| Discussion This study confirms previous findings (HALSE & WEIR 1971, MARCELLOS & SINGLE 1971, PUGSLEY 1971, SYME 1973, Halloran 1975, 1977) of the presence of vernalization response in most Australian spring wheats. This indicates that it is most likely of adaptive significance in conferring yield potential to wheat in Australia and that it has been indirectly retained, and most likely selected upon, in conjunction with yield selection. This study reveals that it is not of a uniform level in Australian wheats and there are indications of differences in the quantitative nature of the response - a threshold response whereby all vernalization is removed after four weeks of cold (3C) and a cumulative response which declines more gradually over six weeks of cold, and in the case of Sherpa up to eight weeks of cold. In studies on the nature of vernalization response in the near-isogenic lines of Triple Dirk (BERRY et al. 1980) threshold and cumulative responses were assigned to the action of vrn 3 and/or vrn 4 and vrn 1 genes, respectively. The gene vrn 2 was found to intensify both types of response. It could be speculated, therefore, that these types of response exhibited by those Australian wheats within the range of responses of the Triple Dirk lines were due to them possessing either the vrn 3 and/or vrn 4 genes in combination with either the vrn 2 or the vrn 1 gene. In terms of the absolute level of response none of the Australian wheats appear to possess both vrn 1 and vrn 2 carried by Triple Dirk C because of their much lower days to ear emergence values at 0 weeks vernalization compared with this line. However, certain wheats, e.g., Bindawarra possess a vernalization response much higher than Triple Dirk D (Vrn 1 vrn 2) or Triple Dirk B (vrn 1 Vrn 2) and much lower than Triple Dirk C (vrn 1 vrn 2). It is possible that such wheats possess different vrn alleles from the Triple Dirk lines at either of these two loci or at both loci or that there are other loci involved in determining their vernalization responses. Evidence has been produced that more than two loci are involved in the control of vernalization response in wheat (HALLORAN & BOYDELL 1967, Pugsley 1971, Gotoh 1979). The presence of significant differences in days to ear emergence between some of the wheats after eight weeks vernalization, when all vernalization response had been removed, indicates the possible existence of a factor(s) influencing development rate other than vernalization and photoperiod. This component of development has been termed basic development rate (FLOOD & HALLORAN 1982). Further studies are necessary of the interrelationship of basic development rate with vernalization and photoperiod responses, in influencing flowering time in wheat. Acknowledgements This research was undertaken while one of us (R.G.F.) was the holder of the William Farrer Memorial Research Scholarship (1979-1981) and on study leave from the Victorian Department of Agriculture. |
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