Seed Production and Apomictic Stability in Tetraploid Bahiagrass.

Bahiagrass (Paspalum notatum Flügge) is a warm-season perennial grass widely used for forage and low maintenance turf applications in Florida. Diploid races reproduce sexually, while tetraploid races reproduce by apomixis. Apomixis is a clonal reproductive mode and plants that reproduce exclusively by apomixis are known as obligate apomicts. However, most tetraploid ecotypes are classified as facultative apomicts because apomixis and sexuality coexist simultaneously. Bahiagrass is primarily established from seed and it has been reported that defoliation following nitrogen (N) application can increase seed yield in bahiagrass. However, such response was found to be month-dependent and varied among cultivars. The objectives of this study were: i) determine the time to terminate defoliation and optimum N rate that will enhance seed yield and quality, and ii) evaluate if the mode of reproduction influences seed production in 8 tetraploid bahiagrass genotypes (5 dwarf, 2 hybrids and ‘Argentine’). The study was conducted on a bahiagrass sward planted in 2011 in a RCBD in a strip-split-plot. Main effects were the bahiagrass genotypes, while subplots were mowing dates (May and June); and sub-subplots N fertilization (ammonium nitrate at 0, 60, or 120 kg N ha^-1). Reproductive pathways were determined by cytoembryological analysis of ovules at anthesis. Inflorescences were collected at 3 different times in 2012: a) spring (only the dwarf genotypes), b) summer and c) fall. Only seed heads from the plots mowed in May were collected in the summer and fall. Mature seed heads were harvested twice in July 2012 and number of seed heads (NSH), percent seed set (SS), percent germination (GERM) and the reproductive efficiency (RE=seed set*germination/100) were measured. Five dwarf genotypes were classified as facultative apomictic and the expression of apomixis was higher during peak flowering, whereas the two hybrids and Argentine reproduced by obligate apomixis. Nitrogen fertilization did not affect the reproductive mode in any genotypes. The dwarf genotypes produced higher NSH, and NSH increased with N applications at 60 kg N ha^-1. NSH was higher in the plots mowed in May and the first harvest event yielded more seed heads. SS varied between 6-23%. Argentine and the dwarf genotypes had higher SS, and overall SS was higher in the plots mowed in May and from the first harvest . GERM varied between 26-69%, and higher values corresponded to obligate apomicts (except Hybrid 14). The plots mowed in June showed higher GERM, but there were no effects of either N or harvest event on germination. RE ranged between 2-16%. Argentine had a significantly higher RE (16%), and RE was higher in the plots mowed in June and in the first harvest event. In conclusion, all genotypes produced viable seeds, although high variability for seed production and viability was found. Time to terminate defoliation and N applications enhanced seed production, but the response of such practices differed between obligate and facultative apomicts. The lower RE found in the dwarf genotypes may be compensated by the higher NSH produced.