Growth-promoting characteristics of potential nitrogen-fixing bacteria in the root of an invasive plant Ageratina adenophora

Root endophytic nitrogen-fixing bacteria (reNFB) have been proposed as important contributors to the invasiveness of exotic legumes; however, the reNFB of invasive nonlegumes has received less attention. In particular, the growth-promoting effect of reNFB on invasive plants remains unknown. In this study, 131 strains of potential nitrogen-fixing bacteria were isolated and purified from the roots of the invasive plant, Ageratina adenophora, in Southwest China. Phylogenetically, these reNFB were categorized into three phyla at 97% sequence identity that included Proteobacteria (92.4%), Actinobacteria (4.6%), and Firmicutes (3.1%). The dominant isolates ranked by number were Pseudomonas (80 isolates, 61.1%), Rhizobium (12 isolates, 9.2%), and Duganella (11 isolates, 8.4%). The community composition and diversity of A. adenophora reNFB were markedly different across study regions. The capacity of these reNFB to accumulate indolyl-3-acetic acid (IAA), solubilize phosphate, and produce siderophores was determined. All 131 isolates of reNFB accumulated IAA, 67 isolates solubilized phosphate, and 108 isolates produced siderophores. Among the three dominant genera of reNFB, Pseudomonas had the highest phosphorus solubilization and siderophore production, while the accumulation of IAA in the genus Duganella was the lowest. Interestingly, the calculated reNFB Shannon diversity index of each A. adenophora individual was negatively correlated with the capacity of reNFB to produce growth-promoting products. Six randomly selected isolates from three dominant genera were further used to conduct inoculation experiments, and all isolates showed significant positive growth-promoting effects on A. adenophora seedlings. The contribution of reNFB to the root biomass was higher than that to the shoot biomass. Our results suggest that reNFB, similar to soil or nodular nitrogen-fixing bacteria, can potentially promote plant growth and may play an important role in the invasion of nonleguminous plants. More detailed studies on the correlation between reNFB and invasive plants are necessary.

Prospects for Improving Mediterranean Grasslands in Lebanon through Seeding, Fertilization and Protection from Grazing

SUMMARYGrassland productivity was studied for four years near Terbol, Lebanon. In the first year pasture availability and plant numbers were monitored along transects, and in the following three years the effects of sowing four Mediterranean annual legumes (three cultivars of subterranean clover and a local ecotype of Medicago rigidula) and top-dressing with super-phosphate were studied. The results indicated that natural grasslands were dominated by annual grasses (Aegilops, Hordeum, Bromur, Lolium and Poa). Legume density was low, which resulted in poor legume productivity, especially in winter. Exotic legumes only resulted in a slight improvement in pasture productivity even when phosphate fertilizer was added. However, partial protection from grazing (for one or two months in late winter and spring) more than doubled the number of legume seeds in the seed bank compared with full protection and open grazing. The build up of seeds in the soil is an essential step towards the improvement of productivity in these degraded pastures, which form a large part of the land surface in Lebanon and on which small ruminant production largely depends.

Laboratory estimates of the nutritive value of some herbaceous native legumes

Forty-four accessions of herbaceous native legumes from the genera Glycine and Galactia were grown in a glasshouse in a soil-sand-nutrient mixture that contained adequate phosphorus, potassium, calcium, magnesium, sulfur and molybdenum for plant growth. They were grown as individual plants with four replications of each. All plants were cut to soil level on August 18; regrowth was cut on October 7, November 22 and December 30 (harvests 1,2 and 3). The herbage was dried and analysed for nitrogen (g N/kg OM), phosphorus (g P/kg OM) and organic matter digestibility (% OMD). There were significant differences between the accessions for values of dry matter yield (P<0.001), nitrogen content (P<0.01), phosphorus content (P<0.001) and OMD (P<0.05). The mean nitrogen content declined at each harvest (40.6,33.7 and 31.7 g N/kg OM, respectively; P<0.05) and OMD of the herbage at harvest 3 was less than that at 1 and 2 (70.4, 71.1 and 67.6%, respectively; P<0.05). Herbage from harvest 1 had significantly greater P values than that from harvests 2 and 3 (4.7,4.2 and 4.4 g P/kg OM; P< 0.05). Plant yield was correlated with both nitrogen yield (r = 0.98; P<0.001) and phosphorus yield (r = 0.93; P<0.01) but not with OMD. Nitrogen content and digestibility were correlated (r = 0.48; P<0.01) but neither attribute was significantly correlated with phosphorus content. Samples of native herbaceous legumes belonging to the genera Desmodium and Kennedia and of some exotic legumes were collected from a nursery and analysed for N, P and OMD. The N and P values of all these legumes were in the range 31.9-39.2 g N/kg OM and 3.0-4.1 g P/kg OM, and compared favourably with those of the legumes grown in the glasshouse. The OMD of the exotic legumes (range 61.3-85.2%) compared favourably with those of the native legumes grown in the glasshouse but the natives Desmodium spp. and Kennedia spp. (range 26.7-35.4%) were very much lower than the Glycine spp. and Galactia spp.

Prospects for the exploitation of biological nitrogen fixation

Agricultural productivity is largely determined by the availability of nitrogen, added as fertilizer or introduced by microbial nitrogen fixation. A doubling of the World’s population early in the next century will require a doubling of the effective agricultural nitrogen input. Economic and environmental constraints will probably preclude multiplying the input of industrial nitrogen fertilizer appropriately; so biological fixation, which is an exclusively microbiological process, must be exploited more effectively. Short term prospects include the expanded use of existing systems, such as exotic legumes, grasses and woody symbioses. In the medium term, plant breeding and genetic manipulation of the appropriate bacteria should yield more effective symbioses. In the long term, new nitrogen-fixing systems might be developed: somatic hybridization of plants might yield new symbiotic systems; manipulation of genetic information for nitrogen fixation into the plant genome might yield plants able to fix nitrogen independently of bacteria.

Possibilities for the enhancement of biological nitrogen fixation

Modern developments in plant science and knowledge of nitrogen fixation have opened exciting medium and long term possibilities: (1) Extended exploitation of existing systems such as exotic legumes; non-leguminous shrubs; the newly discussed grass associations; wider use of ‘green manures’. (2) Augmentation of the effectiveness of existing systems by altering the complement of nitrogen fixation ( nif ) genes; altering the genetic control systems to avoid repression by fixed nitrogen; avoiding wastage of nitrogen fixing capacity in hydrogen evolution; altering the efficiency of uptake of essential metals such as Mo and Fe; altering the character of the plant partner in symbioses. (3) Constructing new nitrogen fixing systems by preparing somatic hybrids of desirable crop plants with natural nitrogen fixing plants; introducing nif into new commensal or symbiotic microbes. (4) Transfer of nitrogen fixing ability to plants by ( a ) constructing DNA viruses or transferable plasmids carrying nif genes as possible vectors; ( b ) associating nif genes with plant mitochondrial or chloroplast DNA; ( c ) seeking uptake of nitrogen fixing microbes as potential organelles. (5) Introducing nif genes into rumen microbes; use of nitrogen fixing microbes as animal fodder. All these projects require extensive basic research; the fact that evolution has not so far provided such systems may mean that unforeseen obstacles to their construction may now exist.