Carbohydrates are often reported to have multiple, functional roles in mediating a wide range of plant growth and environmental responses (Calenge et al., 2006). This is as true of fructans, the water-soluble carbohydrate (WSC) polymers found in several plant groups including the Poaceae (Chalmers et al., 2005), as it is of other carbohydrates. The capacity to accumulate fructans in these temperate plants has been postulated to have evolved to meet a need to adapt to cold winters and dry summers (Hendry, 1993).
Regrowth rates following defoliation have often been linked to vegetative carbohydrate reserve levels (White, 1973; Donaghy & Fulkerson, 1997). In perennial ryegrass (Lolium perenne) the fructans in the leaf sheaths constituting tiller base vegetative reserves may be mobilized to fuel reserve-driven growth (Prud’homme et al., 1992). More generally reserve-driven growth rates have sometimes been shown to be higher when WSC reserves are higher (Turner et al., 2001). This may also apply to early spring growth, which can be higher in ryegrass varieties bred for elevated herbage sugar content. However, these effects are sometimes small and short-lived as photosynthate supply is not often limiting. Indeed, some authors have suggested that nitrogen reserves may be more important (Volenec et al., 1996).
Fructans are widely believed, and frequently quoted, to be involved in resistance to environmental stresses such as cold and drought, although direct correlations have not always been shown (Vijn & Smeekens, 1999). They have been cited as osmoprotectants, in common with proline and glycine-betaine, with postulated roles in membrane stabilisation (van den Ende et al., 2005). It is certainly true that fructan content generally increases during such stresses, but accumulation purely as a byproduct of a reduced growth rate is very different from an active functional role in stress resistance. Species, ecotype and variety comparisons have commonly been used to compare cold tolerance and fructan content in temperate grasses and cereals; in many of these comparisons the more cold-tolerant plants have been shown to be those with higher fructan and/or carbohydrate content (Thorsteinsson et al., 2002; Kerepesi et al., 2004). However, in most cases the genetic backgrounds of the materials being compared have been very different in many other respects, reducing the significance of any conclusions. Moreover, Eagles & Williams (1992) concluded from their work on ryegrass that fructan was not important for cold tolerance. Studies of the same types have been carried out with respect to drought stress. Volaire et al. (1998) reported a positive correlation between the proportion of sugar in the form of polymeric fructan and drought resistance while, by contrast, Thomas & James (1999) concluded that WSC did not appear to be a scarce resource even under severe temperate drought in perennial ryegrass, and that lack of sugar was unlikely to limit survival and regrowth in these circumstances.
As forage fructan content has been demonstrated to have an important role in animal nutrition (Miller et al., 2001), selecting for increased WSC has had high priority in many forage grass breeding programmes. Temperate grasslands support most of the world's milk and meat production so the nutritional value of fodder has a major impact on the efficiency and profitability of livestock production. In excess of 80% of agricultural forage seed usage in the UK is ryegrasses (Burgon et al., 1997) and perennial ryegrass is the most important species. Should increases in the fructan content of perennial ryegrass forage also have positive effects on traits such as regrowth (after grazing and mowing), stress resistance and persistence, then the selection of appropriate alleles for forage quality would have added value.
Further work is necessary to clarify the role of fructan in physiological processes in plants. The objectives of the studies reported here were to characterize regions of the perennial ryegrass genome that have basic control over selected growth and drought-stress traits, and, by comparing quantitative trait loci (QTL) positions, to explore potential functional relationships with fructan content. As the aim was to characterize repeatable effects the strategy of combining replication with year-to-year variation in environmental conditions following the principles advocated by Borevitz & Chory (2004), and employed in a previous WSC QTL study (Turner et al., 2006) and by Price et al. (2002), was also used in this work. Perennial ryegrass populations demonstrate considerable variation for WSC content and fructan QTL have previously been identified in an F2 mapping family (Turner et al., 2006). A range of growth and drought traits have now been measured on the same family. Evidence of co-location for QTL positions for carbohydrate content and physiological traits would be consistent with a close integration of the genetic regulation of the traits and the importance of fructan in these physiological processes, although it should not be forgotten that co-location could simply reflect tight linkage of genes controlling very different traits.