Nitrogen Fixation in Grasses Inoculated with Spirillum lipoferum.

Field-grown pearl millet (Pennisetum americanum) and guinea grass (Panicum maximum), lightly fertilized and inoculated with Spirillum lipoferum, produced significantly higher yields of dry matter than did uninoculated controls. Up to 42 and 39 kilograms of nitrogen per hectare were replaced by inoculation for pearl millet and guinea grass, respectively. The data demonstrate that nitrogen fixation by these grass-Spirillum systems is efficient and is achieved at a reasonable energy cost to the plant.

Characterization of biomass production, cytology and phenotypes of plants regenerated from embryogenic callus cultures of Pennisetum americanum x P. purpureum (hybrid triploid napiergrass).

Five hundred and twenty-four plants of a triploid, sexually sterile hybrid napiergrass (Pennisetum americanum x P. purpureum; 3x=21) were regenerated from embryogenic callus cultures obtained from segments of young inflorescences. Replicated field trials were conducted for two consecutive years to compare the biomass yield, phenotype and cytology of tissue culture regenerants (TC) and vegetatively propagated (V) plants. In the first year total biomass yield of TC plants was significantly greater than V plants but there was no significant difference in the second year. TC plants had more tillers compared to V plants. V plants did not show any morphological variability. The TC population also exhibited a high degree of phenotypic stability (96%). There were 23 phenotypic variants in the TC population of 524, most of them being more dwarf and late-flowering. Detailed morphological analysis of the TC-variant plants suggests that they very likely arose from only a few variant cell lines. Cytological analysis indicated stability of the triploid status in randomly selected regenerants. Two of the morphological variants were hexaploids (6x=42). It is concluded that embryogenic callus cultures can provide useful alternative for the rapid propagation of hybrid napier-grass which is commonly propagated by cuttings.

Correlation and path-coefficient analyses of seed-yield components in pearl millet x elephantgrass hybrids.

Correlation and path-coefficient analyses have been successful tools in developing selection criteria. Since increased seed yield is an important goal in our pearl millet x elephantgrass [Pennisetum glaucum (L.) R.Br. x P. purpureum Schum.] hexaploid breeding program, we used correlation and path-coefficient analyses on seed data. This study was conducted to develop appropriate selection criteria by determining the direct and indirect effects of seed-yield components on seed yield plant(-1). Number of tillers plant(-1), panicles tiller(-1), seeds panicle(-1), 100-seed weight, and seed yield plant(-1), were estimated for individual plants in seven families. Phenotypic (rp) and genetic correlations (rg) were calculated, and path analyses (phenotypic and genetic) were carried out according to predetermined causal relationships. Phenotypic and genetic correlations differed in several cases due to large environmental variance and covariance. Phenotypically, all components were positively and significantly associated with seed yield plant(-1). Genotypically, only seeds panicle(-1) and 100-seed weight were significantly correlated. These two components were also positively correlated (r p=0.55, r g=0.63), so simultaneous improvement for both components would be feasible. Panicles tiller(-1) and seeds panicle(-1) were negatively correlated (r g=-0.97). In the path analyses, all direct effects of the components on seed yield plant(-1) were positive. Phenotypic indirect effects were not as important as genetic indirect effects. The components seeds panicle(-1) and 100-seed weight influenced seed yield plant(-1) the greatest, both directly and indirectly.

Plant yield and nitrogen content of a digitgrass in response to azospirillum inoculation.

Two Australian soils, a vertisol (pH 6.8, 0.299% N) and a sandy yellow podzol (pH 6.2, 0.042% N), were used with digitgrass, Digitaria sp. X46-2 (PI 421785), in a growth room experiment. Comparisons were made between plants inoculated with live and autoclaved bacterial suspensions of Australian and Brazilian isolates of Azospirillum brasilense. Seedlings were inoculated on days 10 and 35. Acetylene-reducing activity was measured five times during the experiment. Dry matter yields of the digitgrass on the podzol (low N) inoculated with live bacteria were 23% higher than those of the controls. On the vertisol (high N), yield increases from inoculation with live bacteria were 8.5%. The higher-yielding plants had significantly lower percent nitrogen, but when total nitrogen of the tops was calculated, the inoculated plants had a higher total N than did the controls (P=0.04). Acetylene-reducing activity was variable in the experiment, ranging from 0.5 to 11.9 mumol of C(2)H(4) core day. Live bacterial treatment induced a proliferation of roots, possible earlier maturity, higher percent dry matter, and a higher total N in the tops.

Responses of Sorghum and Pennisetum Species to the N(2)-Fixing Bacterium Azospirillum brasilense.

Three field inoculation experiments, two in Florida and one in New Mexico, were conducted with Azospirillum brasilense Cd. Each of the Florida experiments evaluated two crop species. One species in each of the Florida experiments responded to inoculation with a significant dry matter yield increases of 11 to 24% and nitrogen yield increases of 9 to 39%. No inoculation response was noted in the New Mexico experiment. The responding species were Sorghum bicolor (L.) Moench (sorghum) and the interspecific hybrid between Pennisetum americanum (L.) K. Schum. (pearl millet) and P. purpureum Schumach. (napiergrass). Nonresponding species were pearl millet (Florida) and Sorghum sudanense (Piper) Staph. (New Mexico). Acetylene reduction activity of inoculated plots in Florida was low, showing no increase over the natural uninoculated background rates and, in one case, was negatively correlated with yield. Acetylene reduction activity was not measured in New Mexico. In Florida, A. brasilense populations were found to decline from 5 x 10 to 5 x 10 bacteria g of soil in about 3 weeks (quadratic regressions). Continued decline to less than 10 by week 5 indicated that the inoculated bacteria did not become established in the soil in high numbers. The A. brasilense population declined at about the same rate in the New Mexico experiment. The erractic inoculation responses in these experiments are similar to those observed in earlier work at the University of Florida. The lack of acetylene reduction activity response to inoculation and the rapid population decline of the inoculated bacteria suggest that N(2) fixation is not the major mechanism causing yield responses after inoculation.

Isolation of Azospirullum from diverse geographic regions.

We have isolated Azospirillum (Spirullum lipoferum) from roots of grasses of several genera collected from a number of tropical and subtropical-temperate locations. Pure cultures were obtained from a small percentage of samples; no higher percentage was secured from tropical than from other grasses. Acetylene reduction and distinctive growth in N-free soft agar deeps were inadequate to identify this genus, although helpful in initial screening. Fluorescent antibody tests with antiserum against characterized strains were helpful. There is some evidence that this genus of bacteria may be favored in the rhizoplane.