SUMMARYThe contents and composition of fructans in underground organs (rhizophores) of Vernonia herbacea (Veil.) Rusby, a perennial herb native to the Brazilian cerrado, were analyzed at different phenological stages, Fructopolysaccharides were always present in higher concentrations than oligosaccharides but variations in amounts of both fractions occurred together, A marked decline in total fructan and an increase in free fructose were observed between sprouting and fiowering, suggesting the occurrence of hydrolysis in association with the growth of new aerial shoots and flower development. There were higher amounts of total fructan at the end of summer and at dormancy, alternating with lower amounts at early dormancy. In spite of these variations the mean molecular mass of the fructo-polysaccharides remained nearly constant throughout the cycle, except for the higher value found in vegetative plants at the end of summer. The maintenance of the mean molecular mass of fructo-polysaccharides in the flowering phase concurrent with a marked reduction in total fructans suggests a peculiar mechanism of depolymerization in this plant, while the variation pattern found for fructan contents over the phenological cycle points to their role as reserve compounds in Vernonia herbacea.

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The underground reserve organs of yacon (Polymnia sonchifolia Poep. Endl.), similarly to other economically important Asteraceae, accumulate more than 60%, on a DW basis, of inulin type β(2‐1) fructans, mainly oligomers (GF2–GF16). Although sucrose:sucrose 1‐fructosyl transferase (1‐SST), fructan:fructan 1‐fructosyl transferase (1‐FFT) and fructan 1‐exohydrolase (1‐FEH) were properly described and characterized from a number of plant species, detailed information about their activities in different organs during development are rather scarce in the literature. In the present work 1‐SST, 1‐FFT and 1‐FEH activities were measured monthly in rhizophores and tuberous roots of yacon plants during their complete growth cycle under field conditions. Results showed that 1‐SST activity in rhizophores was always higher than 1‐FFT activity and increased up to 8 months of cultivation, decreasing to initial values at the end of the growth period. In the tuberous roots 1‐SST activity was also higher than 1‐FFT but varied differently. The higher values were found at the beginning of tuberization (3‐month‐old plants) and at the flowering phase (7‐month‐old plants). Results also showed that synthesizing activities in yacon plants were always higher in rhizophores than in the tuberous roots, while hydrolysing activity predominated in the latter, mainly when 1‐kestose and nystose were used as substrates. 1‐FEH from yacon plants showed low efficiency when commercial inulin from Helianthus tuberosus was utilized as substrate. The analysis of the enzymatic activities performed during growth of yacon clearly indicated the most appropriate source organ and phase of development to obtain the highest enzymatic activities for purification purposes and for the production of fructo‐oligosaccharides (FOS). Furthermore, the results suggested that the relative levels of activities of 1‐SST, 1‐FFT and 1‐FEH could be involved in the chain length distribution of the fructan molecules found in rhizophores and in tuberous roots of this species.

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