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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

The effect of calcium on the nodulation of lucerne was studied using EGTA, a specific calcium-chelator. First, the effects of the chelator were tested on hydroponically grown plants at pH 7.0. Optimal numbers of nodules were obtained in nutrient solution containing 0.2 mM CaCl2. When 0.4 mM EGTA was given additionally, nodulation was completely inhibited. Nodulation was restored specifically with CaCl2, but not with MgCl2. For studies in an acid soil (pH-H2O 5.2), lucerne seedlings were grown in rhizotrons. 67% of the seedlings became nodulated when the soil around the seed was neutralized locally with 1.0 μmol of K2CO3 in drops of 12 μL volume. When native calcium was removed with 2 μmol of EGTA, nodulation was reduced to 12%. However, addition of EGTA to soil resulted in a drop of pH from 6.1 to 5.2. A phosphate buffer could also not keep soil-pH sufficiently stable. Such pH-decreases could be avoided by placing agar blocks containing 6 μmol of EGTA for three hours on freshly developed roots. This treatment reduced nodulation from 87% to 32%, with soil-pH lowering only from 6.2 to 6.0. Nodulation could be restored by adding 2 μmol of CaCl2. The depletion of soil-calcium could depress nodule formation only during the first day after inoculation.

The nodulation of lucerne was studied in soil (pH-H2O 5.2) with seeds either inoculated with Rhizobium meliloti (R), or inoculated and pelleted with lime (RP). For comparison, experiments were done in the field and in two types of micro-cosmos: pots and rhizotrons. In the field experiments, lime-pelleting improved the establishment of seedlings and augmented the nitrogen yield of the first harvest. These positive responses in plant growth were the consequence of a better nodulation on the upper 10 mm of the seedling tap root. The number of seedlings carrying crown nodules increased from 18% (R) to 56% (RP) at 26 days after sowing. In both, pots and rhizotrons, lime-pelleting also increased crown nodulation: in pots from 32% (R) to 60% (RP), and in rhizotrons from 5% (R) to 90% (RP). Rhizotrons, made of plastic petri dishes, allowed for continuously following of early root developments and nodule formation. Crown nodulation could already be measured after 14 days. Based on these experiments, it was concluded (i) that crown nodulation is an adequate parameter to quantify the benefit of lime-pelleting, and (ii) that rhizotrons, because of the more pronounced effects and shorter incubation time, are more suitable to study the nodulation responses in the soil caused by the addition of rhizobia and lime.

Pisum sativum ecotype fulvum forms ineffective nodules with Rhizobium strains, isolated from effective nodules of the cultivated pea in Europe. Rhizobium strains isolated from nodules of fulvum peas in Israel are fully effective on this host plant, but in association with the cultivated pea they induce nodules of poor N2-fixing activity. The distribution of these fulvum-specific Rhizobium strains is restricted to regions where the fulvum pea occurs naturally. Rhizobium strains from other geographical regions induce mainly ineffective, or partially effective nodules on fulvum plants.

Nodulation of pea and broad bean plants grown in the light was found to be reduced when the roots were exposed to far-red light for 5–15 minutes daily during 5 consecutive days following inoculation with nodule bacteria. Similar results were obtained following a single exposure to far-red light during a period of 15 minutes at the 3rd or 4th day after inoculation. When the roots were exposed to far-red light either before inoculation or during the first two days afterwards there were either no effects or only slight effects on nodulation The inhibitory effect of far-red light on nodulation was partly reduced by subsequent exposure to red light, provided that the same part of the plant was exposed to both red and far-red light,viz either the root or the shoot. When different parts of the plant were exposed to red and far-red light respectively, there was no interaction between the two kinds of light on nodulation. Plants whose roots were exposed to far-red light did not subsequently show stem elongation. Nodules were found to develop on the roots of pea plants grown in the dark, provided that the plants were kept at or below 22°C. At 25°C nodulation was almost absent. Nodulation was decreased by addition of kinetin and IAA. In contrast to plants grown in the light pea plants grown in the dark, inoculated with either an effective or ineffective strain of Rhizobium, developed equal numbers of nodules. Exposure to red light slightly increased the percentage of nodulated plants but decreased the number of nodules per plant. Exposure to far-red light slightly decreased both the percentage of nodulated plants and the number of nodules per plant. The effect of far-red light was counteracted by red light andvice versa.

Nodulation of pea plants was inhibited by low concentrations of Fe(III)-EDTA and to a smaller degree by K2-EDTA. Using the divided root system, it was demonstrated that the inhibitory effect was only observed on that part of the root system which was in direct contact with the chelate. A simple device for the application of Fe(III)EDTA to either the upper or the lower part of the root system is described.

A number of experiments was carried out with pea plants growing in a nutrient solution of low pH, inhibitory to nodule formation but not to plant growth when combined nitrogen was available. It has been found that on transferring plants from a neutral culture solution to an acid solution of pH 4.5 at different periods of time after inoculation, nodulation is reduced when the exposure to low pH takes place during the first days after inoculation, and particularly on the second or third day. Once nodulation has been initiated, nodule growth and nitrogen fixation proceed normally in an acid solution. Failure of nodulation at low pH, under the conditions of these experiments, was not due to the inhibition of bacterial growth. From these results an acid-sensitive period, occurring during the second or third day after inoculation, was demonstrated.

An outline is given of the possibility selecting Rhizobium strains capable of performing a relatively good symbiosis with leguminous plants, growing under stress conditions. A Rhizobium strain capable of nodulating pea plants in acid soils is described. A pea cultivar, resistant to the majority of Rhizobium strains when growing at 20°C, is found to require a short period of a higher temperature for successful nodulation. The effect of non-photosynthetic light and its possible significance under natural conditions is discussed.

Root formation was obtained on the petioles of detached leaves of several leguminous plants, particularly on the primary leaves of bean. Root formation is easily obtained in artificial light at a temperature of 22 to 24°C. In the greenhouse it is optimal in early spring and late autumn. During hot summer seasons no roots but callus was formed on the petioles. Root formation was inhibited when the pulvinus was left on the petiole.

Pea cv. Iran was found to be ‘resistant’ to a large number of Rhizobium strains, when growing at 20°C, but nodulation is normal at 26°C. An exceptional Rhizobium strain was found which forms nodules on this pea cultivar both at 20°C and 26°C.