Requirements for High-Intensity Light • 21 



would normally cause flowering if the plants were subsequently 

 placed on long-day conditions, was entirely ineffective. Before such 

 a dark period could be effective, the plants had to be exposed to 

 at least a few hours of high-intensity light; within limits, the 

 effectiveness of the dark period was then directly related to the light 

 energy given before it. This "high-intensity light reaction" clearly 

 differs from the low-intensity reaction sufficient to interrupt a dark 

 period, since it requires light energies some 10,000 times higher for 

 maximum effect. It has since been shown that C0 2 must be present 

 for the high-intensity light to have its effect; in addition, feeding 

 the leaves with carbohydrates or organic acids can at least partially 

 replace the high-intensity light requirement (see Liverman, 1955). 

 Such results suggest that this requirement is largely a requirement 

 for products of photosynthesis. 



Another high-intensity light requirement has also been reported 

 in Xanthium. To be maximally effective, an inductive dark period 

 must be followed as well as preceded by a period of high-intensity 

 light. Lockhart and Hamner (1954), for example, found that if 

 only a brief light flash was given to end the inductive dark period 

 and this was then followed by another dark period before the 

 plants were replaced in long-day conditions, flowering was com- 

 pletely or partially inhibited. A period of high-intensity light given 

 before the second (inhibitory) dark period rendered it ineffective, 

 but low-intensity light did not. Both auxin (see Chapter Six) and 

 high temperature increased the effect of the second dark period. 

 Subsequently, Carr (1957) found that sucrose given to the leaf 

 during the second dark period almost nullified the inhibition, 

 allowing flowering to take place. He thus suggested that the "second 

 high-intensity light requirement," like the first, is a requirement 

 for photosynthetic products. 



While experiments of this sort show that high-intensity light 

 periods can have profound modifying effects on photoperiodic 

 induction, these are probably due to effects of photosynthate as an 

 energy source and on the translocation of the flowering stimulus 

 (see Chapter Five) rather than on photoperiodism proper. Even 

 Xanthium, on which the most detailed work of this kind has been 

 done, can eventually initiate flowers in total darkness (Hamner, 

 1940). Thus the primary role of the dark period in photoperiodism 

 is not contradicted by these data. 



