Impediments to hybridization between Napiergrass (Pennisetum purpureum Schum.) and other Pennisetum species

Authors: DOWLING, CHARLIE - Texas A&M University; Burson, Byron; JESSUP, RUSSELL - Texas A&M University

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/20/2017
Publication Date: 8/1/2017
Citation: Dowling, C.D., Burson, B.L., Jessup, R.W. 2017. Impediments to hybridization between Napiergrass (Pennisetum purpureum Schum.) and other Pennisetum species. Crop Science. 57:2043-2050.

Interpretive Summary: Napiergrass is a very productive, robust grass that grows throughout the tropical and sub-tropical regions of the world. Some plants have enough winter hardiness to survive winters in the Gulf coast region of the United States. However, plants with more cold-tolerance are needed to expand the area where the grass can be grown as both a forage crop and a biofuel crop in the southern United States. Napiergrass was crossed with three closely related, winter hardy grasses (flaccidgrass, oriental fountaingrass, and buffelgrass) to transfer cold-tolerance into napiergrass. Approximately 40,000 napiergrass flowers were pollinated with pollen from these three species, but only two hybrids were produced and they died shortly after germination. A study was begun to determine why these species did not cross with napiergrass and produce hybrids. The same plants were crossed again and flowers were collected at specific times following pollination and examined with a microscope. The pollen from the different donors germinated on the napiergrass stigmas and the pollen tubes grew into the ovaries. Next, the events inside the ovary were observed. It was determined the embryo and endosperm did not develop and viable seed were not produced. This is why hybrids were not recovered. A different approach will need to be used to develop a cold-tolerant napiergrass.

Technical Abstract: Napiergrass (Pennisetum purpureum Schum.) is a robust, perennial, warm-season grass that grows throughout the tropical and sub-tropical 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 the area where the grass can be grown as a dual-use forage and biofuel crop in the southern United States. A male sterile napiergrass accession was pollinated with several genotypes of flaccidgrass (Pennisetum flaccidum Griseb.), oriental fountaingrass (Pennisetum orientale L.C. Rich.), and buffelgrass [Cenchrus ciliaris (L.) syn. Pennisetum ciliare Link] to transfer cold-tolerant genes to napiergrass. More than 40,000 florets were pollinated but only two weak seedlings were recovered and they died. To determine what prevented hybridization, controlled pollinations were made between the same genotypes and pollen germination and tube growth in the pollinated pistils were observed using fluorescent microscopy. Most pollinations did not exhibit barriers to hybridization except the pollen tubes of one oriental fountaingrass accession did not grow into the napiergrass styles. In most crosses, some tubes became disoriented in the ovary and grew in an irregular manner but some tubes entered the micropyle. Since pollen-pistil incompatibilities are not the primary reason 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 days following pollination. This is why hybrids were not recovered from the crosses between these species and alters strategies for developing cold-tolerant napiergrass.