PHYSIOLOGY OF GENE ACTION IN HYBRIDS 105 



the rate of development during the embryo and seed maturation i)eriod is 

 of any critical importance with respect to the development of hybrid vigor 

 during post-embryonic growth. 



EARLY SEEDLING GROWTH AND HETEROSIS 



There have been few studies of early postgermination growth in plants in 

 relation to heterosis. It would seem that the usual failure to find higher 

 growth rates during the grand period of growth, or longer continued growth 

 periods in heterotic hybrids, would suggest that the answer to the develop- 

 ment of hybrid vigor lies for the most part in the early postgermination 

 growth stages. The work of Ashby and his co-workers (Ashby, 1930, 1932, 

 1936; Hatcher, 1939, 1940; Luckwill, 1937, 1939) emphasized that the hybrid 

 advantage in their materials was either present in the resting embryo or be- 

 came manifest in early postgermination growth. Its development was defi- 

 nitely not a characteristic of the later growth phases. This observation has 

 now been made for many cases of hybrid vigor (Whaley, 1950). There are 

 some instances in which hybrid vigor seems to be the result of longer-con- 

 tinued growth on the part of the hybrid. These probably have a dilTerent 

 explanation from the majority of cases. 



We have been concerned lately in our own laboratory with an analysis 

 of the early postgermination growth of corn inbreds and single and double 

 cross hybrids (Whaley, 1950) . Studies of growth during the first ten to twelve 

 days after germination have revealed that the hybrid advantage is largely 

 the result of the heterotic hybrid plants reaching a high growth rate earlier 

 than do the inbreds. Almost without exception, the development of the hy- 

 brid advantage takes place very rapidly in the early stages of germination 

 and growth. Rarely have we seen evidence of the hybrids having higher 

 growth rates during any later part of the developmental cycle. Neither are 

 the hybrid growth periods extended appreciably beyond those of the in- 

 breds. In most instances the hybrids mature somewhat more rapidly than 

 the inbreds — a fact of common observation among plant breeders. 



Since the attainment of the maximum growth rate takes place more 

 quickly during the early stages of development, the hybrids do have a longer 

 maximum growth rate period. During this period the early advantage is 

 compounded, to give a considerably greater maturity advantage. When 

 both the inbred lines and the hybrids used in our studies are considered, it is 

 apparent that the rapid attainment of high early growth rates is correlated 

 with relatively low embryo weights. This apparent higher efficiency of small 

 embryos and its importance in relation to hybrid vigor requires further study. 



On the basis of the data at hand one can suggest that the hybrid advantage 

 lies in the more rapid unfolding of certain metabolic processes, a suggestion 

 which receives support from the recorded studies of later growth. 



