Storage in the 3 upper internodes of the wheat stem appears to have periods of increased and decreased accumulation (Fig. 1). In barley similar reductions were noticed at booting and at the milky ripeness stage, the most active periods for consumption of supplied assimilates (40). During booting internodes elongated and fructans decreased, but fructan decreased in the elongation zone of fescue (Festuca arundinaceae Schreb) leaves (41). In elongating barley leaves fructans peaked in the growth zone and then decreased in the zone just after the growth zone (42). The decrease observed in stem tissues could be related to use of carbon for microsporogenesis and pollen shed rather than for stem elongation. However, the same increases and decreases would be observed if fructans were utilized to control sucrose content. (43). Since large quantities of fructan appear to be non-lethal or non-punitive, an adaptive advantage for accumulation is unnecessary. Better to let fructan accumulate than to allow sucrose content to rise and therein become a signal molecule influencing gene regulation and thereby alter carbon distribution (44). But their accumulation could have evolved added benefits to the plant in adaptation to cold (45, 46) or drought (47). If sucrose builds in the vascular tissue then transport to the sink, storage in the stem, is inhibited by repression of the gene encoding the sucrose symporter which loads sucrose into the phloem at the source (44). This repression does not occur if sucrose concentration is kept low by incorporation into fructan.
The vasculature of the plant serves as the conduit for transport of sucrose, the principal transport form of carbon in cereals, from sources to sinks. Fructans are the chief storage form of carbon in the vegetative tissues of most small grains, rice being the notable exception. In leaf blade tissues fructans serve as short-term reserves of carbon accumulating to buffer sucrose concentrations in a diurnal cycle similar to the role of starch (48). The carbon source for fructan stored in the internode is sucrose transported to the internode in the phloem. The vasculature of a monocot where there is no secondary vascular meristem can influence the distribution of resources. In wheat the vascular bundles coming out of the leaf join the stem at the node of origin and at nodes one, two, and three below the node of leaf attachment (Fig. 3 after 49). Transported sucrose from the flag leaf can contribute to fructan accumulation in the internode above through bridging strands and to the first and second internodes below that of insertion (49, 50). When the grain demand is less or assimilate supply is increased then fructan storage goes up in nearly all of the internodes of the stem as visualized in the first 10 to 20 days after anthesis in Fig. 1. Figure 1 illustrates the total water soluble ketose sugars in different internodes however a similar pattern can be observed in fructan accumulation when supply of assimilates is increased by elevating the CO2 level (20). Elevated CO2 (680 |amol/mol compared to 360) significantly increased the post anthesis maximum accumulation of fructan in the upper most stem internode and in the two internodes below (20) as might be expected from the connections of the leaf vascular bundles to that of the stem. Under elevated CO2 the period of fructan accumulation was twice as long as well. Contribution on a mass balance approach was estimated at 1218% irrespective of CO2 treatment (20). This estimate is similar to that found in Schnyder's review (1) and is close to the 10-15% found in Wardlaw's review (51).
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