WILLIAM J. ROBBINS 



Columbia University and New York Bofanical Garden 



Chapter 7 



Hybrid Nutrifionol 

 Requirements 



Hybrid vigor has been recognized for more than a century. It has been con- 

 sidered from a genetic, morphological, developmental, physiological, and 

 commercial standpoint. Although a great deal of information has been ac- 

 cumulated about the phenomenon, we are still unable to define exactly why 

 a hybrid grows better than the parents from which it comes. 



It is obvious that the cause is physiological — the hybrid functions more 

 effectively or for a longer period of time, and accumulates a greater mass of 

 cell substance. Its metabolic efficiency is greater (East, 1936). It would be 

 illuminating if we could locate specifically the physiological processes which 

 are responsible for the greater vigor of the hybrid — recognizing that they may 

 be numerous and complex rather than single and simple, and that they may 

 not be the same for all examples of hybrid vigor. 



For many years I have been interested in the factors which determine why 

 one plant species, variety, or strain grows slowly in a given environment 

 where another flourishes. I have dealt mainly with microorganisms, especial- 

 ly the filamentous fungi, because the external env ironment can be more easily 

 controlled and photosynthesis is not a complicating factor. From my ex- 

 perience, as well as from the work of others, it is clear that in many instances 

 growth — the accumulation of cell substance — is limited by the efficiency of 

 the organism's metabolic machinery, especially the activity of one or more 

 enzyme systems. Whether this concept can be applied also to the phenome- 

 non of hybrid vigor is still to be determined. However, it is a hypothesis 

 which deserves exploration. 



Let us begin with a simple example of growth-limitation. Aspergillus niger 

 grows well in a liquid medium of sugar, mineral salts, and asparagine. In the 

 same medium Phycomyces Blakesleeanus will not grow at all. 



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