Associative N2-fixation in plants growing in saline sodic soils and its relative quantification based on15N natural abundance

The interactive effect of low P supply (0, 10, 20 and 40 M) and plant age on nodule number, mass and functioning (ureide analysis technique), vegetative growth and pod production were investigated in glasshouse-grown nodulated cowpea (Vigna unguiculata L.cv. Kausband) in sand culture. Compared with 40 M P, P stress (0 M P) or very low (10 M P) supply markedly impaired nodulation, allantoin and amino-N concentrations and weight of N solutes in xylem exudates. Consequently, P stress reduced top growth and pod yields by 48 and 90%, respectively. N solutes in xylem exudates and total plant N assayed by Kjeldahl technique (as estimates of N₂ fixation) responded similarly to P supply. However, the relative ureide index [(ureide-N/ureide N+amino-N)×100] remained constant (99%), irrespective of P supply, indicating the plants' complete dependency on symbiosis for growth, without implying that growth was markedly increased by N₂ fixation. Although P concentrations in plant tops, roots and nodules increased with P supply, N concentrations in these plant tissues were unaffected by P supply. The concentrations of N and P in the nodules were 2–2 1/2 times higher than in plant tops. P application interacted strongly with plant age, with the largest P effect evidently achieved at the early podding stage. The significance and implications of these results are discussed.     

Effective nodulation of rhamnaceous actinorhizal plants induced by air dry soils

Air dry soil samples stored at room temperature for more than one and a half years, were used as inocula for actinorhizal plants. Seedlings of Colletia paradoxa, Discaria americana, D. articulata and D. trinervis (Rhamnaceae) cultivated in nitrogen-free nutrient solution, were inoculated by adding dry soil to the solution. All the soil samples tested were able to induce nodulation, showing the presence of infective propagules of Frankia. Healthy growth of nodulated plants suggested the occurrence of nitrogen fixation.     

An inflatable minirhizotron system for root observations with improved soil/tube contact

Commonly used minirhizotrons consisting of a transparent tube inserted into the soil seldom attain good contact between the tube and the soil, which leads to root growth occurring in a gap rather than in the soil. A new system is described involving an inflatable flexible rubber wall, made from a modified motorcycle tube. Pressure ensures a proper tube/soil contact so that the environmental circumstances for root growth along the tube more closely correspond to those in the undisturbed soil. Before the endoscope slide is introduced into the minirhizotron for taking pictures, the inflatable tube is removed, so that there is no-often opaque-wall between the endoscope and the roots. This improves the picture quality and facilitates the analysis of root images.     

Influence of maize root mucilage on soil aggregate stability

This study was undertaken to determine the effects of root exudates on soil aggregate stability. Root mucilage was collected from two-month old maize plants (Zea mays L.) Mucilage and glucose solutions were added at a rate of 2.45 g C kg⁻¹ dry soil to silty clay and silt loam soils. Amended soils, placed in serum flasks, were incubated for 42 d with a drying-wetting cycle after 21 d. Evolved CO₂ was measured periodically as well as the water-stable aggregates and soluble sugar and polysaccharide content of the soil. In mucilage-amended soils CO₂ evolution started with a lag phase of 2–3 days, which was not observed in glucose-amended soils. There was then a sharp increase in evolved CO₂ up to day 7. During the second incubation period there were only small differences in evolved C between treatments. Incorporation of mucilage in both soils resulted in a spectacular and immediate increase in soil aggregate stability. Thereafter, the percent of water-stable aggregates quickly decreased parallel to microbial degradation. On completion of the incubation, aggregate stability in the silty clay soil was still significantly higher in the presence of mucilage than in the control. This work supports the assumption that freshly released mucilage is able to stick very rapidly to soil particles and may protect the newly formed aggregates against water destruction. On the silty clay, microbial activity contributes to a stabilization of these established organo-mineral bounds.     

Influence of nitrate concentration at the root surface on yield and nitrate uptake of kohlrabi (Brassica oleracea gongyloides L.) and spinach (Spinacia oleracea L.)

The objective of this study was to determine if plant roots have to take up nitrate at their maximum rate for achieving maximum yield. This was investigated in a flowing-solution system which kept nutrient concentrations at constant levels. Nitrate concentrations were maintained in the range 20 to 1000 μM. Maximum uptake rate for both species was obtained at 100 μM. Concentrations below 100 μM resulted in decreases in uptake rate per cm root (inflow) for both spinach and kohlrabi by 1/3 and 2/3, respectively. However, only with kohlrabi this caused a reduction in N uptake and yield. Thus indicating that this crop has to take up nitrate at the maximum inflow. Spinach, however, compensated for lower inflows by enhancing its root absorbing surface with more and longer roots hairs. Both species increased their root length by 1/3 at low nitrate concentrations.     

Strain-specific salt tolerance and chemotaxis ofAzospirillum brasilense and their associative N-fixation with finger millet in saline calcareous soil

Three salt-tolerantAzospirillum brasilense strains were isolated from the roots of finger millet grown in saline calcareous soil and characterized. The effect of various salts on growth and N₂ase activity of these strains was tested and strain STR1 was found more tolerant at higher concentrations of Cl^-, SO₄ ² and HCO₃ ^-. Bicarbonate was found to be the most toxic. The content and concentrations of root exudates of finger millet genotypes were different and chemotaxis to sugars, amino acids, organic acids and root exudates was strain specific. Under salt stress, significant interactions between strains and genotypes of finger millet resulted in different responses of N₂ase activity, endo- and exorhizospheric population, dry weight of root, shoot and grain yield.     

Penetrometer resistance,root penetration resistance and root elongation rate in two sandy loam soils

Root penetration resistance and elongation of maize seedling roots were measured directly in undisturbed cores of two sandy loam soils. Root elongation rate was negatively correlated with root penetration resistance, and was reduced to about 50 to 60% of that of unimpeded controls by a resistance of between 0.26 and 0.47 MPa. Resistance to a 30° semiangle, 1 mm diameter penetrometer was between about 4.5 and 7.5 times greater than the measured root penetration resistance. However, resistance to a 5° semiangle, 1 mm diameter probe was approximately the same as the resistnace to root penetration after subtracting the frictional component of resistance. The diameter of roots grown in the undisturbed cores was greater than that of roots grown in loose soil, probably as a direct result of the larger mechanical impedance in the cores.     

Formation of nodules on non-nodulating soybean T201 after treatment with 2,4-dichlorophenoxyacetate

The formation of effective root nodules on a non-nodulating line (T201) of soybean (Glycine max (L.) Merr.,) was induced by a treatment with 2,4-dichlorophenoxyacetate (2,4-D). The induced nodules, inoculated with mixed Bradyrhizobium japonicum strains A1017 and IRj2101, had a normal internal structure, red in colour and the cells being filled with bacteroids. Externally, the induced nodules were of unusual shape, being paired or gourd-like in form and were attached to thickened roots. The nodules were capable of acetylene reduction (3.1–3.5 moles g^-1 fresh weight nodules h^-1), allowing the growth of plants with dark green leaves.     

Role of magnesium in combination with liming in alleviating acid-soil stress with the aluminium-sensitive sorghum genotype CV323

An experiment to study the effects of Mg nutrition on root and shoot development of the Al-sensitive sorghum (Sorghum bicolor (L.) Moench) genotype CV323 grown in pots of sandy loam under different acid soil stress is reported. This experiment had a factorial design: four rates of liming were combined with four rates of Mg fertilization. When no Mg was added, the pH of the soil solutions (collected in ceramic cups) increased from 4.0 (unlimed) to 4.2, 4.7 and 5.9 at the increasing rates of liming. After 30 days of growth dry matter yields of the limed treatments were 40%, 115% and 199% higher than that of the unlimed treatment. Without liming and at the highest liming rate, adding Mg did not affect plant biomass significantly. At the two intermediate levels of liming, however, 11.3 mg extra Mg per kg soil increased dry matter yield to the same levels as found at the highest liming rate. Concentrations of Mg in the soil solution rose after Mg was added and fell when lime was added, but adding both Mg and lime increased Mg concentrations in the plant shoots. In plants of the limed treatments, dry matter yield was correlated closely with the Mg concentration in the shoot. This was not so in the unlimed treatment. Furthermore, in the unlimed treatments root development was inhibited, but reduced Mg uptake by the plants resulted mainly from the direct effect of Al- (or H-) ions in the soil solution rather than from impaired root development. It is concluded that Mg fertilization counteracted the interfering effects of Al- and H ions on Mg uptake.     

Phosphorus efficiency of plants

Föhse et al. (1988) have shown that P influx per unit root length in seven plant species growing in a low-P soil varied from 0.6×10^-14 to 4.8×10^-14 mol cm^-1s^-1. The objective of this work was to investigate the reasons for these differences. No correlation was found between P influx and root radius, root hairs, cation-anion balance and Ca uptake. However, when root hairs were included in mathematical model calculations, the differences of P influx could be accounted for. These calculations have shown that in soils low in available P, contribution to P uptake by root hairs was up to 90% of total uptake. The large contribution of root hairs to P uptake was partly due to their surface area, which was similar to that of the root cylinder. However, the main reason for the high P uptake efficiency of root hairs was their small radius (approx. 5×10^-4 cm) and their perpendicular growth into the soil from the root axis. Because of the small radius compared to root axes, P concentration at root hair surfaces decreased at a slower pace and therefore P influx remained higher. Under these conditions higher I_max (maximum influx) or smaller K_m values (Michaelis constant) increased P influx. The main reasons for differences found in P influx among species were the size of I_max and the number and length of root hairs. In a soil low in available P, plant species having more root hairs were able to satisfy a higher proportion of their P demand required for maximum growth.