Ah, synchronicity. While Luigi was fleetingly confused about rhizobia and other bacterial symbionts of pigeonpeas, I was pondering one of the more interesting blog posts — and papers — I have read in a long time, also about rhizobia. Those are the bacteria that “infect” leguminous plants, forming nodules on the roots. In the nodules the bacteria “fix” nitrogen gas, from the air, into a form plants can use. In exchange, as it were, the plants supply the bacteria with a safe home and some of the food the plants have photosynthesized. Some rhizobia do a better job than others, and many are completely useless at fixing nitrogen. Better yet, the plants know, and send more food to the nodules fixing the most nitrogen.
Now, the tricky part.
Modern agriculture does not usually apply nitrogen to leguminous crops. But there can be considerable carry-over from the preceding crop. So, two possibilities arise. Maybe soybeans no longer respond to better nitrogen-fixing bacteria by sending more food their way, because they don’t really need the nitrogen. Or maybe more soil nitrogen means that the plant can afford to starve out all but the very best nitrogen fixers.
But why am I repeating all this? You cannot possibly do better than head over to Ford Denison’s blog, where he does a much better job than me of explaining the significance of his results. The paper is also discussed in Nature News.
Spoiler (aka don’t bother me with the details): modern varieties do very poorly when inoculated with a mixture of good and bad nitrogen fixers. It is as if they simply cannot tell the difference and feed both equally.
Stunning new idea: If modern varieties tolerate low quality rhizobia, then low quality rhizobia are going to proliferate in the soil, doing nobody any good. So why not deliberately breed legume crops to impose very strict sanctions against poorly-performing rhizobia strains? Long term this would enrich the soil with top-notch fixers.