Abstract
Grain filling in sunflower (Helianthus annuus L.) mainly depends on actual photosynthesis, being the contribution of stored reserves in stems (sucrose, hexoses, and starch) rather low. Drought periods during grain filling often reduce yield. Increasing the capacity of stem to store reserves could help to increase grain filling and yield stability in dry years. Fructans improve water uptake in soils at low water potential, and allow the storage of large amount of assimilates per unit tissue volume that can be readily remobilized to grains. Sunflower is a close relative to Jerusalem artichoke (H. tuberosus L.), which accumulates large amounts of fructan (inulin) in tubers and true stems. The reason why sunflower does not accumulate fructans is obscure. Through a bioinformatics analysis of a sunflower transcriptome database, we found sequences that are homologous to dicotyledon and monocotyledon fructan synthesis genes. A HPLC analysis of stem sugar composition revealed the presence of low amounts of 1-kestose, while a drastic enhancement of endogenous sucrose levels by capitulum removal did not promote 1-kestose accumulation. This suggests that the regulation of fructan synthesis in this species may differ from the currently best known model, mainly derived from research on Poaceae, where sucrose acts as both a signaling molecule and substrate, in the induction of fructan synthesis. Thus, sunflower might potentially constitute a fructan-bearing species, which could result in an improvement of its performance as a grain crop. However, a large effort is needed to elucidate how this up to now unsuspected potential could be effectively expressed.
Highlights
Sunflower (Helianthus annuus L.) is an important crop worldwide, ranking 4th among oil producing species 1 which produces high quality oil for human consumption
Because reserve assimilates stored in vegetative organs of the plant before anthesis may be mobilized during grain filling, they may buffer grain yield against adverse conditions for photosynthesis during that period
Our results suggest that the inability of sunflower to substantially accumulate fructan is not attributable to the lack of functional genes encoding for enzymes of fructan synthesis, neither to insufficient sucrose levels to induce fructan synthesizing enzymes in stems
Summary
Sunflower (Helianthus annuus L.) is an important crop worldwide, ranking 4th among oil producing species 1 which produces high quality oil for human consumption. Borrell et al (1989) reported that post-anthesis stem reserves may have contributed at least 21% of final grain yield of semi-dwarf wheat, while Gebbing et al (1999) reported similar average values for two cultivars of this species These percentages are increased during periods of drought (Blum, 1998; Gupta et al, 2011). Because reserve assimilates stored in vegetative organs of the plant before anthesis may be mobilized during grain filling, they may buffer grain yield against adverse conditions for photosynthesis during that period. This is a general phenomenon observed in cereals (Gebbing et al, 1999), and other crops including sunflower (Hall et al, 1989).
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