Sprouting of the fructan- and starch-storing geophyte Lachenalia minima: Effects on carbohydrate and water content within the bulbs.
The carbohydrate pool within the bulbs of Lachenalia minima W.F. Barker (Hyacinthaceae) consists of similar amounts of fructans and starch. This study was conducted to examine the changes within the pool of non-structural carbohydrates that occur during sprouting under field conditions. The bulbs were watered over a period of 23 days to simulate the onset of the rainy season. Even though there was no significant change of the total fructan content, the distribution and the composition of the fructan fraction within the different leaf scales of the bulbs altered during sprouting. The major changes occurred in the innermost scales, the total fructan content increased from 300 (day 0) to 607 (day 23) g kg-1 dry mass and high-performance anion-exchange chromatography analysis revealed a significant increase of fructans with low degree of polymerization (DP). With respect to starch, the most pronounced difference accompanying the transition to growth was also in the innermost scales. In contrast to fructans, starch content decreased from 241 (day 0) to 60 (day 14) g kg-1 dry mass. These results demonstrate that starch, and not fructan, is used as the carbon and energy source for sprouting. The water content data suggest the involvement of fructans in water relations. The preferential accumulation of low DP fructans and sucrose within the innermost scales directs the water flow to where it is most needed for growth. Similar changes were obtained for bulbs in the dry soil, but transformation rates were much slower and occurred to a lesser extent, indicating that these reactions were not triggered but were accelerated by water.
Fructan Hydrolysis Drives Petal Expansion in the Ephemeral Daylily Flower
Dry weight, water content, soluble carbohydrate content, and carbohydrate composition of daylily (Hemerocallis hybrid cv Cradle Song) flower petals were monitored in the 3 d leading up to full opening and in the first day of senescence. Timing of events was related to the time (hour 0) when flower expansion was 60% complete. Petal dry weight increased linearly from hour -62 (tight bud) to hour 10 (fully developed flower), then fell rapidly to hour 34 as senescence advanced. Increase in water content was proportional to dry weight increase from hour -62 to hour -14, but was more rapid as the bud cracked and the flower opened, giving an increase in fresh weight/dry weight ratio. Soluble carbohydrate was 50% of petal dry weight up to hour 10, then decreased during senescence to reach 4% by hour 34. Up until hour -14, fructan accounted for 80% of the soluble carbohydrate in the petals, whereas hexose accounted for only 2%. Fructan hydrolysis started just prior to bud crack at hour -14, reaching completion by hour 10 when no detectable fructan remained, and fructose plus glucose accounted for more than 80% of the total soluble carbohydrate. The proportion of sucrose remained constant throughout development. Osmolality of petal cell sap increased significantly during fructan hydrolysis, from 0.300 to 0.340 osmolal. Cycloheximide applied to excised buds between hour -38 and hour -14 halted both fructan hydrolysis and flower expansion. The findings suggest that onset of fructan hydrolysis, with the concomitant large increase in osmoticum, is an important event driving flower expansion in daylily.
Carbohydrate Metabolism of Salt-tolerant Fructan Grasses as Exemplified with Puccinellia peisonis
Summary The physiological role of carbohydrates, particularly of water-soluble fructose polymers (fructans), in metabolism was studied in Puccinellia peisonis, a salt-tolerant, caespitose grass growing at the banks of salty pools (“Lacken”) in the Seewinkel area (Burgenland, Austria). High resolution steric exclusion chromatography (SEC) made it possible to determine the molecular-weight distribution of the homologous fructan series and to quantify the individual components. The seasonal changes in the concentration of free monosaccharides, sucrose, oligo- and polyfructans (in 4 fractions with increasing molecular weight) and starch, which always occurred in small quantities, were studied in relation to the plant's phenology. Results showed that high-molecular fructans (with a degree of polymerization, DP of more than 18) mainly served as storage components, whereas fructose oligomers, similar to the raffinose series in many dicotyledonous species, provided resistance to freezing. Fructose and sucrose contributed much to the osmotic potential of the plant, which as most monocotyledonous species is highly efficient in excluding excess ions from the substrate. The resulting osmotic values, however, are too low to bridge dry periods in summer. In such cases decomposition of the water-soluble fructans may lead to a rapid increase of the osmotic potential. Moreover, the possible role of low molecular-weight fructans (with DP 7 to 10) as effectors of osmoregulation is discussed. The multiform functions of a polymeric series of simple carbohydrates may help to explain the success of Poaceae in growing on dry biotopes.