No abstract

‘Florigraze’ rhizoma peanut (Arachis glabrata Benth.) is a perennial legume of high forage quality adapted to warm climates, but there has been no comprehensive evaluation of its responses to grazing management. In 1988 and 1989, the effects of grazing frequency and intensity on Florigraze persistence and herbage accumulation (HA) were evaluated on a loamy, siliceous, hyperthermic Grossarenic Paleudults soil. All 12 combinations of three levels of residual dry matter after grazing (RDM, 500, 1500, and 2500 kg ha‐') and four grazing cycle lengths (GC; 7, 21, 42, and 63 d between grazings, including a 0.5‐ to 2‐d grazing period) were replicated twice. Data were analyzed by fitting multiple regression equations starting with a second order polynomial model. In 1988, rhizoma peanut HA ranged from 6130 to 10 240 kg ha− and increased linearly as GC and RDM increased. There was a GC‐by‐RDM interaction for rhizoma peanut HA in 1989, whereby at low RDM, increasing GC increased HA, but GC had less effect as RDM increased. In 1989, rhizoma peanut HA of at least 8800 kg ha− was estimated to occur with GC of 42 d or longer when RDM was 1500 kg ha− or greater. Rhizoma peanut percentage in HA was greatest with high RDM and long GC, but values of 80% or greater in the second year were estimated for RDM as low as 1300 kg ha− when GC was 63 d, or with GC as low as 7 d when RDM was above 2300 kg ha−. Lowest values were obtained with low RDM and short GC. These data indicate that unlike most tropical legumes, rhizoma peanut is productive and persistent over a relatively wide range of grazing management practices.

The tetraploid germplasm of bahiagrass, Paspalum notatum Flüggé, is an unexploited source of variability that can be used for the genetic improvement of this species as forage or turf. The objectives of this research were to develop a segregating population by hybridizing induced sexual and apomictic clones and to characterize the resulting population for mode of reproduction, seed fertility, growth habit, seasonal growth pattern, and frost resistance. A population of 591 hybrids had a 4:1 ratio between sexual and apomictic (facultative + obligate apomictic) plants and an 8:1 ratio between others (facultative apomictic + sexual) and obligate apomictic plants. The general fertility of the progeny was higher than that of the induced sexual parents. Significant differences were observed for self‐ and cross‐fertility among apomictic progeny and for self‐fertility among sexual progeny. Most sexual progeny set the same amount of seed when self‐ or cross‐ pollinated, indicating an absence of self‐incompatibility. Marked variation was observed for growth habit, seasonal growth patterns, and frost resistance among the progeny. Heritability estimates obtained for these characteristics indicate that selection can be successfully used for the genetic improvement of this species.

Manipulating apomixis for fixing tetraploid bahiagrass (Paspalum notatum Flüggé) hybrids exhibiting superior agronomic characteristics would be a valuable tool for the genetic improvement of this species. The objectives were to create a second generation of hybrids by crossing sexual first-generation tetraploid hybrids (FGTH) and unrelated apomictic FGTH or ecotypes, determine the segregation for mode of reproduction, and estimate the resulting genetic variation and heterosis for several agronomic characteristics. The segregation for mode of reproduction was analyzed using mature embryo sac observations. Field measurements and visual ratings were used to evaluate the growth habit, production of inflorescences, cool-season growth and freeze resistance of hybrids. A ratio of 4.6:1 between sexual and apomictic hybrids was observed. Only 3% of the progeny was classified as highly apomictic. The low proportion of highly apomictic progeny restricts the feasibility of this breeding approach. Large genetic variation was observed among hybrids for growth habit, cool-season growth and freeze resistance. The proportion of hybrids exhibiting heterosis was high for plant height, cool-season growth, and freeze resistance. The tetraploid bahiagrass germplasm contains a large diversity for traits of agronomic importance that can be exploited for the genetic improvement of this species.

No abstract

Crop improvement in bahiagrass (Paspalum notatum Flüggé) is limited by apomixis in most natural tetraploids, however, diploid sexual types occur. Production of sexual tetraploids by chromosome doubling will allow hybridization with apomictic tetraploids. Diploid bahiagrass (Paspalum notatum Flüggé) embryogenic callus tissue was exposed to three concentrations of three antimitotic chemical agents, colchicine, oryzalin and trifluralin. Callus was generated to plants and ploidy was evaluated by stomata size, mitotic chromosome counts, and flow cytometry. A total of 310 plants were verified as tetraploid of 1,432 plants that reached transplanting size. All treatments yielded 4x plants. The mean percentage success over all treatments was 22%, with means of 31% for oryzalin, 24% for colchicine and 16% for trifluralin. The high rates of success indicate that all agents can be successfully used to double chromosome numbers in bahiagrass. The percentage of 4x plants ranged from 9% (20 lM trifluralin) to 43% (20 lM oryzalin). Several treatments adversely affected regeneration. Mitotic chromosome counts are difficult and labor intensive in bahiagrass. Therefore, leaf stomata measurements were used as a preliminary screen. Data gave a bimodal distribution with overlapping tails and based on chromosome counts would have given an error rate of 12%. Flow cytometry analysis of regenerated plants resulted in mean nucleus fluorescence distributions consistent with control diploid or tetraploid values. These values agreed with chromosome counts, and this method is recommended for determining bahiagrass ploidy level. Research goals and available resources should be taken into consideration when selecting a treatment for chromosome doubling in bahiagrass.

Bahiagrass (Paspalum notatum Flüggé) is one of the major forage grasses used for beef cattle production in the southern USA. Description of the reproductive characteristics of bahiagrass germplasm is needed for genetic improvement of this forage crop. The objective was to determine and compare the mode of reproduction of diploid and tetraploid germplasm of bahiagrass. Reproductive behavior was determined by embryo sac observations and a series of controlled pollination studies in the greenhouse and field in 2004 and 2005. Two sexual diploid populations (‘Pensacola’ and ‘Tifton 9’) were not different in self‐ or cross‐fertility, indicating that the general fertility of the crop was not affected by phenotypic mass selection. Sexual diploids, as a group, were determined to be primarily cross‐pollinated, with low but variable levels of self‐fertility. A group of 20 artificially induced tetraploids was determined to be primarily sexual and cross‐pollinated. Fertility of these autotetraploids was lower than the original diploids. Two selected sexual tetraploids produced similar amounts of seed when self‐ or cross‐pollinated. In contrast, six other tetraploids, including the cultivars Argentine and Wilmington, the experimental hybrid Tifton 7, and three plant introductions 315732, 315733, and 315734 reproduced primarily by apomixis. The sexual or apomictic expression, the general fertility, and other agronomic characteristics of these groups of tetraploids define their usefulness for genetic improvement research.

RESEARCH S t. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze] is widely adapted as a lawn grass in warm, tropical and subtropical regions of the world (Sauer, 1972). The grass is highly resistant to weed infestation (Busey, 2003), grows well in a broad range of soil conditions, and exhibits good performance under shade conditions relative to other warm-season grasses (Busey and David, 1991; Busey et al., 1982b). Broad leaf blades and rapid stolon production allow the grass to form a coarse-textured monostand that is well suited for sod production, home lawns, and commercial landscapes. All these characteristics make St. Augustinegrass a valued turfgrass in the southern United States. The genus Stenotaphrum is comprised of seven species, all indigenous to coastlines from East Africa to islands of the South Pacific (Busey, 1995; Sauer, 1972). Pembagrass [S. dimidatum (L.) Brongn.] is the most closely related species to St. Augustinegrass. Evidence of introgression from this species in some polyploid St. Augustinegrass introductions has been observed (Busey, 1993, 1995). The base chromosome number of S. secundatum is