Evaluation of coated seeds as a Rhizobium delivery system for field pea

Greenhouse and field experiments were conducted with field peas (Pisum sativum, L.) in soils of pH 4.4 to 6.8 to determine the best rate of inoculation with rhizobium and to evaluate pre-inoculated (coated) seeds as an alternative to the traditional seed inoculation method of using sticking agents. Inoculation rates higher than 105 cells seed–1 were usually required for high nodulation, nitrogen fixation and grain yields. Therefore, Canadian standards, which require that 105 nodulating rhizobia be delivered per seed for large-seed legumes like peas, may need to be increased. Counts of rhizobia on coated seeds were about 3 log units lower than those on freshly inoculated seeds, but coated seeds significantly outperformed standard seed-inoculated seeds in nodulation and crop yield in acid soils (pH 4.4 and 4.7). However, field pea yields were too low to have commercial value at these low pH levels. In soils with higher pH, standard inoculation resulted in greater nodulation and yield, but the differences were not always significant. It is concluded that the use of coated seeds provides a yield advantage for field pea grown on acid soils, but liming would probably be a better option. Use of coated seeds on other soils will depend on the trade-off between the time and money saved in inoculation in order to seed early and a possible reduction in yield due to insufficient numbers of rhizobia being applied. Key words: Nitrogen fixation, nodulation, Pisum sativum, pre-inoculated seeds

Soil nitrogen and lodgepole pine seedling responses to five years of legume cover

Legumes were tested for their ability to increase soil N content and improve growth of lodgepole pine (Pinus contorta Dougl. ex Loud.) seedlings in west-central British Columbia. A trial with alsike clover at varying densities were tested at one site, while three legume species (alsike clover, birdsfoot trefoil, and white clover) were tested on a second site. After five years of legume cover, mineralizable N mass of the forest floor were 0.5 to 4.5 times those of controls. Total N of the forest floor more than doubled in the seeding density study compared with controls, but was insignificant in the multiple species study. Despite observed increases in soil nitrogen, lodgepole pine growth was not enhanced by the legume treatments. Factors such as N immobilization, root distribution, low S levels, and competition for B may have limited the response of lodgepole pine seedlings to additions of biologically fixed-N. Key words: nitrogen-fixation, legumes, lodgepole pine, soil nitrogen

Rhizobia that nodulate tree legumes: specificity of the host for nodulation and effectiveness

Rhizobial specificity, defined in terms of nodulation and nitrogen-fixing effectiveness characteristics of a group of rhizobia on a host legume, serves as a basis for predicting the need to inoculate, selecting species for most probable number plant-infection assays, and preparing rhizobial inoculants suitable for a range of legume species. A series of inoculation experiments were performed under growth room and greenhouse conditions to delineate rhizobial specificity of a variety of tree legumes. Gliricidia sepium, Calliandra calothyrsus, and Leucaena leucocephala nodulated effectively with rhizobia isolated from each of the three genera. With a few exceptions, Sesbania grandiflora and Robinia pseudoacacia nodulated effectively only with rhizobial strains isolated from each genus respectively. A range of specificity was found among species that nodulate with Bradyrhizobium. Whereas Acacia mearnsii nodulated with most strains but fixed N2 effectively with relatively few, Acacia mangium and Lysiloma latisiliqua were specific for both nodulation and effectiveness. Key words: nitrogen fixation, effectiveness, nodulation, rhizobia, tree legumes.

Temperature response of an associative N2-fixing Pseudomonas species in pure culture

The growth rate and nitrogenase (acetylene reduction) activity of a pseudomonad isolated from the rhizosphere of the grass species Deschampsia caespitosa were investigated in the 0–35 °C temperature range. Growth was detected from 5 to 30 °C with or without provision of fixed N and its optimum growth temperature was about 25 °C, establishing its psychrotrophic response to temperature. Maximum nitrogenase activity was also found at about 25 °C. Low but significant nitrogenase activity was detected below 15 °C. This organism was confirmed to be a microaerobic diazotroph and it was sensitive to a dissolved O2 concentration of ca. 12 μM. It is probably dependent on the appropriate environment in the rhizosphere of the host plant in order to persist with sustained nitrogenase activity during cold and dormant periods. Key words: nitrogen fixation, Pseudomonas, psychrotroph, temperature.

Starch content and activities of starch-metabolizing enzymes in effective and ineffective root nodules of soybean

Starch content and activities of some enzymes of starch metabolism were determined in wild-type, N2-fixing (fix+) nodules and in two non-N2-fixing (fix−) nodules induced by Bradyrhizobium japonicum mutant strains, T5-95 and T8-1, on soybean (Glycine max L.) roots. The T5-95 nodules are similar to wild type in ultrastructure, but the T8-1 nodules are different in that the bacteroids are not released from the infection thread. After initial accumulation to relatively high concentration, starch was depleted during nitrogen fixation in fix+ nodules. However, in fix− nodules, the accumulated starch was not metabolized. The activity of starch-bound starch synthase (EC 2.4.1.21) declined in fix+ nodules but remained high in fix− nodules. The activity of α-amylase (EC 3.2.1.1) was only slightly higher than wild type in T5-95 but was four times higher than wild type in T8-1 nodules. The activity of starch phosphorylase (EC 2.4.1.1) increased in all nodule types from 14 to 21 days postinfection. A positive correlation was observed between the capacity of nodules to fix N2 and their capacity to degrade starch. Collectively, these results support the concept that starch accumulated during early stages of nodule development is metabolized to supply energy for nitrogen fixation and to meet the metabolic demands of bacteroids. Key words: nitrogen fixation, starch content, effective and ineffective nodules, starch synthase, starch phosphorylase, α-amylase.

Leghemoglobin variability in the genus Phaseolus

Leghemoglobins are oxygen-binding proteins in the legume root nodule that generally show structural heterogeneity. Fifty accessions of the genus Phaseolus were screened for leghemoglobin electrophoretic variation. Heterogeneity was observed in two of the six Phaseolus species investigated; the single leghemoglobin components found in samples of Phaseolus vulgaris and Phaseolus acutifolius nodule extracts, as well as in other Phaseolus species, appear to be the exception among nitrogen-fixing plants that have been evaluated for leghemoglobin profiles. Interspecific hybrids were produced to transfer existing leghemoglobin variability into the P. vulgaris background. Leghemoglobins were also shown to be inherited codominantly in interspecific hybrids. The amino terminal amino acid sequence of leghemoglobin II of a P. vulgaris × Phaseolus filiformis hybrid and leghemoglobins I and II from Phaseolus lunatus nodules were determined. The 37 amino terminal amino acids of these components were identical to the published sequence for P. vulgaris. Key words: nitrogen fixation, interspecific hybrids, electrophoresis.