Abstract
Early maturing varieties of soybean have a high yield potential in Europe, where the main biotic threat to soybean cultivation are root lesion nematodes (Pratylenchus spp.). Nitrogen fixation in root nodules by highly efficient inoculants of Bradyrhizobium japonicum is an incentive to grow soybean in low-input rotation systems. We investigated density-dependent effects of Pratylenchus penetrans on nitrogen fixation by co-inoculated B. japonicum. Less than 130 inoculated nematodes affected the number and weight of nodules, the density of viable bacteroids in nodules, and nitrogen fixation measured as concentration of ureides in leaves. With more inoculated nematodes, the percentage that invaded the roots increased, and adverse effects on the symbiosis accelerated, leading to non-functional nodules at 4,000 and more nematodes. When P. penetrans invaded roots that had fully established nodules, growth of nodules, density of bacteroids, and nitrogen fixation were affected but not the number of nodules. In contrast, nodulation of already infested roots resulted in a high number of small nodules with decreased densities of bacteroids and nitrogen fixation. P. penetrans invaded and damaged the nodules locally, but they also significantly affected the nodule symbiosis by a plant-mediated mechanism, as shown in an experiment with split-root systems.
Highlights
Maturing varieties of soybean have a high yield potential in Europe, where the main biotic threat to soybean cultivation are root lesion nematodes (Pratylenchus spp.)
The objectives of this study were to investigate whether the root lesion nematode P. penetrans affects the symbiosis of soybean plants with the nitrogen-fixing bacteria, and how this depends on the population density of the nematode in soil and in the roots
The roots of soybean were co-inoculated with B. japonicum and various numbers of P. penetrans to investigate density dependent effects of the nematodes on nodulation, symbiotic N2 fixation, and plant growth
Summary
Maturing varieties of soybean have a high yield potential in Europe, where the main biotic threat to soybean cultivation are root lesion nematodes (Pratylenchus spp.). Nitrogen-fixing capacity as measured by the acetylene reduction assay was significantly reduced by P. penetrans in a phytotron experiment compared to plants without nematodes, but not in a greenhouse experiment[14] These experiments were performed with high numbers of infective stages of the nematodes that do not reflect normal field densities. The density-dependent effects of root-lesion nematodes on the symbiosis of soybean plants with inoculants of B. japonicum are still unclear, and whether the nodule formation or the functioning of established nodules is affected by the nematodes. The objectives of this study were to investigate whether the root lesion nematode P. penetrans affects the symbiosis of soybean plants with the nitrogen-fixing bacteria, and how this depends on the population density of the nematode in soil and in the roots. As we observed migration of the nematodes into the nodules, we investigated whether the detrimental effect of the nematodes on nodule functioning is caused by the local damage of the nodules, or by a systemic mechanism
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