Growth, photosynthesis and transpiration in Psophocarpus tetragonolobus (L.) DC cultivar 'UPS 99'

Two methods (whole-plant growth analysis and gas exchange) were used to measure the response of Psophocarpus tetragonolobus (L.) DC cultivar 'UPS 99' to the environment. This plant had an optimal temperature for root growth of 25°C, its rate of acetylene reduction (when inoculated with Rhizobium, strain 'RRIM 56') was maximal at 30°C and it required an atmospheric temperature of about 35°C for optimal shoot growth. Maximum water-use efficiency was ca. 33 mg CO2·g H2O(-1). The rate of photosynthesis reached a plateau at 900 vpm CO2-this condition also gave the lowest rate of transpiration. Under normal conditions, the light compensation point was at 1.7 klx, while that for CO2 was 60 vpm. Photorespiration diminished gross photosynthesis of P. tetragonolobus by forty percent. Water stress (as measured by sensitivity to slightly increased CO2 levels) caused rapid closure of stomata, and the response was 'remembered' for up to five days.

Dynamics of Rhizobium competition for nodulation of Pisum sativum cv. Afghanistan

Nodulation of Pisum sativum cv. Afghanistan by Rhizobium leguminosarum strain Tom can be blocked by the nonnodulating R. leguminosarum strain PF2. This system was used to study the dynamics of rhizobial binding to seedling root surfaces. Appreciable numbers of 35S-labeled rhizobia bound to the root surface within 2h after inoculation. PF2 colonized the root surfaces earlier and in greater numbers that did Tom. Radioactive extracellular polysaccharides and lipopolysaccharides from either organism failed to bind to the seedling roots in measurable quantities.

Isolation and metabolism of Vigna unguiculata root nodule protoplasts

Axenic cultures of bacteroid-containing protoplasts were isolated from root nodules of Vigna unguiculata L. Walp. Dimensions of the protoplasts were 35 to 135 μm long x 35 to 95 μm wide. Yields were about 30 to 50 mg dry weight per gram fresh weight of nodules. About 5x10(8) protoplasts packed into 1 ml of basal medium under the influence of gravity. When incubated in hypertonic, nitrogen-free media, freshly isolated protoplasts began to reduce acetylene to ethylene after a lag period of 24 to 48 h. Various additions to the basal medium showed that the system possessed functional glycolytic and tricarboxylic acid pathways. Endogenous application of various intermediary metabolites stimulated both acetylene reduction and respiration, though not often equally. As acetylene reduction, but not respiration, was inhibitable by both asparagine and glutamine, the system appears suitable for the study of mechanisms controlling symbiotic nitrogen fixation.

Development of the nitrogen fixing apparatus in the legumes, Centrosema pubescens Benth., and Vigna unguiculata L. Walp

The sequence of events leading up to the establishment of symbiotic nitrogen-fixation were studied in two tropical legumes, Centrosema pubescens Benth, and Vigna unguiculata L. Walp. Parameters measured included fresh and dry weights, chlorophyll and leghaemoglobin contents, as well as the activities of NADH-nitrate reductase (EC 1.6.6.1), and nitrogenase (nitric-oxide reductase-EC 1.7.99.2) in plants that were inoculated with suitable rhizobia or which were watered with potassium nitrate. Dry weight and photosynthetic activity of both species followed the sigmoidal pattern which is characteristic of most plants. Growth was little different in either a qualitative or quantitative sense whether nitrogen was supplied as nitrate or through dinitrogen fixation. Although the biochemical sequence of events was dependent on the limiting sensitivities of the individual assays used, the data suggest that nitrate reductase is the first measurable enzymatic activity in the nodules (and roots), followed by acetylene reduction and leghaemoglobin in that order. It is possible therefore, that low levels of symbiotic nitrogen fixation occur in the nodules in the absence of leghaemoglobin. Nitrate reductase activity in C. pubescens nodules was negatively exponentially correlated with nitrogenase activity of the same nodules, suggesting a changing metabolism in old nodules. These data are discussed in terms of environmental and physical factors known to control nitrogen fixation.

Comparative structural and immunochemical properties of leghaemoglobins

Circular dichroism studies on leghaemoglobins from snake bean, lupin, serradella and other plants show that, in common with soybean (reported earlier) they have a similar overall polypeptide chain conformation and haem environment and orientation. Immunochemical studies, on the other hand, suggest that the antigenic determinants on the surface of the leghaemoglobins vary considerably. Thus, firstly the alpha-helix content of the leghaemoglobins as a class is very similar (60-65%) and approaches that of the myoglobins, secondly, the sign, magnitude and shape of their circular dichroism spectra in the near ultraviolet, Soret and visible regions suggest close similarities in the environment and orientation of a structurally important tryptophan residue and of the haem moiety, and thirdly, there is comparatively weak haem-protein interaction. The extent of immuno cross-reactivity was found to be best deomonstrated using the Farr radioimmunoassay procedure. The results were (a) 5 leghaemoglobins from one plant (soybean) crossreacted completely but with varying affinities. (b) The extent of cross reactivity between leghaemoglobins from different plants was compared to that within a single plant; the reaction of antiserum to a soybean leghaemoglobin with a serradella leghaemoglobin was weak, with a snake bean leghaemoglobin still weaker (and incomplete) while lupin leghaemoglobins showed no cross reactivity at all. (c) The "rapid" attenuation of cross reactivity among different plant leghaemoglobins is explicable in terms of the extensive amino acid substitutions which have been demonstrated in the literature and in the present studies. (d) In view of this rapid divergence it is not surprising that sperm whale and horse heart myoglobins showed no cross reactivity with soybean leghaemoglobins. In summary, amino acid substitutions in the leghaemoglobin family are conformationally but not immunochemically conservative.

Molecular properties of lupin and serradella leghaemoglobins

1. Leghaemoglobins were extracted from the root nodules of lupin (Lupinus luteus L.) and serradella (Ornithopus sativus Brot.) plants and fractionated into different leghaemoglobin components on DEAE-cellulose-acetate columns. 2. The first two fractions eluted from columns loaded with either lupin or serradella leghaemoglobins were in the Fe(3+) oxidation state. 3. These components have protohaem IX as the prosthetic group and glycine as the N-terminal amino acid. 4. Other properties are: lupin component I, pI5.08, molecular weight 19000; lupin component II, pI5.13, molecular weight 20600; serradella component I, pI5.00, molecular weight 17500; serradella component II, pI5.05, molecular weight 19100. 5. Leghaemoglobins are thus heterogeneous with respect to size and charge.

Control of leghaemoglobin synthesis in snake beans

1. The finding that the plant is the genetic determinant of leghaemoglobin production in legume nodules was further tested by inoculating snake beans with two strains of Rhizobium selected to give large genetic differences. Carbohydrate requirement patterns, immunological techniques and DNA base ratio determinations were used to demonstrate genetic differences between the two rhizobial strains. 2. Partially purified preparations of the haemoglobins from the nodules produced by the two strains showed no differences when examined by electrophoresis, isoelectric focusing or ion-exchange chromatography. 3. Two different leghaemoglobins from each type of nodule were separated by chromatography on DEAE-cellulose. One of these was isolated in the Fe(3+) form and accounted for two-thirds of the total leghaemoglobin. When it was examined in the analytical ultracentrifuge and by amino acid analysis, this major component did not vary with the inoculant rhizobial strain. The molecule had an s(20,w) of 1.88S, a diffusion coefficient of 10.7x10(-7)cm(2).s(-1) and a mol. wt. of 16700. 4. These results strongly support the hypothesis that the mRNA for leghaemoglobin is transcribed from plant DNA.