Impediments to Hybridization between Napiergrass and different Pennisetum Species

Napiergrass [ Pennisetum purpureum (Schum.) Morrone] is a robust, perennial, warm‐season grass that grows throughout the tropical and subtropical regions of the world. Some genotypes have sufficient winter hardiness to survive winters in the Gulf Coast region of the United States. However, germplasm with increased cold tolerance is needed to expand where the grass can be grown as a forage and biofuel crop in the southern United States. A male‐sterile napiergrass accession was pollinated with several genotypes of flaccidgrass ( P. flaccidum (Griseb.) Morrone], P. orientale (Rich.) Morrone, and buffelgrass [ Cenchrus ciliaris L. syn. P. ciliare (L.) Link] to transfer cold tolerance to napiergrass. More than 38,500 florets were pollinated but only two weak, short‐lived napiergrass × P. orientale seedlings were recovered. Controlled pollinations were made to determine what prevented hybridization, and pollen germination and tube growth in the pollinated pistils were observed using fluorescent microscopy. Most did not exhibit barriers to hybridization; however, pollen tubes of one P. orientale accession did not grow into the napiergrass styles. Pollen tubes became disoriented in the ovary and grew in an irregular manner in most crosses, but some tubes entered the micropyle. Since pollen–pistil incompatibilities were not the primary cause for failure of hybridization, post‐fertilization events in the embryo sacs were observed. The egg cell and polar nuclei were not fertilized in many sacs, and when they were, the embryo and endosperm began deteriorating within 3 to 5 d following pollination. Thus, hybrids were not recovered from crosses between these species because of post‐fertilization barriers, suggesting alternative strategies to wide hybridization may be required for the development of cold‐tolerant napiergrass.