1760 CHEMISTRY OF CHLOROPLAST PIGMENTS CHAP. 37B 



Table 37B.I 



Synthesis of Organic Magnesium Compounds by Etiolated 

 Barley Seedlings in Light (after Smith 1947) 



Weight per cent of dry matter before and after exposure 



Total Mg Total ether-sol. Mg 



Chi Mg 



Leaves Before After Before After After 



Attached(71hr. exposure).... 0.16 0.22 5.2X10-" 2 X 10"^ 1.8 X 10"* 

 Detached (46 hr. exposure) .. . 0.19 0.216 5.4X10-' 1.2X10-2 l.llxiO-« 



The fastest increase in ether-soluble magnesium was noted in the first 

 two hours of exposure to light; during this time, the total formation of 

 ether-soluble magnesium compounds ran considerably ahead of that of 

 chlorophyll, indicating the accumulation of other magnesium-containing 

 organic molecules^perhaps chlorophyll precursors (c/. section 2 below). 

 After a day or two, chlorophyll formation caught up with that of total 

 organic magnesium compounds; in attached leaves, in particular, over 

 90% of the total ether-extractable magnesium was found, at that time, 

 to be present in chlorophyll. 



Leaves left in the dark as controls formed no more ether-soluble mag- 

 nesium compounds after germination, indicating that light is needed not 

 only for the formation of chlorophyll, but also for the accumulation of the 

 other magnesium-containing organic compounds. 



Smith (1949) gave time curves for the formation of total ether-soluble 

 magnesium and chlorophyll magnesium in etiolated barley seedlings at 

 three different temperatures. 



At 0° C, the maximum amount of ether-soluble magnesium compounds 

 formed was only 6% of that formed at 19° C. ; the synthesis reached satura- 

 tion in a few hours, after which the concentration of ether-soluble mag- 

 nesium began to decline. The chlorophyll synthesis fared even worse — 

 the amount s.yntheeized never exceeded 15% of total ether-soluble mag- 

 nesium, and started to decline already after two hours of exposure. 



At 7° C, a fast initial formation of a small amount of chlorophyll was 

 noticeable in the first hour (without much change in the total ether-solu- 

 ble magnesium) ; a delay ensued in the next few hours, after which the 

 formation of ether-soluble magnesium compounds got under way and con- 

 tinued steadily even after 40 hours' exposure. The formation of chlorophyll 

 followed, and gradually caught up with, that of the total ether-soluble 

 magnesium compounds; after 40 hours, 70% of total ether-soluble mag- 

 nesium was in chlorophyll. 



At 19° C, the chlorophyll formation, after a slight initial lag, caught up 

 in a few hours with the total formation of ether-soluble magnesium com- 

 pounds (in agreement with the results shown in Table 37B.I for attached 



