Growth of the peduncle
Elongation of the peduncle is similar to leaf elongation in grasses in that it is restricted to its tissues at the base of the internode, which is enclosed by the flag leaf sheath. At anthesis, the peduncle grew linearly in length at a rate of 28 mm d−1 (c. 1.2 mm h−1). Leaf elongation rate of wheat seedlings was reported to be higher and reached values up to 3 mm h−1 (e.g. Christ, 1978; Hu et al., 2000). In those studies, leaf elongation rate was measured at 20°C, which is within the optimum temperature range for growth of most temperate grasses (20–25°C; Cooper & Tainton, 1968). During peduncle elongation, mean daily air temperature ranged between 17 and 25°C. Lower temperatures during night might have caused a reduction of the mean peduncle elongation rate. Whether the peduncle elongation rate is always lower than leaf elongation rate is unclear. A high leaf elongation rate is crucial for the rapid development of light intercepting area in seedlings. Peduncle elongation is probably not primarily associated with the development of photosynthetically active area, but with exposure of the ear above the leaves. An increased distance between the upper leaves and the ear could lessen the risk of pathogen infection of the ear by ‘leaf borne’ diseases. This could be achieved by a high elongation rate, an extended duration of elongation, or a combination of both.
Elongation of the peduncle is followed by cell wall deposition (i.e. structural growth) to strengthen the newly built tissue (Lev-Yadun et al., 1999), which increases DM of the stem after anthesis (Gebbing et al., 1998). The lower sucrose concentration in the basal Pl segments until 10 DAA and the delayed accumulation of fructan in Pl1 and Pl2 might be attributable to this utilization of carbohydrates in structural growth. At mid grain-filling, Pu and Pl were approximately equal in mass (data not shown). Thus, the differences in WSC storage in these parts were almost exclusively attributable to differences in carbohydrate concentration.
Fructan and sucrose contents in the peduncle during grain filling
In the present field experiment the well described pattern of fructan accumulation in vegetative plant parts after anthesis and the mobilization of these reserves during later stages of grain filling was found (e.g. Blacklow et al., 1984; Kühbauch & Thome, 1989, for a review see Schnyder, 1993). The highest fructan content was observed at 16 DAA which is well in the range of 10–20 DAA given for temperate climate and favourable growth conditions (Schnyder, 1993). Higher carbohydrate (fructan) contents than in the peduncle were usually observed in the lower inserted internodes (Bonnett & Incoll, 1993; Willenbrink et al., 1998). But in most studies the peduncle was taken as a whole. In the present study the observed fructan concentrations in Pl were similar to fructan concentrations reported for the penultimate internode (Willenbrink et al., 1998).
At 14 DAA no diurnal changes in fructan concentration could be found, whereas the sucrose concentration changed significantly. This corroborates the assumed role of fructan in the wheat stem as a long-term reserve (Schnyder, 1993), which is not subject to turnover (Winzeler et al., 1990). Mobilization of fructans during late grain filling is associated with an increased activity of fructan exohydrolase (Simpson & Bonnett, 1993; Willenbrink et al., 1998). It has often been concluded from sink-source manipulation that this mobilization of reserves might buffer insufficient photosynthate production during late grain filling (Kühbauch & Thome, 1989).
Fructans do not accumulate in Pu
As expected from photosynthesising organs like leaves (Kalt-Torres et al., 1987) or glumes (Gebbing & Schnyder, 2001) the sucrose content in Pu showed significant diurnal variation. The photosynthetic activity of the peduncle is likely to contribute to stem photosynthesis observed by Evans & Rawson (1970). The increase of sucrose concentration during the light might be attributable to temporary sucrose storage in photosynthesising tissues, whereas the following decrease during the dark period is likely to be associated with export of sucrose or with respiratory metabolism.
The rate of sucrose accumulation during the light period of 1.1 mg g−1 f. wt h−1, was substantial. For comparison: in excised wheat leaves subjected to continuous light, sucrose concentration increased by c. 1.5 mg g−1 f. wt h−1 (at 300 µmol m−2 s−1 PPFD and 20°C (Penson & Cairns, 1994)), but in these leaves export of photosynthate was blocked completely by excision.
In both experiments, fructan concentrations were significantly higher in Pl than in Pu. Results indicate an abrupt discontinuity in fructan concentration between Pl6 and Pu during the fructan storage phase (16–23 DAA). Unfortunately, this discontinuity was not studied in spatial detail, but graphical interpolation (i.e. connecting the data points of Pl6 and Pu) excludes a linear decrease of the fructan concentration along these two compartments. Furthermore, changes of fructan concentrations in Pu were small between 10 and 23 DAA, whereas in Pl, the fructan concentration increased by a factor of c. 3. This also indicates a nonlinear decrease of fructan concentration between Pu and Pl, otherwise the fructan concentration in Pu would also have increased .
Much knowledge about fructan metabolism in grasses has been derived from experiments with excised leaf blades kept in a sucrose solution or exposed to continuous light (e.g. Penson & Cairns, 1994). Usually, fructan concentrations in leaf blades of grasses are low (Housley et al., 1989). But such treatments induce a rapid accumulation of WSC (mainly sucrose) followed by a change in gene expression (Winters et al., 1994) associated with fructan synthesis and subsequent accumulation of fructan (Wagner et al., 1983, 1986, Cairns & Pollock, 1988; Penson & Cairns, 1994). Thus, in these experiments fructan accumulated in tissues of high photosynthetic activity. Fructan accumulation is supposed to be a mechanism to sustain sucrose gradients between different cell types (Pollock et al., 2003). An increase of the sucrose concentration is assumed to be the signal for switching from export of carbohydrates to pathways associated with storage of fructan (Winters et al., 1994; Müller et al., 2000). Thus, fructan accumulation is induced by a certain sucrose concentration, which is in line with the definition of the fructan pool in leaves as an extension of the sucrose pool (Wiemken et al., 1995). The triggering sucrose concentration is considered to be c. 14–20 mg g−1 f. wt (Cairns & Pollock, 1988; Penson & Cairns, 1994).
In the present experiment, linear regression (Fig. 3a) showed that high (triggering) sucrose concentrations were reached after 10 h of light and stayed above this level for the next 10 h. In a study with excised leaves of Festuca, the application of inhibitors indicated that de novo synthesis of fructan synthesising enzymes started within 8 h after onset of sucrose accumulation and later (> 8 h) applications of inhibitors did not prevent accumulation of fructan (Winters et al., 1994). The peduncle contains the conducting tissues which transport all the photosynthate exported from the leaves to the growing grains, and part of the sucrose in the peduncle may be associated with the phloem (Willenbrink et al., 1998). The sucrose concentration in Pl increased during the latter part of the light period, which could be attributed to an increased flux of sucrose through the transport pathways, because photosynthesis of Pl is supposed to be negligible (Wardlaw, 1965). This may also hold for part of the sucrose accumulation observed in Pu. When this increase of sucrose in Pl was subtracted from the sucrose in Pu, thus correcting the sucrose concentration in Pu for this ‘transport sucrose’, the residual concentration of sucrose in Pu was still higher than 15 mg g−1 f. wt for more than 9 h (data not shown).
At 10 DAA the sucrose concentrations were similar in Pu and in Pl6. These concentrations were observed at 09:00 h and it is likely that the sucrose concentration in Pu had increased further during the day as in the growth chamber experiment. Thus, in both experiments the sucrose concentration seemed to be sufficient to induce fructan accumulation in both peduncle parts. But it is not known whether a higher sucrose concentration in Pu than in Pl is required to trigger fructan synthesis.
At least in part, the observed increase in sucrose concentration occurred in tissues which do not store fructan. For example the transport tissues are probably less sensitive to sucrose concentration in terms of triggering fructan synthesis, because besides one exception in Agave deserti (Wang & Nobel, 1998) fructans are virtually absent from the phloem. We are not aware of any study which determines the fraction of transport tissues in Pu and Pl. But the outer diameter of the peduncle decreases in the upper region (data not shown) and, assuming a constant conductivity for assimilates along the peduncle (i.e. a constant size of the transport pathways), this fraction of transport tissues might be higher in Pu than in Pl. Thus, there may be more of this nonfructan storing tissues in the upper part of the peduncle. Recent developments in single cell sampling and carbohydrate analysis showed different patterns of carbohydrate accumulation in different cell types of barley leaves (Koroleva et al., 1998). To our knowledge, such experiments with different cells of the peduncle have not yet been conducted.
The present study showed a strong spatial separation of the long-term storage of fructan within the peduncle. It remains unclear why fructan did not accumulate in the exposed photosynthetically active peduncle part. Fructan storage can rapidly be induced in photosynthetically active tissues, when the export of carbohydrates is restricted. But in most fructan storing species, under more ‘natural’ growth conditions, long-term fructan storage occurs mainly in tissues of low or negligible photosynthetic activity. The present findings corroborate the circumstantial evidence for a negative relationship between ‘photosynthesis’ and (long-term) fructan storage. Although still rather speculative, the assumption that metabolic steps associated with photosynthesis may hinder the long-term storage of fructan deserves further investigation.