Plant growth, nodulation and Rhizobium competition in alfalfa and siratro rhizosphere in vermiculite: sand culture of varying proportions
Kapulnik, Y.
Greenhouse experiments were conducted to determine the effect of different physical structures of the rooting matrix on root development, rhizobial growth, nodulation and nitrogenase activity in the rhizosphere of Medicago sativa L. and Macroptilium atropurpureum. Eight different soil physical structures were obtained by altering the proportion of sand to vermiculite. Increasing the sand portion in the rooting matrix decreased root dry weight and plant growth by 33% and 25%, respectively. Under these conditions the number of nodules on the taproot of Medicago sativa L. inoculated with Rhizobium meliloti and of Macroptilium atropurpureum inoculated with Bradyrhizobium sp. decreased significantly, with no apparent change in the total number of nodules per plant. Competition studies with low inoculum size (10(6) c.f.u./mL) of two efficient Bradyrhizobium strains (280A and 2212A at 1:1 ratio) in M. atropurpureum rhizosphere, resulted in a significant reduction in number of nodules induced by strain 280A. This effect was associated with an increase in the sand portion in the rooting matrix and with a reduction of strain 280A growth in the rhizosphere. The number of nodules containing strain 2212A and its population in the rhizosphere remained constant in the differently proportioned sand:vermiculite mixtures. It was concluded that the main effect of rooting matrix composition was on root function, thus affecting rhizobial activity in the rhizosphere and the nodules' function.
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