed by far-red radiation, as shown in Fig. 4 (Toole et al., 

 1953). 



The photocontrol of seed germination is closely interrelated with 

 the temperature of germination (Table I). Light-sensitive lettuce seeds 

 respond differently to light at different temperatures and will not 

 respond to it at high temperatures (Toole et al., 1957). When fully 

 promoted seeds are held at high temperatures, the seeds gradually 

 lose the ability to germinate in darkness at 20 °C. A thermal reversal 

 of the pigment occurs, and the seeds revert to the nongerminating 

 condition. The reversal is more rapid at 35° than at 30° (Table II). 



