HETEROSIS IN A NEW POPULATION 443 



3 where rains are generally well distributed throughout the rainy season. 

 However, in many areas of Zone 3, the corn is often subjected to long periods 

 of drought. Since drought generally reduces the total length of time for 

 growth, varieties are needed which are not only drought resistant, but also 

 earlier in maturity than Celaya. 



So far, no good hybrids earlier than Celaya have been obtained from a 

 recombination of early Celaya lines. It became necessary to look elsewhere 

 for material which would give the desired earliness and drought resistance 

 when combined with Celaya. The two races Conico Norteno and Tabloncillo, 

 which overlap Celaya in its distribution in Zone 3, were found to be un- 

 desirable because of their high susceptibility to both root and ear-rots, al- 

 though they were early in maturity. In the search for suitable material, a 

 race called Bolita, found in a small valley in Oaxaca about 500 miles from the 

 Bajio, has shown considerable promise. It probably originated in the Valley 

 of Oaxaca through the hybridization of Tabloncillo and an early maturing 

 tropical race called Zapalote Chico (Fig. 27.3). The Valley in Oaxaca where 

 Bolita probably originated has the same elevation and has a climate similar 

 to parts of the Bajio. Bolita, when grown in the Bajio, was found to be early 

 maturing, very resistant to ear-rots, and generally resistant to root rots. Its 

 yield capacity, however, was considerably below that of Celaya in years 

 with good rainfall distribution. 



Through a method of reciprocal testing of lines of Bolita with Celaya and 

 lines of Celaya with Bolita, Si lines of Bolita were isolated, which when 

 combined with certain Si lines of Celaya, produced hybrids superior to both 

 Bolita and Celaya in the drier areas of Zone 3. One of these hybrids, made 

 with a single cross of two Si Celaya lines (C 90 X C 67) as a female parent 

 and a synthetic of four Si Bolita lines as a pollinator, is now being pro- 

 duced for large scale testing. Preliminary data obtained on this hybrid, 

 called Celita, are given in Table 27.12. 



In the first three localities where Celita was tested, the rainfall was either 

 well distributed or supplemented by one irrigation in a period of extreme 

 drought. Under these conditions as evident in Table 27.12, Celita was about 

 equal in yield with the standard variety Celaya. But at Irapuato under ex- 

 treme drought conditions, Celaya yielded only 741 kilos per hectare (about 

 12 bushels per acre) while Celita yielded 1441 kilos per hectare, or about 23 

 bushels per acre. Also as indicated in Table 27.12 by the differences in per 

 cent dry matter at harvest, Celita was considerably earlier in maturity than 

 Celaya. Celita is also fairly resistant to root rots and much more resistant 

 to ear-rots than the best hybrids made with Celaya lines. 



It appears, therefore, that the hybrid Celita, under conditions normal 

 for Celaya, is equal to it in yield, but under severe drought conditions it is 

 greatly superior. This hybrid also is superior to Bolita under both normal 

 and dry conditions although the data are not presented in the table. Here 



