Journal of Agricultural Science, 141: 149-154. (2003)
Earliness per se and its dependence upon temperature in diploid wheat lines differing in the major gene Eps-Am1 alleles
M. L. APPENDINO AND G. A. SLAFER

SUMMARY

Differences in development among wheat cultivars are not only restricted to photoperiod and vernalization responses. When both requirements are fully satisfied differences may still arise due to earliness per se. It is not clear at present to what extent this trait is 'intrinsically' expressed (a constitutive trait) independently of the environmental conditions so that it might be selected under any thermal condition or if it may be altered to the extent of showing a crossover interaction with temperature in which the ranking of wheat genotypes may be altered. The present study assessed the influence of temperature on the intrinsic earliness for lines of diploid wheat characterized for their differences in a major gene for intrinsic earliness, but also possibly differing in their genetic background for other factors controlling this polygenic trait. To do so the lines were grown individually in two temperature regimes (16 and 23 ºC) under long days having previously been fully vernalized. Multiple comparisons analyses were carried out among lines of the same allelic group for the Eps-Am1 gene. Results indicated that within each group there were lines that did not differ in their earliness per se, others differed but without exhibiting any line × temperature interaction and finally different types of interaction were shown, including cases where the ranking of lines was altered depending on the growing temperature. It is thus possible that the selection of a genotype based on its earliness per se in an environment might not represent the same performance in another location where temperature varied significantly.

INTRODUCTION

Differences in developmental patterns among wheats (Triticum aestivum, L.) are essential for improving adaptation (Slafer & Whitechurch 2001) and yield potential (Slafer et al. 1999). Understanding the genetic and physiological bases of these developmental patterns is critical for their rational use in breeding (either conventionally or assisted by molecular biology tools). In wheat the main environmental factors affecting development are photoperiod, vernalization and temperature (Pirasteh & Welsh 1980; Fischer 1984; Hay & Kirby 1991; Slafer & Rawson 1994). Major differences in time to heading among cultivars are ascribed to their responses to these factors. The largest differences are mainly attributed to different sensitivities to photoperiod and vernalization (Miralles & Slafer 1999). Consequently, both the genetics and physiology of these responses have been extensively studied in hexaploid wheat (Flood & Halbran 1986; Law 1987; Slafer & Rawson 1994; Worland 1996; Slafer & Whitechurch 2001; Snape et al. 2001). However, the differences between cultivars are not restricted by any means to these two sensitivities: even after all vernalization and photoperiod requirements are fully satisfied, there is still variation in time to heading (Slafer & Rawson 1994). These differences in development between cultivars have been termed earliness per se or intrinsic earliness (Hoogendoorn 1985; Masle et al. 1989; Penrose et al. 1991; Worland et al. 1994; Slafer 1996), and both terms will be used interchangeably hereafter. Although it has been shown that there is a wealth of variation in earliness per se in different geographical regions of Europe or America (Worland et al. 1994; Appendino et al. 2003), there are several genetic and physiological aspects that remain unexplained for this trait. To gain a better understanding of these aspects may be instrumental in fine-tuning developmental patterns for a particular photoperiod and vernalization responsiveness.

As part of our lack of physiological knowledge of this trait (see Slafer 1996 for a detailed description of non-validated assumptions), it has not been clearly established to what degree the intrinsic earliness/ lateness of a genotype is actually 'intrinsic'. This means that we do not know whether it is a constitutive trait expressed independently of the environment (Hoogendoorn 1985; Worland et al. 1994) or if it can be modified by the growing temperature (Slafer & Rawson 1995a). Broadly speaking, there are two possible major types of genotype x temperature interaction: in one of them the magnitude of differences among cultivars would change with the thermal conditions, but their ranking in earliness would not be affected. The other case represents a cross-over interaction in which the ranking of genotypes may be altered. For instance, in the most extreme case, a genotype known to be 'intrinsically early' in a particular condition may become 'intrinsically late' in a different condition (Slafer 1996). Similarly, and again for the extreme cases of the possible interactions, a genetic factor conferring intrinsic earliness may be expressed as a genetic factor for intrinsic lateness in a different environment.

As the screenings for intrinsic earliness require to be conducted with full satisfaction of the photoperiod and vernalization requirements, they are mostly (virtually always) conducted under controlled conditions, in which for practical and economic reasons temperatures are substantially higher than those actually experienced in the field by the crop. In this context, it seems quite relevant to elucidate whether the growing temperature may alter the expression of intrinsic earliness in wheat.

Part of the lack of knowledge on the likely genotype x temperature interaction in intrinsic earliness may be due to the complexity of the hexaploid genome of bread wheat in which this trait has been mostly studied. Using simple, diploid wheat may help to better understand the nature of this interaction. Another reason may be the quantitative nature of the trait, with many genetic factors controlling it located on different chromosomes (Scarth & Law 1983; Miura & Worland 1994; Laurie et al. 1995; Suárez et al. 1995; Worland 1996; Kato et al. 1999; Snape et al. 2001; Bullrich et al. 2002). This evidences a complex genetic base for this trait. Analysing the genotype x temperature interaction for intrinsic earliness in a set of diploid lines built up to strongly differ in the constitution of one of these genes (but with variation among lines for the constitution of other possible intrinsic earliness genes) may provide material better suited to investigate to what degree the thermal condition of the growing plants may alter their intrinsic earliness.

In a previous study aimed to locate a particular vernalization gene (Dubcovsky et al. 1998), two genotypes of Triticum monococcum (the spring type DV92 and the winter type G3116) showed a behaviour suggesting they might strongly differ in their intrinsic earliness. The objectives of the present study were (i) to illustrate the actual difference in intrinsic earliness between two lines of Triticum monococcum known to strongly differ in their responsiveness to vernalization and (ii) to assess the influence of temperature on the intrinsic earliness for lines of diploid wheat characterized for their differences in a major gene for intrinsic earliness, but also differing in their genetic background for other factors controlling this polygenic trait.


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