744 



A. M. Mayer and A. Poljakoff-Mayber 



Table 3. The combined effect of gibberellic acid and coumarin on germination of 

 'Grand Rapids' lettuce seeds, at 25° C. in the dark. 



''Added in proof: Mayer, A. M. Joint action of gibberellic acid and coumarin 

 in germination. Nature. 184: 826, 827. 1959. 



through oxidative systems. Thiourea inhibits certain oxidative en- 

 zymes in lettuce (40, 42) and causes others to function at an earlier 

 stage (60). Coumarin may be linked to this process via its interference 

 with the phosphorus metabolism. Another possible link for this in- 

 teraction is the coumarin-destroying enzymes. As already mentioned, 

 lettuce seeds can metabolize coumarin and it is possible that this 

 ability is modified by thiourea, as thiourea affects phenolase activity 

 and inhibits its catecholase activity more than its cresolase, i.e., hy- 

 droxylating, activity (39, 42, 44). Thus, other hydroxylating en- 

 zymes containing metal as an active group may be similarly affected. 

 It is interesting to note that gibberellic acid reverses the inhibi- 

 tion of germination caused by coumarin (Mayer, unpublishecP). This 

 reversal is a function of both coumarin and gibberellic acid concen- 

 trations (Table 3). Such a reversal seems to be absent in growth in- 

 hibition of lettuce seedlings caused by coumarin. But Kato (23) re- 

 ports also partial reversal of growth inhibition on cucumber shoots 

 caused by coumarin. Both coumarin and gibberellin can be consid- 

 ered as naturally occurring growth substances. It is tempting to sug- 

 gest that exogenous coumarin affects in some way the growth sub- 

 stance metabolism or turnover. lAA itself need not necessarily be 

 involved, although the observation that coumarin inhibits lAA oxi- 

 dase of pea epicotyls (6) suggests that it may be. The observation by 

 Housley and Taylor (21) that the /iJ-inhibitor from potatoes contains a 

 coumarin derivative — scopoletin — may be relevant in this connec- 

 tion. 



SUMMARY 



Coumarin appears to be a widely distributed compound occurring 

 in many plants and seeds. It can stimulate or inhibit growth and 

 germination, depending on the plant species, the concentration, and 

 the presence or absence of compounds modifying its action. 



