High temperatures during late floral bud stages decrease fertilization in strawberry (Fragaria ₓ ananassa): pollen-pistil interaction and anatomical evidences

Abstract

High temperature (HT) effects on pistil tissues and female gametophyte have been scarcely investigated in crops species. During strawberry flowering, HT can induce fruit malformations due to poor pollen performance in pistils, reducing the fertilization level. In this study, the cultivars ‘Earlibrite’ and ‘Fortuna’ were exposed to ambient temperature (AT) or HT (>30 °C for 6-hour day-1) at late flower bud development over the duration of 3 or 5 days. To evaluate the capacity of heated pistils to support and guide pollen tubes, we examined the performance of unheated pollen grains and their path along the pistil, as well as the morpho-anatomy of reproductive tissues, in apical and basal pistils on the receptacle. In both cultivars, HT significantly induced a decrease in the number of adhered and germinated pollen grains, and of pollen tubes in the style and the ovule micropyle. After HT treatment, microscopic observations revealed loss of stigmatic papillae turgidity and fertilization failures in the ovary due to abnormal pollen tube paths. The latter finding was anatomically related to the incidence of high immature female gametophytes in apical pistils and unviable female gametophytes in basal pistils. Ovule examinations also suggested the occurrence of facultative apomixis. This is the first report of impaired pistil functions when strawberry flower buds are exposed to brief episodes of HT at late stages, as revealed by in vivo poor pollen performance, abnormalities in pollen-pistil interaction and fertilization failures within 24 h after pollination.