PHYSIOLOGY OF GENE ACTION IN HYBRIDS 109 



growth-promoting activities of extracts from inbred and hybrid corn grains. 

 Robbins' evidence indicates that a substance or substances, which he has 

 designated as factor Z, may be synthesized in greater amounts by the hy- 

 brids than by the inbreds. He has stated that factor Z can be fractionated into 

 Zi, which is hypoxanthine; and Z2, a still unidentified fraction. Robbins' 

 work suggests that among the advantages possessed by heterotic hybrids 

 may be the ability to synthesize certain growth substances which the in- 

 breds either cannot synthesize or cannot synthesize as well. 



Further evidence of a slightly different nature is provided by the root 

 culture work of Robbins (1941b) and of Whaley and Long (1944). Robbins 

 used cultures of a strain of Lycopersicou pimpiiieUijolium Mill., a strain of 

 L. esculeutum Mill., and their Fi hybrid, in solutions supplemented by thia- 

 min, thiamin and pyridoxine, or thiamin, pyridoxine, and nicotinamide. 

 Robbins found that the Fi roots grew much more rapidly and produced 

 more dry matter than those of either parental line. He was able to show 

 further that one parental line made a greater response to the presence 

 of pyridoxine than did the other, while the roots of the second parental line 

 made a greater response to nicotinamide than those of the first. This suggests 

 the combination of complementary factors from the parents in the hybrid. 

 Whaley and Long (1944) obtained essentially the same results with a cross 

 involving two inbred lines of L. esculeutum. 



In the University of Texas tissue and organ culture laboratory, we have 

 been exploring certain aspects of this problem. While the results are not suf- 

 ficiently complete for publication, some facts are already clear. Among the 

 roots of many inbred lines of tomatoes which we have been culturing, there 

 are marked differences in growth responses associated with the availability 

 or non-availability of thiamin, pyridoxine, niacin, and certain other sub- 

 stances. These differences appear definitely to be inherited and they can be 

 studied in either the inbred lines or hybrids. 



It is still too early to say what the inheritance pattern is, but consideration 

 can be given to some aspects of the growth response patterns. One of the 

 most significant revelations is that the responses of most of the roots to a 

 specific substance are conditioned not only by the availability of that sub- 

 stance, but by the availability of the other substances and by the gen- 

 eral composition of the culture medium. Heterosis in tomato root cultures 

 is, like heterosis in whole plants, definitely relative, and conditioned, not 

 only by the environment, but, with respect to any specific gene action, by 

 the background of other gene actions taking place in the developing or- 

 ganism. 



Heterosis in tomato root cultures is definitely related to the inheritance of 

 the capacity to synthesize or utilize such substances as thiamin, ])yridoxine, 

 and niacin. This is not to suggest that heterosis in whole plants of tomato 

 may have its basis in the genetic recombination of factors concerned in the 



