DISCUSS/ ON 671 



llaiulmski aiul I showed tli;il ;ill)iiu) com. in wliidi Baiulurski was unable 

 to liiul aiiv carotenes at all, was (|m"tc adixc pholotropically. We did not 

 publish the piiototropir thita, l)ecau>e the Indian corn used has a long. 

 iMidulating (irst internode, and we couldn't get any good cpiantitative photo- 

 tropism data. .Secondly, Wallace .uicl Haijcrmann have a sunllower which 

 thev find is analytically free of carotenes, but certaiidy phototropically active. 

 Thirdly, there is the evidence from Phycomyces grown on a medium in 

 which the main carbon .source is lactate. Growth, especially in the presence 

 of diphenylamine, leads to a depression of the carotene level to about l"j, 

 or less of the normal with, however, the full retention of phototropic sensi- 

 lixitN. I believe it would be fair to include all ol this evidence in an 

 evaluation of the photoreceptor problem. 



Dr. Thimann: In general, my answer to Dr. Galston's question is that 

 none of these studies of albino plants is decisive. My reasons for saying 

 this are that in general the phototropic sensitivity is spoken of as still present, 

 l>ut no attempts at all are made to compare quantitatively the sensitivity 

 of these plants to those of their pigment-containing controls. An example 

 of this is the white Helinnthus, which Dr. Galston mentioned, and which 

 we have also grown. It certainly contains very little carotenoid, and it 

 certainly is phototropically sensitive, but only very weakly so. It is not 

 easy to make measurements with it since it produces very large mutations. 

 The phototropic sensitivity is a very small curvature superimposed on the 

 large differences of the controls, so no measurements are really possible. 

 I have corresponded with Dr. Wallace about this, because in his paper 

 with Dr. Habermann he speaks of these plants as being normally photo- 

 tropic. It turned out that he exposed these plants to light for 24 to 36 

 hours, so you can see what sort of energy dose they got. As a result of this 

 they yielded a positive response. In general, this sort of thing is true. When 

 Phycomyces is grown on diphenylamine, for instance, the amount of caro- 

 tenoids is reduced to some 5% or so of that of the control. But unless one 

 carefully measures the phototropic sensitivity, perhaps, for all we know, 

 the phototropic sensitivity may be 5% of the control, too. So unless we 

 have measurements of both kinds I fear those data are almost worthless. 



Dr. Galston: May I add one thing to that. The data which Bandurski 

 obtained on carotene led him to believe that there could not be more than 

 0.1% of the normal carotene content in the albino maize with which he 

 worked, and yet the phototropic sensitivity was still present. Now it is com- 

 pletely impossible to prove that any tissue is completely free of carotene. All 

 one can do is to set an upper limit to the carotene present. I would say 

 that this one case at least must be taken seriously. 



Dr. Thimann: Well, I should like to see the phototropic data, and I 

 would like to ask Dr. Galston whether he has any comparable data on the 

 riboflavin content to indicate whether it remains normal. 



Dr. Galston: In that one we did do the riboflavin assay, and it is normal. 



