PHYSIOLOGY OF GENE ACTION IN HYBRIDS 107 



efTective use of these elements. Lyness (1936) studied heterotic Fi hybrids 

 resulting from crosses between a low phosphorus-absorbing capacity inbred 

 and a high phosphorus-absorbing capacity inbred. He found the heterotic 

 Fi plants to have high phosphorus-absorbing capacity. These results sug- 

 gested that phosphorus-absorbing capacity in corn, in some instances at 

 least, acts genetically as a dominant factor. Lyness also noted the relation- 

 ship between high phosphorus absorption and the extent of root develop- 

 ment. He supposed that the extent of root development might be responsible 

 for varietal differences in phosphorus absorption, a supposition which is sup- 

 ported by later studies. The work of DeTurk et at. (1933) suggested that more 

 than simply phosphorus-absorbing capacity is involved. This work revealed 

 that the actual phosphorus content patterns of two Fi hybrids of corn were 

 quite different. By estimating the amount of phosphorus in various chemical 

 fractions, De Turk and his coworkers were able to demonstrate marked phos- 

 ])horus pattern difTerences and to associate these pattern differences with 

 various phosphate fertilizer treatments. 



In our laboratory we have made a study of the phosphorus-absorbing ef- 

 ficiency of corn inbreds and hybrids, and have attempted to correlate the 

 findings of this study with developmental changes in the vascular system 

 and with general growth (Whaley et al., 1950; Heimsch et al., 1950; Rabideau 

 et al., 1950). The data indicate that heterotic hybrids definitely absorb more 

 radioactive phosphorus than their inbred parents. This advantage in ab- 

 sorption on the part of the hybrid is associated with more rapid early de- 

 velopment, with earlier attainment of maturity, and with certain features of 

 vascular organization. The greater absorption can be referred at least in 

 part to better early development of the root system in the hybrids, and to a 

 generally higher level of metabolic activity which presumably creates a 

 greater phosphorus demand. The greater absorption of phosphorus by the 

 hybrids is certainly one of the factors which compounds the heterotic effects, 

 but it seems doubtful that it is a primary factor in the development of hybrid 

 vigor. 



Harvey's (1939) studies of nitrogen metabolism among inbreds and hy- 

 brids of both corn and tomato showed differences from one line to another 

 with respect to the ability to use nitrate and ammonium nitrogen. The ex- 

 periments were of such a nature as to make it clear that such differences in 

 nutritional responses were results of differences in genetic constitution. The 

 behavior of hybrids produced from the inbreds reflected a combination of the 

 characteristics of the inbreds. Significantly, Harvey's study revealed that not 

 only did differences exist among his inbreds and hybrids with respect to the 

 ability to use different types of nitrogen, but that there were distinct genetic 

 differences in the responses of the plants to various levels of nitrogen avail- 

 ability. 



Somewhat similar differential responses to potassium availability were 



