Achene structure, development and lipid accumulation in sunflower cultivars differing in oil content at maturity

Sunflower cultivars exhibit a wide range of oil content in the mature achene, but the relationship between this and the dynamics of oil deposition in the achene during grain filling is not known. Information on the progress, during the whole achene growth period, of the formation of oil bodies in the components of the achene and its relationship with variations in final oil content is also lacking.

The biomass dynamics of achene components (pericarp, embryo, oil) in three cultivars of very different final oil concentration (30-56 % oil) were studied. In parallel, anatomical sections were used to follow the formation of oil and protein bodies in the embryo, and to observe pericarp anatomy.

In all cultivars, oil bodies were first observed in the embryo 6-7 daa after anthesis (daa). The per-cell number of oil bodies increased rapidly from 10-12 daa until 25-30 daa. Oil bodies were absent from the outer cell layers of young fruit and from mature pericarps. In mature embryos, the proportion of cell cross-sectional area occupied by protein bodies increased with decreasing embryo oil concentration. The sclerenchymatic layer of the mature pericarp decreased in thickness and number of cell layers from the low-oil cultivar to the high-oil cultivar. Different patterns of oil accumulation in the embryo across cultivars were also found, leading to variations in ripe embryo oil concentration. In the high-oil cultivar, the end of oil deposition coincided with cessation of embryo growth, while in the other two cultivars oil ceased to accumulate before the embryo achieved maximum weight.

Cultivar differences in mature achene oil concentration reflect variations in pericarp proportion and thickness and mature embryo oil concentration. Cultivar differences in protein body proportion and embryo and oil mass dynamics during achene growth underlie variations in embryo oil concentration.