HYBRID NUTRITIONAL REQUIREMENTS 123 



lites may be non-diffusible or very labile substances such as enzyme pro- 

 teins, which could only be introduced into the cell through inserting a nucleus 

 and its genes. The original hypothesis may be in error. We may not be 

 dealing with limiting quantities of essential metabolites but with inhibitors. 

 We might assume that the growth of one or both of the parents is limited by 

 autogenic inhibitors, and the presence of both kinds of nuclei in a common 

 cytoplasm results in the neutralization in some fashion of the inhibitors. 



Emerson (1948) has succeeded in producing heterocaryons in which one 

 kind of haploid nucleus neutralizes the effect of the other. The augmented 

 growth of the heterocaryon, as compared to that of strains which are 

 homozygous, reminds one, says Emerson, of instances of single gene heterosis 

 in maize reported by Jones. 



The importance of internal factors in heterosis is suggested by the results 

 I obtained on the growth of the excised roots of a heterotic tomato hybrid 

 and its inbred parents (Robbins, 1941b). The hybrid roots and the roots of 

 the two inbred parents were grown in liquid culture which contained mineral 

 salts and cane sugar. This basal medium was supplemented with thiamine, 

 with thiamine and pyridoxine, and with thiamine, pyridoxine, and nicotina- 

 mide. 



Growth of the roots of the hybrid exceeded that of either of the inbred 

 parents in all three types of media (Fig. 7.4). Growth of one parent was im- 

 proved by the addition of pyridoxine to the thiamine solution, but a further 

 supplement of the medium with nicotinamide had little effect. Growth of the 

 second inbred parent was little affected by the addition of pyridoxine to the 

 thiamine medium, but was improved by the further addition of nicotinamide 

 to the thiamine and pyridoxine solution. 



These results suggest that the greater vigor of growth of the heterotic 

 hybrid is determined in part by its greater ability to synthesize pyridoxine 

 and nicotinamide. That is evidently not the whole story, because its growth 

 exceeded that of the inbred parents in media containing all three vitamins. 



Although heterosis may be considered and should be considered from the 

 genetical standpoint, it should also be studied from the physiological stand- 

 point. I have suggested that it may be important to devote attention to the 

 question of v/hat the internal factors are which limit growth, what they are 

 in inbreds, and how they are removed in heterotic hybrids. We should con- 

 sider in such investigations the role of essential metabolites, of growth in- 

 hibitors, and of other specific chemical compounds which materially modify 

 growth. Microorganisms might be utilized as tools for the detection of growth 

 stimulators or growth inhibitors. 



