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Cuticular conductance to water (gcw ) is difficult to quantify for stomatous surfaces due to the complexity of separating cuticular and stomatal transpiration, and additional complications arise for determining adaxial and abaxial gcw . This has led to the neglect of gcw as a separate parameter in most common gas exchange measurements. Here, we describe a simple technique to simultaneously estimate adaxial and abaxial values of gcw , tested in two amphistomatous plant species. What we term the 'Red-Light method' is used to estimate gcw from gas exchange measurements and a known CO2 concentration inside the leaf during photosynthetic induction under red light. We provide an easy-to-use web application to assist with the calculation of gcw . While adaxial and abaxial gcw varies significantly between leaves of the same species we found that the ratio of adaxial/abaxial gcw (γn ) is stable within a plant species. This has implications for use of generic values of gcw when analysing gas exchange data. The Red-Light method can be used to estimate total cuticular conductance (gcw-T ) accurately with the most common setup of gas exchange instruments, i.e. a chamber mixing the adaxial and abaxial gases, allowing for a wide application of this technique.

Contents 'Summary' 986 I. 'Introduction' 987 II. 'Pioneering metabolic studies of day respiration with 14C' 987 III. 'Metabolic flux pattern of day respiration' 988 IV. 'Significance of day respiration for leaf N assimilation' 991 V. 'Significance of day respiration for leaf gas exchange' 992 VI. 'Is day respiration influenced by CO2 mole fraction?' 995 VII. 'Significance of day respiration at the plant and ecosystem levels' 997 VIII. 'Conclusions and perspectives' 998 References 998 Summary It has been 75 yr since leaf respiratory metabolism in the light (day respiration) was identified as a low-flux metabolic pathway that accompanies photosynthesis. In principle, it provides carbon backbones for nitrogen assimilation and evolves CO2 and thus impacts on plant carbon and nitrogen balances. However, for a long time, uncertainties have remained as to whether techniques used to measure day respiratory efflux were valid and whether day respiration responded to environmental gaseous conditions. In the past few years, significant advances have been made using carbon isotopes, ‘omics’ analyses and surveys of respiration rates in mesocosms or ecosystems. There is substantial evidence that day respiration should be viewed as a highly dynamic metabolic pathway that interacts with photosynthesis and photorespiration and responds to atmospheric CO2 mole fraction. The view of leaf day respiration as a constant and/or negligible parameter of net carbon exchange is now outdated and it should now be regarded as a central actor of plant carbon-use efficiency.

Mitochondrial respiration often appears to be inhibited in the light when compared with measurements in the dark. This inhibition is inferred from the response of the net CO2 assimilation rate (A) to absorbed irradiance (I), changing slope around the light compensation point (Ic ). We suggest a model that provides a plausible mechanistic explanation of this 'Kok effect'. The model uses the mathematical description of photosynthesis developed by Farquhar, von Caemmerer and Berry; it involves no inhibition of respiration rate in the light. We also describe a fitting technique for quantifying the Kok effect at low I. Changes in the chloroplastic CO2 partial pressure (Cc ) can explain curvature of A vs I, its diminution in C4 plants and at low oxygen concentrations or high carbon dioxide concentrations in C3 plants, and effects of dark respiration rate and of temperature. It also explains the apparent inhibition of respiration in the light as inferred by the Laisk approach. While there are probably other sources of curvature in A vs I, variation in Cc can largely explain the curvature at low irradiance, and suggests that interpretation of day respiration compared with dark respiration of leaves on the basis of the Kok effect needs reassessment.

This work aims at developing an adequate theoretical basis for comparing assimilation of the ancestral C3 pathway with CO2 concentrating mechanisms (CCM) that have evolved to reduce photorespiratory yield losses. We present a novel model for C3 , C2 , C2 + C4 and C4 photosynthesis simulating assimilatory metabolism, energetics and metabolite traffic at the leaf level. It integrates a mechanistic description of light reactions to simulate ATP and NADPH production, and a variable engagement of cyclic electron flow. The analytical solutions are compact and thus suitable for larger scale simulations. Inputs were derived with a comprehensive gas-exchange experiment. We show trade-offs in the operation of C4 that are in line with ecophysiological data. C4 has the potential to increase assimilation over C3 at high temperatures and light intensities, but this benefit is reversed under low temperatures and light. We apply the model to simulate the introduction of progressively complex levels of CCM into C3 rice, which feeds > 3.5 billion people. Increasing assimilation will require considerable modifications such as expressing the NAD(P)H Dehydrogenase-like complex and upregulating cyclic electron flow, enlarging the bundle sheath, and expressing suitable transporters to allow adequate metabolite traffic. The simpler C2 rice may be a desirable alternative.

Stable carbon isotope ratios (δ(13) C) of terrestrial plants are employed across a diverse range of applications in environmental and plant sciences; however, the kind of information that is desired from the δ(13) C signal often differs. At the extremes, it ranges between purely environmental and purely biological. Here, we review environmental drivers of variation in carbon isotope discrimination (Δ) in terrestrial plants, and the biological processes that can either damp or amplify the response. For C3 plants, where Δ is primarily controlled by the ratio of intercellular to ambient CO2 concentrations (ci /ca ), coordination between stomatal conductance and photosynthesis and leaf area adjustment tends to constrain the potential environmentally driven range of Δ. For C4 plants, variation in bundle-sheath leakiness to CO2 can either damp or amplify the effects of ci /ca on Δ. For plants with crassulacean acid metabolism (CAM), Δ varies over a relatively large range as a function of the proportion of daytime to night-time CO2 fixation. This range can be substantially broadened by environmental effects on Δ when carbon uptake takes place primarily during the day. The effective use of Δ across its full range of applications will require a holistic view of the interplay between environmental control and physiological modulation of the environmental signal.

'Summary' 1077 I. 'Introduction' 1078 II. 'Atmospheric CO2 concentrations through time' 1079 III. 'Plant sensitivity to CO2 at geological timescales' 1080 IV. 'Plant sensitivity to CO2 over the last 200 yr' 1081 V. 'Plant sensitivity to long-term experimental manipulation of CO2' 1084 VI. 'Simple formulation of stomatal conductance in land surface models for simulating long-term CO2 response' 1087 VII. 'Conclusions' 1088 'Acknowledgements' 1089 References 1089 Summary The rate of CO2 assimilation by plants is directly influenced by the concentration of CO2 in the atmosphere, ca. As an environmental variable, ca also has a unique global and historic significance. Although relatively stable and uniform in the short term, global ca has varied substantially on the timescale of thousands to millions of years, and currently is increasing at seemingly an unprecedented rate. This may exert profound impacts on both climate and plant function. Here we utilise extensive datasets and models to develop an integrated, multi-scale assessment of the impact of changing ca on plant carbon dioxide uptake and water use. We find that, overall, the sensitivity of plants to rising or falling ca is qualitatively similar across all scales considered. It is characterised by an adaptive feedback response that tends to maintain 1 − ci/ca, the relative gradient for CO2 diffusion into the leaf, relatively constant. This is achieved through predictable adjustments to stomatal anatomy and chloroplast biochemistry. Importantly, the long-term response to changing ca can be described by simple equations rooted in the formulation of more commonly studied short-term responses.

Summary • The 13C isotopic signature (δ13C) of CO2 respired from plants is widely used to assess carbon fluxes and ecosystem functioning. There is, however, a lack of knowledge of the metabolic basis of the δ13C value of respired CO2. • To elucidate the physiological mechanisms driving 12C/13C fractionation during respiration, the δ13C of respired CO2 from dark-acclimated leaves during the night, from darkened leaves during the light period, and from stems and roots of Ricinus communis was analysed. The δ13C of potential respiratory substrates, the respiratory quotient and the activities of phosphoenolpyruvatecarboxylase (PEPc) and key respiratory enzymes were also measured. • It is shown here that the CO2 evolved from darkened light-acclimated leaves during the light period is 13C-enriched, and that this correlates with malate accumulation in the light and rapid malate decarboxylation just after the onset of darkness. Whilst CO2 evolved from leaves was generally 13C-enriched (but to a lesser extent during the night), CO2 evolved from stems and roots was depleted compared with the putative respiratory substrates; the difference was mainly caused by intensive PEPc-catalysed CO2 refixation in stems and roots. • These results provide a physiological explanation for short-term variations of δ13C in CO2, illustrating the effects of variations of metabolic fluxes through different biochemical pathways.

• Day-to-day variability in the carbon isotope composition of phloem sap (δ 13 C hd ) and ecosystem respiratory CO 2 (δ 13 C R ) were measured to assess the tightness of coupling between canopy photosynthesis (δ 13 C hd ) and ecosystem respiration (δ 13 C R ) in two mature Nothofagus solandri (Hook. f.) forests in New Zealand. • Abundant phloem-tapping scale insects allowed repeated, nondestructive access to stem phloem sap 1-2 m above ground.δ 13 C hd was compared with δ 13 C predicted by an environmentally driven, process-based canopy photosynthesis model. Keeling plots of within-canopy CO 2 were used to estimate δ 13 C R . • By including a lag of 3 d, there was good agreement in the timing and direction of variation in δ 13 C hd and predictions by the canopy photosynthesis model, suggesting that δ 13 C hd represents a photosynthesis-weighted, integrative record of canopy photosynthesis and conductance. • Significant day-to-day variability in δ 13 C R was recorded at one of the two forests. At this site, δ 13 C R reflected variability in δ 13 C hd only on days with

Wheat plants were grown at a day/night temperature of 18/13°C under glasshouse conditions. Twenty-two d after anthesis, one set of plants was shaded to 50% of the normal photon fluence rate, another was ‘degrained’ by selective spikelet removal which left only the grains in the five central spikelets; a further set was left as control. Individual plants were harvested at days 22, 30 or 42 after anthesis. Extracts from the peduncle and the penultimate internode were prepared to determine the activities of sucrose phosphate synthase, sucrose synthase, fructan exohydrolase and acid invertase, and to assess the concentration of hexose sugars, sucrose and fructans. Measurements were also made of ear and individual grain weights, and stem f. wt and d. wt. There was a decline in the amount of fructans with time, more pronounced in ‘shaded’ (source-limited) than in control plants. By contrast, in ‘degrained’ (sink-limited) plants, the amount of fructans in the stem initially rose, then decreased, with a concomitant increase in the amount of fructose. The shifts in sugar content of the wheat culm reflected both the sink demand of the ear and source activity. The activity of fructan exohydrolase correlated with the carbohydrate changes. Under limited photosynthate assimilation, the mobilization of fructans from the internodes towards the ear was related to an increase in this enzyme, whereas the other enzymes played a less direct role in the mobilization of fructan reserves from the wheat stem.

A partially purified enzyme preparation from leaves of Lolium temulentum L. was previously shown to catalyse the net synthesis of oligofructans and polyfructans from sucrose. Here the same preparation is shown to catalyse the hydrolysis of both sucrose and oligofructans. The magnitude and properties of these hydrolytic activities have been determined. The significance of these catabolic activities for studies of fructan polymerization both in vitro and in tissues in a physiologically anabolic state are discussed. The preparation hydrolysed 1-kestose, 6-kestose, neokestose, inulin oligosaccharides of low degree of polymerization (DP 4 and 5) and endogenous oligofructans from L. temulentum, with the concomitant release of monosaccharide. The preparation also released reducing sugar at low rates from high molecular weight inulin but had no detectable activity against bacterial levan. Simultaneous incubation of sucrose and Neosugar (a commercially available mixture of predominantly β-2, 1 linked tri-, tetra- and penta-saccharides) showed that sucrose was preferentially hydrolysed by the preparation, with Neosugar fructans being protected from hydrolysis at sucrose concentrations > 30 mol m -3 . The kinetic properties for hydrolysis of both sucrose and Neosugar were determined. For sucrose and Neosugar respectively, Michaelis constants at 30 °C and pH 6.0 were 7.7±0.5 and 14.1±1.1 mol m -3 (as terminal fructose) and maximum velocities were 6.5±0.1 and 2.7±0.1 mg g -1 fr. wt h -1 (equivalent to 10.0 and 4.2 nkat g -1 as reducing sugar release). Maximal temperatures for activity were 45 and 44 °C, and Arrhenius activation energies were 39.9 and 46.9 kJ mol -1 . Preincubations for 1 h at 49 and 48 °C caused 50 % loss of activity in subsequent assays at 30 °C. The pHs for maximal activity for the two substrates were 5.2±0.1 and 5.5±0.1. Using size exclusion chromatography (SEC), an activity catalysing the formation of fructan oligosaccharides and another catalysing sucrose hydrolysis, were not fully resolved, but exhibited distinct profiles of elution indicating M r of 57 and 133 kD respectively. When assayed for the hydrolysis of Neosugar, the SEC eluate exhibited two peaks of activity indicative of M r values of 57 and 133 kD.

To test the hypothesis that fructan structure can be used as a taxonomic character for the Poaceae we examined the accumulation of the linear, β-2,6-linked fructan series with a terminal glucose residue within the tribes Aveneae and Poeae. Only Dactylis glomerata L. (Poeae) has been shown to contain this series, making it unique among fructan structures and a potential taxonomic marker. To this end, 13 members of the tribes Aveneae and Poeae were surveyed for the presence of this fructan series by comparing extracts of water-soluble carbohydrate by TLC with an extract of D. glomerata. The fructans of four species were similar to those of D. glomerata, as determined by TLC. Further comparison by anion-exchange chromatography and linkage analysis demonstrated that Phalaris aquatica L., Puccinellia stricta (Hook. f.) C. H. Blom and Holcus lanatus L. contained a linear, β-2,6-linked series of fructan with a terminal glucose residue but that Lagurus ovatus L., although containing predominantly β-2,6-linked fructan was composed of more than one series of fructan, one with a terminal glucose residue and one with an internal glucose residue. A linear β-2,6-linked series of fructans with a terminal glucose residue was the dominant fructan accumulated by some species of genera belonging to both the Aveneae (Phalaris and Holcus) and the Poeae (Dactylis and Puccinellia). As both of these tribes also contain genera that accumulate fructans with an internal glucose residue, e.g. Avena and Lagurus (Aveneae) and Lolium (Poeae), structure of fructan cannot be used to distinguish these tribes. Review of the literature, however, showed that it is possible to separate the supertribes Triticodae and Poodae on the basis of the structure of the fructan that they accumulate.

The products formed by a fructan:fructan fructosyltransferase (FFT) activity purified from Lolium rigidum Gaudin were identified after gas chromatography-mass spectrometry of partially methylated alditol acetates, electrospray ionization-mass spectrometry and reversed-phase high-performance liquid chromatography. The FFT activity synthesized oligofructans up to degree of polymerization (DP) 6, but did not synthesize fructans of DP > 6 even when assayed with (1,1,1)-kestopentaose for up to 10 h. The FFT activity when assayed with 1-kestose or 6(G)-kestose synthesized fructan with fructosyl residues almost exclusively linked by beta-2,1-glycosidic linkages. When assayed with 1-kestose, the FFT activity synthesized tetrasaccharides and pentasaccharides with an internal glucosyl residue. The predominant tetrasaccharide was (1&6(G))-kestotetraose and the predominant pentasaccharide was (1&6(G),1)-kestopentaose. By comparison, tetrasaccharides and pentasaccharides extracted from L. rigidum also contained predominantly beta-2,1-glycosidic linked fructans with an internal glucosyl residue. The only exception was that one of the pentasaccharides contained beta-2,1- and beta-2,6-glycosidic linked fructosyl residues. This pentasaccharide was not synthesized by the FFT activity. The role of this FFT activity in formation of oligofructans in L. rigidum is discussed.

Fructan:fructan fructosyltransferase (FFT) activity was purified about 300-fold from leaves of Lolium rigidura Gaudin by a combination of affinity chromatography, gel filtration, anion exchange and isoelectric focusing. The FFT activity was free of sucrose:sucrose fructosyltransferase and invertase activities. It had an apparent pI of 4.7 as determined by isoelectric focusing, and a molecular mass of about 50000 (gel filtration). The FFT activity utilized the trisaccharides 1-kestose and 6(G)-kestose as sole substrates, but was not able to use 6-kestose as sole substrate. The FFT activity was not saturated when assayed at concentrations of 1-kestose, 6(G)-kestose or (1,1)-kestotetraose of up to 400 mM The rate of reaction of the FFT activity was most rapid when assayed with 1-kestose and was less rapid when assayed with 6(G)-kestose, (1,1)-kestotetraose or (1,1,1)-kestopentaose. The FFT activity when assayed at a relatively high concentration of enzyme activity (approximately equivalent to about half the activity in crude extracts per gram fresh mass) did not synthesize fructan of degree of polymerization > 6, even during extended assays of up to 10 h. When assayed with a combination of 1-kestose and uniformly labelled [C-14]sucrose as substrates, the major reaction was the transfer of a fructosyl residue from 1-kestose to sucrose resulting in the re-synthesis of 1-kestose. Tetrasaccharide and 6(G)-kestose were also synthesized. When assayed with 6(G)-kestose and [C-14]sucrose as substrates, the major reaction of the FFT activity was the synthesis of tetrasaccharide. However, some synthesis of 1-kestose and re-synthesis of 6(G)-kestose also occurred. When 6, kestose was the sole substrate for the FFT activity, synthesis of tetrasaccharide was 2.7 to 3.4-fold slower than when 1-kestose was used as the sole substrate. Owing to differences in the fructan:sucrose fructosyltransferase activity of the FFT with each of the trisaccharides, net synthesis of tetrasaccharide by the FFT was altered significantly in the presence of sucrose. The magnitude of this effect depended on the concentration of the trisaccharides. In the presence of sucrose, 6(G)-kestose could be a substrate of equivalent importance to 1-kestose for synthesis of tetrasaccharide.

SUMMARY Five fructan exohydrolase activities from Lolium rigidum Gaudin were separated and partly purified by a combination of salt precipitation, affinity chromatography, gel-filtration chromatography, anion-exchange chromatography and isoelectric focusing. The activities were identified by incubating enzyme fractions with fructan from both Lolium rigidum and Cichorium intybus. On the basis of activities when (6,6,6)-kestopentaose and (1,1,1)-kestopentaose were used as substrates, it was concluded that three of the activities hydrolyzed β-2, 6-glycosidic linkages faster than β-2,l-glycosidic linkages and two activities hydrolyzed β-2,l-glycosidic linkages faster than β-2, 6-glycosidic linkages. Fructan exohydrolases that hydrolyze β-2,l-glycosidJc linkages faster than β-2, 6-glycosidic linkages, and fructan exohydrolases that hydrolyze β-2, 6-glycosidic linkages faster than β-2,1- glycosidic linkages have not previously been identified together in a temperate grass. All fructan exohydrolases were inhibited markedly by sucrose. It is proposed that the classification of β-fructofuranosidases be reconsidered because EC 3.2.1. 80 is presently used to designate all fructan exohydrolases irrespective of the rates at which they hydrolyze β-2, 6 or β-2, 6-glycosidic linkages in fructans.

SUMMARY Fructan pentasaccharides were purified, in quantities suitable for use as substrates for enzyme assays, from Neosugar-p-(Meijj Seika Kaisha Ltd. Japan), tubers of Helianthus tuberosus L., L., and stems and leaf sheaths of Triticum aestivum L by a combination of gel-filtration and RP-HPLC. Fructan of higher molecular mass (mean DP = 30) was purified from Leaves of Lolium rigidum Gaud, that had been induced to accumulate fructan and characterized along; with the commercially available fructan from Cichorium intybus L. (Sigma, St Louis, USA) (mean DP = 33). The fructan pentasaccharide purified from H. tuberosus was found to contain exclusively 2, 1-linked fructose and terminal fructose and terminal glucose, and was identified as (1, 1, 1)-kestopentatise. The fructan pentasaccharide purified from Neosugar-P also contained (1,1,1)-kestopentaose. although the presence of fructan Klinked glucose and 1 % 2, 6-linked fructose indicated that a small proportion of other kestopentaoses were present, The fructan pentasaccharide purified from T aestivum consisted of almost exclusively 2,6-linked fructose and terminal glucose and terminal fructose and was considered to contain predominantly (6,6,6)-kestopentaose. The presence of 1 % 2,1,6)-linked fructose indicated the sample also contained a small proportion of branched kestopentanse. The high molecular mass fructan from C. intybus was found to comprise linear molecules containing only 2,1-linked fructose, terminal glucose and terminal fructose- High molecular mass fructan from L. rigidum contained predominantly 2. h-linked fructose, had predominantly internal glucose, indicated by 2 %, 1.6-linked glucose, low levels of branching, indicated 2 % 2,1,6-linked fructose residues; and 1% of the residues were 2,1 -linked fructose.

SUMMARY In grasses, fructan reserves are mobilized from vegetative plant parts during seasonal growth, after defoliation during grazing and from stems during seed filling. Well-illuminated leaves show a diurnal pattern of fructan accumulation during the light and mobilization during the dark. In expanding leaves, fructans are accumulated in cells of the elongation zone and when mobilized are considered to contribute assimilate for synthetic processes. Even in leaves which do not contain high fructan concentrations, high rates of fructan turnover occur. The process of fructan mobilization appears to be regulated in relation to ontogenic events, demand for assimilate during growth and in response to environmental stress. Hydrolysis of fructans in bacteria is catalyzed by both endo- and exohydrolases. However, in higher plants only fructan exohydrolases (FEH) (EC 3.2.1.80) have been reported. FEH has been extracted from only a limited number of grass species. The pH optimum of FEH activities varies between pH 45-5-5, the temperature optimum ranges from 25–40 °C and FEH is considered to be entirely localized in vacuoles. Estimates of the Km for FEH assayed using high molecular weight fructan substrates vary widely and should be considered carefully because most substrates are ill-defined. Many studies indicate that crude and partially-purified FEH activity is highest when assayed using a fructan substrate extracted from the species that was the source of the enzyme activity. Inulin extracted from members of the Asteraceae is generally less readily hydrolyzed and levans from bacteria are relatively poor substrates for FEH from grasses. Glycosidic-linkage-specific hydrolysis has been demonstrated for an FEH activity extracted from barley. This FEH activity hydrolyzed β-2,1-glycosidic linkages more rapidly than β-2,6-linkages. Most other studies are less conclusive because ill-defined fructan substrates were used. Two isoforms of FEH are reported in leaves of Lolium spp., but the roles of isoforms and their kinetic characteristics are not known. FEH activity in different tissues may be regulated by metabolic concentrations, sucrose (5–10 mw) being a strong inhibitor in vitro of FEH from some species. Results of experiments with Dactylis glomerata indicate control of expression of FEH activity at the gene level. In stem bases, FEH activity increased after defoliation. The increase was abolished by applications of inhibitors of protein synthesis and was apparently repressed by application of various sugars. Although the rates of fructan hydrolysis measured in vitro are sufficient to explain the in vivo rates of fructan hydrolysis, it is yet to be shown whether fructan hydrolysis in vivo is due to the activity of FEH exclusively, or FEH and invertase-like activities. The overriding conclusion is that the various studies of FEH from grasses present a confusing and incomplete picture of the function, activity and kinetics of this enzyme. This is due in part to the lack of defined, commercially-available substrates. The chromatographic techniques available to most laboratories do not permit purification of sufficient quantities of high molecular weight fructans of specific degree of polymerization, or fructan oligosaccharides with glycosidic linkages which differ from that of the inulin series for enzyme characterization. It is recommended that a few well-defined oligosaccharides be adopted as substrate standards for future research.

SUMMARY A partial separation of sucrose: sucrose fructosyltransferase (SST) activity from invertase activity extracted from leaves of Lolium rigidum Gaudin was achieved by gel filtration. The SST activity was purified 54-fold with a ratio of SST activity to invertase activity of 1.1:1. The partially purified SST activity eluted with an apparent molecular mass of about 68000, had an optimum pH of 55 and a Km of 0.2 M. When incubated with sucrose concentrations ranging from 10 to 600 mM the partially purified SST activity produced mainly 1-kestose and some 6-kestose (10–15% of trisaccharide produced). SST and invertase activities declined during storage at — 25 °C with invertase activity declining at a faster rate. After the loss of invertase activity in storage, the partially purified SST activity synthesized only 1-kestose. When incubated with 1-kestose the partially purified SST activity synthesized neokestose (1.9 nkat ml−1), nystose (2.2 nkat ml−1) and an unidentified tetrasaccharide (1.4 nkat ml−1) indicating that the fructosyltransferase preparation also had FFT-like activity. The partially purified invertase had significant sucrose-hydrolyzing activity (25 nkat ml−1) and contained no measurable SST activity when incubated with 10–400 mM sucrose, but produced trisaccharide (1-kestose and 6-kestose, ratio 15:1) when incubated with 600 mM sucrose. L. rigidum does not normally accumulate 6-kestose. The optimum pH of the sucrose-hydrolyzing activity was 4–5 and the optimum pH of the fructosyltransferase activity of the invertase preparation was 55. The partially purified invertase did not have significant FFT-like activity. The results support the hypothesis that SST activity synthesizes only 1-kestose, which is a key intermediate in the synthesis of fructan in L. rigidum.