| Discussion There were significant amounts of transgressive segregation for days to ear emergence in the F2 populations particularly for the Condor x Sunset cross in all four combinations of vernalization and photoperiod. When photoperiod would have limited development several plants of the F2 potulation of the Condor x Kogat cross showed transgressive segregation for days to ear emergence (Tables 4 and 5). None of the F2 populations gave segregation ratios which would support simple inheritance. Photoperiod sensitivity in wheat was found to be controlled by two genes (PUGSLEY 1966 ; KEIM et al. 1973) and KLAIMI & QUALSET (1973) proposed a two gene system whth three alleles at each locus. Transgressive segregation in the Condor x Kogat F2, which occurred in both the vernalized and unvernalized populations grown under long day, however, does not necessarily imply complex inheritance for photoperiod response. Transgressive segregation of the entire F2 potulations occurred with the Condor x Sunset cross in three of the four treatments also in the Condor x Thatcher cross when vernalized plants were grown under long day. This was most marked in the vernalized long day treatment (Table 2) in which both vernalization and photoperiod did not limit rate of development. Under such conditions differences in days to ear emergence are likely to be due to differences in basic development rate (FLOOD 1983), i.e., development rate differences in the absence of vernalization and photoperiod influences. If this character was quantitatively inherited it is possible that it could cause transgressiveness for days to ear emergence in the types of environmental regimes of this study. Another possible explanation for the complete transgressiveness in certain of the F2 populations of the study could be the influence of the Condor cytoplasm (as female parent) on the expression of the "development" genes of the respective male parent. While there is little evidence in bread wheat for maternal inheritance, significant effects of alien cytoplasm on developmental responses in wheat have been reported (KINOSHTA et al. 1979 ; WARD et al. 1983). This study reveals the dependence of spikelet number on rate of development in wheat. In the absence of vernalization response, long photoperiod caused substantial reductions in spikelet number compared with short photoperiod, e.g., Kogat showed a reduction from 28 to 16 spikelets under short compared with long photoperiod. Similar effects have been observed by other workers (HALSE & WEIR 1970 ; RAWSON 1970, 1971 ; WALL & CARTWRIGHT 1974). The strong dependence of spikelet number on developmental responses in wheat raises the possibility of manipulating vernalization and/or photoperiod response to raise the yield potential as increased spikelet number. In the present study the cultivars Condor and Kogat both exhibited maximum spikelet numbers of ca. 28 (unvernalized/short photoperiod) but their developmental responses were markedly different. Kogat has no vernalization response but is strongly sensitive to photoperiod while Condor has a weak response to vernalization and a slight response to photoperiod. Without considering the genetic components of spikelet number control in these two cultivars, the production of the same number of spikelets per ear was associated with very different combinations of developmental responses. Thus, it may be possible to increase yield potential by breeding to incorporate developmental responses that confer optimum flowering time for maximum spikelet number. This approach to raising wheat productivity is also advocated by VINCE-PRUE & COCKSHULL (1981) as an alternative to the production of photoperiod insensitive wheats. |
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