Cowpea and hyacinth bean nodulated and grew well at 11–14 hr and poorly at 8 hr light duration. Nodulation and plant growth increased with increase in light intensity from 1.4 to 17.1 W/M2. But the natural light intensity (228.3 W/M2) inhibited nodulation in July and plant growth in December
Nodulation and growth of both crops were best at moderate temperatures and cowpea tolerated warm more than cool temperatures, whereas hyacinth bean showed the reverse situation.
Inoculation improved nodulation in cowpea but not in hyacinth bean. Application of nitrogen did not improve the growth of plants affected by light and temperature.
Green house experiments were done with two purposes: (1) to identify strains of rhizobia effective and acid tolerant in symbiosis with Lablab purpureus, and (2) to determine whether soil acidity or the symbiotic condition increased the phosphate requirement for growth.
Five rhizobial strains were tested in one neutral soil, at two acid soils and two acid soils limed to pH 6.6. In the neutral and limed soils, three of the strains were effective (CB 1024, CB 756, TAL 169), but only two strains (CB 756 and TAL 169) remained effective in acid soil.
Strain CB 756 and plus – N treatment were further compared in a factorial trial involving combination of five levels of P with lime, no lime and CaCl2 treatments, applied to an acid soil. Some of the treatments were also applied to plants inoculated with CB 1024. Between the N-fertilized and CB 756 treatments, there was no clear difference in growth response to applied P, and the critical internal concentration of P for 95% of maximal growth was the same (0.22% shoot by weight). Increasing P beyond levels needed for maximal growth increased nodulation and N concentration in plants inoculated with CB 756. It lowered N concentration in N-fertilized plants. There was evidence suggesting that the P requirement of symbiotic plants increased if the soil was acid, or if CB 756 were replaced by CB 1024 as microsymbiant, but the critical stastical interactions were not significant.
Rizobia play an important role in agriculture by contributing biologically fixed nitrogen. They fix nitrogen as an endosymbiotic bacteroid in the root nodule. The ideal strain of root nodule bacteria for inoculation is one that survives for a long time in soils containing a spectrum of secondary substances released during decay and produces effective nodules. Most of the secondary substances, especially those derived from phenols inhibit nitrogen fixation. Catechol is the central metabolite in phenolic degradation. Two questions arise here whether Catechol and related derivatives inhibit rhizobia and whether Rhizobium catalyze catechol as an energy source. To investigate this, catechol was chosen as the representative aromatic substance along with Rhizobium strain isolated from Lablab purpureus.
The strain of Rhizobium used was fast growing with a generation time of 3.7 hours on YMA medium at 30°C.
Rhizobium of the species was recovered from catechol treated soil after 9 months of incubation. Rhizobium species utilized catechol upto 10 mM as a sole carbon source but 20 mM was toxic. No viable cells were recovered from the medium containing 20mM catechol. Rhizobium species isolated from Lablab purpureus survived for 9 months in soils containing catechol. In synthetic medium, Rhizobium species utilized catechol, upto 10 mM as sole carbon source and catechol 1, 2 – dioxygenase was present in the catechol grown cells. In the presence of organic acids and sugar, catechol was co-metabolized, but catechol 1, 2-dioxygenase induction was inhibited. The ability of Rhizobium species to utilize various phenolic substances provides potential advantage to overcome the phenolic toxicants in soils.
The first root nodules appeared when the first pair of leaves had unfolded. The nodule meristem was opposite to protoxylem with dense cytoplasm and large nuclei. Mature nodules had distal meristem, an infection zone, a pink symbiotic bacteroid zone and a multilayered parenchymatous cortex with 5–11 peripheral vascular bundles. The nodule is polystelic. The ineffective nodules are similar to the effective ones in structure and development, but the bacteroid zones are much reduced and scattered pseudo nodules appear as large swellings containing scattered meristems and prominent scenescent zones in the secondary xylem.
Nodule number and dry weight, leghaemoglobin content, nitrogen content and nitrogen activity were higher in L.purpureus plants duly inoculated with G. mosseae and Rhizobium than inoculated with Rhizobium alone. Plant growth and nodulation were generally increased by supplementary P. Accumulation of plant biomass, total nitrogen and total phosphorus in L.purpureus were greater with dual inoculation than with either Rhizobium or Glomuss mosseae inoculation alone. The percentage organic carbon was lower in all the inoculated plants in comparison with control.
Roots of Lablab purpureus were treated with tri-idobenzoic acid (TIBA), kinetin or with nodulation factors (Nod factors) purified from Rhizobium sp. NGR 234 and grown in the presence of mycrorrhizal inoculum (Glomus mosseae). Colonization by the micorrhiza fungus was increased from <30% to about 65% of root length when roots were treated with the growth regulators. Moreover, treatment of mycorrhizal L.purpureus roots with nod factors or TIBA strongly induced sporocarp formation of Glomus mosseae. In parallel, the pool size of the fungal disaccharide trehalose was significantly affected in roots treated with TIBA and Nod factors alone, and with TIBA combined with all effectors, and increased from 0.06 mg/g dry weight (DW) in control roots to upto 1.7 mg/g DW (TIBA + Kinetin). Conversely, the sucrose pool decreased from 5% DW to less than half in roots treated with Nod factors. Activities of trehalase were significantly enhanced in micorrhizal roots by the treatment with Nod factors of TIBA. When Nod factors and TIBA were added in combination, these activities were strongly enhanced suggesting synergism between these growth regulators.
Seeds of Lablab purpureus cv. PEP were sown in unsterile soil supplemented with different concentrations of N and P in earthern pots. Seven day old seedlings were inoculated with Rhizobium. Supplemental N and P (50 and 15 mg/kg soil) induced a 15% increase in leghaemoglobin content in Rhizobium and Glomus mosseae–inoculated plants over the Rhizobium–inoculated plants. Higher accumulation of plant biomass, total nitrogen, total phosphorus and total chlorophyll content in leaves was observed with dual inoculation over the Rhizobium or Glomus mosseae inoculation, whereas the accumulation was a function of supplemental N and P. Per cent increase in yield attributes, viz., pod number, pod dry weight and seed dry weight, were higher with dual inoculation than either Rhizobium or G. mosseae inoculation. The yield attributes also increased progressively with increasing concentrations of supplemental N and P in all treatments.
Cotton producers in Australia are interested in including legume–green–manure crops in their farming systems. Lablab and lucerne are two crops that have been considered for this role. The object of this study was to determine their biomass production, nitrogen fixation, water use and water–use–efficiency within a 1-year out-of-cotton rotation. Both species were grown under full irrigation and partial irrigation, where periods of moisture stress occurred. During the period of the rotation, Lablab produced more biomass and fixed more nitrogen than lucerne. Its biomass production was increased from 9,655 to 16,024 kg/ha by full irrigation compared with partial irrigation, while lucern biomass similarly increased from 6,563 to 8,040 kg/ha only. Lablab also fixed more nitrogen (177 kg N/ha) than lucerne (111 kg N/ ha). Lucerne used more water than Lablab and thus Lablab had higher water-use-efficiently of biomass production and nitrogen fixation. The study indicates that Lablab produces more green manure with greater water–use efficiency than lucerne with in a 1- year out-of-cotton rotation.