Are Your Nodules Pink?

After my last post I started worrying about my beans' roots' relationships. In the sub-soil chaos would they find the symbiotic relationship they craved? Would Mr. Bean find the perfect Ms. Rhizobia for nurturing nitrogen together? The simplest way to find out is to have a look. And here is the root system of one of the beans. You can see the nitrogen-fixing nodules quite clearly (click to enlarge) although I'm not sure there are as many as I have seen in other situations.

You'd think that with the evidence of obvious nodules I could relax. But it seems it's not quite as simple as that. The presence of nodules does not in itself mean that significant nitrogen fixation is taking place.' When nodules are young and not yet fixing nitrogen, they are usually white or gray inside. As nodules grow in size, they gradually turn pink or reddish in color, indicating nitrogen fixation has started.' In fact the reddishness is caused by a protein similar to the haemoglobin in our blood and performing a similar function in managing oxygen flow to the bacteria. Apart from immaturity there are two main reasons why the nodules would not be pink inside. One is if the plant has reached the fruiting stage in its development. Its whole purpose in developing the relationship with the symbionts has been to store nitrogen to utilise in the formation of its seeds. That's why green manures are cut down before flowering. Leave too late and the nitrogen is no longer in the ground. The other reason for half-hearted nodule formation and colouring is simply that if the plant can access available nitrogen in the soil it will not invest energy is cooperating with the nitrogen-fixing Rhizobia. (It costs a significant amount of photosynthetic effort to sustain the bacteria.) So in an already nitrogen rich soil, don't expect legumes to add to that store - they will use it instead.

And so it's time for the crucial test of my nodules. It was almost painful slicing them with the craft knife to reveal the internal hue of - green?? 'Legume nodules that are no longer fixing nitrogen usually turn green and may actually be discarded by the plant.' Here's my take on what happened. Until the gales, my beans were growing vigorously and looked very healthy. I'm sure had I looked then my nodules would have been nice and pink. After the storm which flattened them, perhaps they were so shocked that the available nitrogen quickly got used up in trying to repair torn tissue. I'm wondering too at what temperature nitrogen-fixation switches off, if at all. Current soil temperature at depth of 4" (10 cms.) is about 4C.

As usual, I come to no serious conclusion. More questions raised than answered. Will the relationship warm up with the weather? Watch this space. Thanks to W.C. Lindemann and C.R. Glover of Mexico State University for background info:Nitrogen Fixation by Legumes