KINETIC ANALYSIS OF PHOTOPERIODISM 



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Fig. 5. Action spectra of the five principal plant photochemical reac- 

 tions. Photosynthesis and chlorophyll synthesis are given in the upper 

 graph. The photosynthesis curve is from data of Chen (1952) for the Hill 

 reaction in spinach chloroplasts. The curve for chlorophyll synthesis is 

 from corn leaf data of Koski, French, and Smith (1951). The lower 

 graph contains the three action spectra of the regulatory photochemical 

 reactions. The phototropic curve is from Shropshire and Withrow (1958) 

 for Avena. The red induction and far-red reversal curves are from With- 

 row, Klein, and Elstad (1957) for the hypocotyl hook opening of the 

 bean seedling. All the curves have been adjusted to an arbitrary value of 

 100 units response at the peak. 



of the plant is, in part, determined by the balance between the 

 carbohydrate supply on the one hand and the inorganic nutrients, 

 especially nitrogen, on the other. However, photosynthesis often can 

 be reduced to an insignificant level by the use of very low intensities. 

 In terms of carbohydrate supply, photosynthesis is not significantly 

 active at intensities below about 100 ft-c, whereas the regulatory 

 photochemical reactions are excited to significant levels by many thou- 

 sand-fold lower values of intensity. 



The regulatory function of chlorophyll synthesis is likewise a 

 possibility and cannot be excluded. Whereas we have shown by two 

 experimental technics (Klein et al., 1957), that photomorphogenesis 

 in seedlings probably does not involve protochlorophyll activation, 

 there is little doubt that chlorophyll synthesis or some closely related 

 process is responsible for morphological changes in the chloroplast 

 itself. 



