LIGHT AND THE PIGMENT 125 



inhibition, while in chrysanthemum it is not. The results with 

 pigweed (Fig. 7-10) are especially interesting, because they also 

 indicate that F-phytochromc is inhibitory and that this can be 

 expressed by only a small amount of pigment, providing it is available 

 for a long enough time. Various other plants and other systems such 

 as germination also behave as though F-phytochrome were the active 

 form. 



Since phytochrome is a protein, it is easy to imagine that F-phyto- 

 chrome must have enzymatic activity. It might lead to the synthesis 

 or destruction of some essential metabolite. In some systems the 

 quantity of phytochrome required for a given action is extremely 

 small, a characteristic common to many enzymes. For example, a 

 dark-grown pea leaf will show a measurable increase in length 

 (0.1 mm) if only one part in 10,000 of R-phytochrome is converted 

 to F-phytochrome (19). This conversion requires only one erg of 

 incident red radiation per square centimeter. The concentration of 

 F-phytochrome causing this response could be less than 100 

 molecules per cell."^ 



2. The Role of R-Phytochrome 



It is difficult to imagine that R-phytochrome can play anything 

 but a passive role in pigweed. Yet the effect of incandescent light 

 upon long-day plants (stem elongation) could be interpreted by 

 assuming that R-phytochrome leads to the production of a stem 

 elongation factor (gibberellin ?). Of course F-phytochrome might 

 inhibit this production, and decreasing it might allow synthesis to 

 proceed. If R-phytochrome does have an effect, then it would appear 

 that optimum flowering in long-day plants requires a balance in the 

 amounts of the 2 pigment forms. The interesting results with 

 Japanese morning glory might also be interpreted in this way, but 

 other, perhaps better, explanations are available. R-phytochrome 

 might play some sort of role in certain systems, but we have no really 

 good evidence that it does. 



' Assuming a concentration of 10~" moles per liter for total photochrome (the 

 concentration at which it was extracted — see above), the tissue was sensitive 

 to 10-^^ moles of F-phytochrome per liter. This is about 6 x 10^- molecules 

 per liter. A liter could contain 1.25 x 10^^ cuboidal cells, 20 microns on each 

 side (a reasonable sized cell for young tissue). Each of these cells would have 48 

 molecules at a concentration of 10~^^ moles per liter. 



