High temperatures during late floral bud stages decrease fertilization in strawberry (Fragaria × 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. HT in strawberry can induce fruit malformations due to poor pollen performance in pistils, reducing the fertilization level. In this study, Fragaria × ananassa cvs. Earlibrite and Fortuna were exposed to ambient temperature (AT) or HT at late flower bud development over the duration of 3 or 5 days. To evaluate the capacity of heated apical and basal 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 anatomy of reproductive tissues. 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. Microscopic observations revealed loss of stigmatic papillae turgidity and fertilization failures due to abnormal pollen tube paths. The latter finding was related to the high incidence of immature female gametophytes in apical pistils and unviable female gametophytes in basal pistils. Facultative apomixis is also described. This is the first report of impaired pistil functions when strawberry buds are exposed to HT, as revealed by in vivo poor pollen performance 24 h after pollination.