Phyllody in sunflowers; 44 years to explain the appearance of aberrant flowers

Abstract

Phyllody in sunflowers (Helianthus annuus L.) is characterized by the appearance of bracts and ray flowers in the centre of the heads. It has been discussed for decades among the sunflower plant breeders and experts without a clear “unique” explanation for it. The erratic appearance of the phenomena has led to consider it either a disease or hybrid defect and the plants with Phyllody as off-types, raising several farmers claims to seed producers. However, the auxins gradient, in the growing Asteraceae heads, determines the differentiation pathway of the undifferentiated cells. i.e., bracts and ray flowers at the border and disk flowers in the centre of the head. Disruption of the natural auxin gradient alters the cell differentiation in the growing heads and causes bracts and ray flowers to grow where only disk flowers should grow. The disruption of the natural head bottom auxin gradients may be caused by small injures in the fast-growing head receptacle. The most common causes of Phyllody are hormonal herbicide damage and Boron (B) deficiency, even temporary. Plants growing with B deficiency have brittle cell wall and membranes while a plant with high B levels produces plastic or elastic cell wall and membranes. Brittle cell walls are susceptible to breaks during growth. Sunflowers genotypes may react to crack or small damages in the sunflower bud in two ways. Some tend to repair the damages by regrowing new organs such as ligules or bracts and forming Phyllody. Others do not regrow and only heal the wound creating the funnel hole head shape.