106 W. GORDON WHALEY 



LATER GROWTH AND HETEROSIS 



It is unfortunate that most studies of the physiology of heterosis have been 

 confined to the later growth period, and consequently do not include that 

 part of the growth cycle during which the important differences seem to be 

 developed. Nonetheless, we can learn much from these studies of later 

 growth as to the nature of the physiological differences which may furnish 

 bases for the development of hybrid vigor. 



The early experiments on physiological differences between inbreds and 

 hybrids were concerned mostly with the responses of the inbreds and the 

 hybrids to different soil conditions. A few examples will serve to indicate the 

 type of investigation and the character of the results. Hoffer (1926) deter- 

 mined the amounts of the constituents of the ash of heterotic hybrid corn to 

 be generally intermediate between those of the parental types. He noted that 

 iron and aluminum were present in the ash of the hybrids in smaller amounts 

 than in the inbreds. His studies showed that although there were marked 

 differences in the absorption of iron and aluminum in different soil types the 

 vigorous hybrids tended to absorb less of both these elements than the less 

 vigorous inbred lines. 



In the same year Kiesselbach (1926) reported distinct differences in water 

 requirements between selfed lines of corn and their heterotic F; hybrids. The 

 low productivity inbreds had much higher water requirements than the 

 vigorous Fi hybrids, when water requirements were calculated on the basis of 

 either water absorbed per gram of ear corn or water absorbed per gram of 

 total dry matter. Barley inbreds and heterotic barley hybrids were shown 

 by Gregory and Crowther (1928, 1931) to make distinctly different responses 

 to various levels of available minerals. These investigators postulated that 

 heterosis in barley might be directly related to differences in the ability of 

 the hybrids and the inbreds to use certain nutrients. This suggestion has had 

 a fairly adequate test, particularly with reference to nitrogen and phos- 

 phorus nutrition. 



The work of DeTurk et al. (1933), Smith (1934), Lyness (1936), Harvey 

 (1939), Burkholder and McVeigh (1940), and Rabideau et al. (1950), has 

 provided a fairly adequate picture of the relation of phosphorus and nitro- 

 gen nutrition to the development of hybrid vigor. Smith demonstrated dis- 

 tinct differences among inbred corn lines with respect to phosphorus nutri- 

 tion, noting that these differences were most apparent when the phosphorus 

 supply was limited. He postulated that the higher phosphate utilization effi- 

 ciency of the hybrids might be referred, at least in part, to the dominant in- 

 heritance in them of a much branched root system. Later studies have shown 

 that the root growth pattern is certainly important in relation to heterosis. 



Smith noted particularly that when inbred lines were inefficient in the 

 utilization of phosphorus or nitrogen, crossing them failed to produce hybrids 

 showing any evidence of physiological stimulation resulting in the more 



