Ultrastructure and histochemistry of the receptive surface and stigma exudate in strawberry (Fragaria × ananassa Duch.)

The stigma ensures successful pollination, seed, and fruit development by capturing pollen for germination and initial tube growth during its receptive period. Detailed stigma structure in relation to receptivity remains underexplored in strawberry. This study evaluates stigmatic function in Fragaria × ananassa ‘Camino Real’ through histochemical, ultrastructural, and pollination analyses. Stigma receptivity was assessed via manual pollinations from 1 day pre-anthesis to 6 days post-anthesis. Four distinct developmental stages were examined using light microscopy and three stages using transmission electron microscopy. F. × ananassa exhibits a wet stigma with a non-papillate epidermis overlying 3–4 subepidermal layers. Epidermal cells initiate active secretion at pre-anthesis, producing protein- and polysaccharide-rich exudate that covers the surface by flower opening. Stigmatic receptivity begins one day before anthesis, with receptive and non-receptive stigmas potentially coexisting in one flower, and continues until 6 days after anthesis. Pollen tubes grow through subepidermal cell cytoplasm, indicating a less common pollen tube pathway. Continuous exudate sustains prolonged receptivity by supporting pollen hydration and tube growth into substigmatic tissue. Unlike other Rosaceae, this study suggests that strawberry stigma exudate derives from epidermal secretion, without epidermal lysis or substigmatic contributions during the receptivity period.