SummaryThirty-eight cultures of rhizobia and 10 non-rhizobia growing in the root zone of clover (Trifolium glomeratum L.), 5 rhizobia and 3 non-rhizobia in that of lucerne (Medicago sativa L.), and 8 rhizobia in that ofSiratro (Phaseolus atropurpureus DO.) revealed a specific relationship between bacteria and host that determined the kind and degree of deformation of the root hairs.With all hosts the markedly curled condition was practically restricted to the host plant associated with virulent homologous rhizobia. Notable exceptions with T. glomeratum were the two cultures of Rhizobium leguminosarum which caused marked curling of the root hairs, and a culture of R. lupini which produced a less marked but similar result. The two less drastic degrees of deformation ("moderately curled" and "branched") were generally most frequent with homologous associations but were found in some degree with most of the rhizobia tested on T. glomeratum, including avirulent R. trifolii. Four rhizobia (all cowpea) and generally all the non-rhizobia (including agrobacteria) were without effect on this host. Tests with M. sativa and P. atropurpureus confirmed the evidence of specificity favouring the homologous association. The several agrobacteria were again without effect when tested on M. sativa.Ooncurrent or prior growth of plants with their homologous rhizobia did not affect the kind or degree of root-hair response to the heterologous rhizobia. Bacteriafree filtrates of R. trifolii and R. meliloti were able to cause branching and the moderate type of curling, but failed to produce the markedly curled condition. The effect of filtrate prepared from a suspension of the homologous strains of rhizobia was greater than that from heterologous strains.
SUMMARYGrowth of Rhixobium trijiolii in a defined medium reflected the supply of Ca2+ and Mg2+ (subsequently Ca and Mg, respectively) in distinctive fashion. Deficiency of Ca, in the presence of sufficient Mg, caused reduction in growth rate, the level of maximum growth and the proportion of viable cells. Such Ca-deprived cells were markedly swollen and vacuolated. On the other hand, although shortage of Mg (Ca sufficient) was without effect on growth rate down to the lowest concentration a t which growth occurred, maximum growth and the proportion of viable organisms were markedly decreased. Mg-deficient organisms were appreciably elongated. Signs of Ca deficiency became apparent at less than 0.025 mM, and Mg deficiency at less than 0.1 mM, most markedly in the range below 0.5 mM. Additionally there was a need for total divalent cations of the order of 0.4-0.6 mM. This could be met by either Ca or Mg provided both were sufficient for their maximum specific effect.
JumimY. The growth and survival in peat culture of 3 strains of Rhizobium representing 3 inoculation groups, were affected by the source of the peat, pH and method of pH adjustment, method of sterilization, drying temperature and moisture content. The choice of a peat for culture production can be made only on actual tests of its suitability as a medium for rhizobial growth and survival. Sterilization always permitted more rapid growth of the rhizobia and favoured their viability during storage. For the cowpea strain sterilization was essential. Sterilization by y radiation was generally superior to autoclaving: temperatures > 100" at the time of drying adversely affected survival and multiplication. Heat of wetting, which kills many of the added rhizobia, and the production of inhibitory substances, which prevents subsequent multiplication and accelerates death during storage, are important factors. Temperatures of 80-100"were safe and sufficient to dry peat spread in a thin layer. A moisture content of 4&50% (on a wet weight basis in the finished culture) was optimal.PEATS AND SOILS rich in organic matter are widely used in. the final stage of the preparation of legume inoculants, and generally constitute a suitable carrier for this purpose. Immediately after mixing a broth culture of Rhizobium with partially dried peat, there is likely to be a period of multiplication followed generally by a relatively slow decline in the number of viable bacteria. For commercial purposes a safe storage period of 6 months can be expected, provided the initial level of rhizobia is sufficiently high and storage is a t 10-15".Multiplication of rhizobia in the freshly mixed peat culture, and the speed with which the bacteria subsequently die, are likely to be affected by such factors as the nature ofthe carrier, whether it has or has not been sterilized, its water content, and conditions of aeration and temperature to which it is exposed (Hedlin & Newton, 1948 ; Newbould, 1951 ; Date, 1959). Calculations (Vincent, 1958) based on published work showed a weekly logarithmic death rate (Ic) ranging from nil in sterilized peat stored at 5" and with minimal water loss to 0-19 at 25" under conditions that permitted considerable loss of water. In packaged commercial cultures in unsterilized peat k varied from * Present address :
Paired rhizobial strains SUDDkd in several nronortions were used to studv LI
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