Summary A number of examples is given demonstrating the co-existence of pea genotypes and their specific Rhizobium strains isolated within the same region. R. leguminosarum strains compatible with the cultivated pea have a narrow symbiotic range and they are widely distributed in European soils. This is presumably due to the narrow genetic base of the cultivated pea and its wide-spread cultivation in European soils. Rhizobium strains capable of nodulating a primitive pea line from Afghanistan were only found in soils of the Middle East and Central Asia. A more restricted distribution of specific Rhizobium strains was found for fulvum peas from Israel. Rhizobium strains effective with the fulvum pea were found in Israeli soils. A good example of co-evolution due to geographical isolation was found in south Turkey. Here a pea line was found which can form an effective symbiosis with local Rhizobium strains but not with strains from other parts of Turkey.
The effects of inoculum level and lime-pelleting were studied in an acid soil with respect to the nodulation and growth of lucerne (Medicago sativa cv Resis) and the population dynamics of Rhizobium meliloti. In small root-boxes (rhizotrons), the in-situ survival of inoculated rhizobia was studied in the micro-environment around the seed for a period of 12 days after sowing. During the initial 24 hours, a strong increase in rhizobial numbers was measured, concomitantly with the development of roots. As a result of lime-pelleting, rhizobial numbers were higher only at 3 days after sowing (P < 0.05). Later, this difference diminished steadily. Addition of lime did not increase the adhesion of the rhizobia to the seedling tap root.Plant responses to inoculation were studied in pots. To obtain optimal nodulation, the soil had to be neutralized around the seed with lime and at least 105 cells of R. meliloti were required. With more than 105 rhizobia per seed, lime-pelleting increased the number of crown-nodulated seedlings from 24% to 77%. Higher numbers of rhizobia could not compensate the effect of lime. A strong correlation was found between crown nodulation, nitrogen content and dry weight of the shoots.
A method for the indirect measurement of soil-pH is described. This method allows the spatial arrangement of soil and rhizosphere to be conserved. The soil is brought into contact with a layer of agar, containing bromocresol purple. A nylon gauze is placed between soil and agar. For quantitative pH measurements, a micro-electrode is inserted into the agar after three hours of contact between soil and agar.The validity of the method was checked by comparing its results with those obtained by standard procedures. At different pH-levels (pH 5.0 to 7.0) in either a sandy or a clay soil, a high correlation (r 2 = 0.98) was found between the two methods. However, in the case of the clay soil, the agar-pH was significantly lower than the standard-pH. In the sandy soil, in the range pH 5.0 to 6.0, the results of both methods agreed very well. The agar method was used to measure the pH dynamics in the rhizosphere of lucerne seedlings, grown in rhizotrons.
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