Presence of inulin in sunflower (Helianthus annuus L.) grown under high irradiance

Abstract

Fructans are important polysaccharides synthesized from sucrose which are present in about 12–15% of angiosperms. Sunflower (Helianthus annuus L.) is considered a non-fructan bearing plant even though its close relative, Helianthus tuberosus, accumulates the inulin type of the polymer in large amounts. Previous work suggested that putative fructan-synthesizing enzymes may be expressed in sunflower, but only very limited amounts of the trisaccharide isokestose were found in stems of plants storing high levels of sucrose due to capitulum removal. The present work is aimed at investigating whether intact sunflower plants may indeed synthesize fructans in any of its parts when grown in conditions that favor sucrose availability. Plants were grown in the field at a low density, resulting in a high light availability and low competition for resources, in comparison with controls (usual crop planting density). Plants were harvested at anthesis. Thinned treatment led to an increase in carbohydrates level especially in the capitulum. Carbohydrates analysis of this tissue in thinned plants revealed, for the first time in this species, the presence of inulin-type fructans. The amount of each member of the series appeared to decline starting from isokestose, being DP = 15 the longest fructan detected. Results suggest that, in sunflower, fructans could be synthesized only when sucrose availability exceeds a high threshold, which may not be attained under usual growing conditions. Given the relationship between fructans and tolerance to abiotic stresses including drought, the present finding opens a new perspective for breeding and management of this crop.