In many papers, in th material and methods to isolate Rhizobium from nodule, the authors collet nodules at flowering stage. Why?
At this moment plants have a high metabolism to develop the flower and polinization right? So needs a higher number of rhizobia at the root to maintain the nitrogern level. Is it right to say that?
Thanks.
I don't think it is essential to collect rhizobia from nodules at the time of flowering. It's just that the nodules are large and fully-operational, with all zones established, at this time. This makes them easier to harvest and full of the bacteria. However, if all you are trying to do isolate rhizobia, you could harvest nodules prior to flowering and still have success. For many tree species of legume, it can be years before they flower, but most will have formed nodules full of rhizobia prior to that.
I propose that the isolation of the rhizobium from the nodule is performed at this phase due to the plant's stage in the nitrogen cycle. The plant and nodules are at thier peak during the flowering phase. During the seed phase of growth, the plant will consume much of the nitrogen produced in order to create the seeds.
One other point is that determinate type nodules tend to be lost after flowering due to sink competition between nodules and seeds. Since most major legume crops, such as soybean and common beans have determinate nodules, it is deemed safest to determine nodule mass and number at flowering.
Rhizobium isolation from nodules is best done at 30-45 days after sowing a legume when the nodules are in active stage and pink (leghaemoglobin). At Flowering which is the peak stage of nitrogen fixation, the nodule is a sink for the photosynthate that drives BNF. For indeterminate types sampling can be done at flowering also. But the thumb rule will be about 4-6 weeks and works best.
I have some experience with root nodules in cycads (coralloid roots). Also in this case I observed (in the field) a direct relationship between formation of nodules and formation of "cones". As for flowers, cones maight need some more nitrogenous compounds.
I don't think it is essential to collect rhizobia from nodules at the time of flowering. It's just that the nodules are large and fully-operational, with all zones established, at this time. This makes them easier to harvest and full of the bacteria. However, if all you are trying to do isolate rhizobia, you could harvest nodules prior to flowering and still have success. For many tree species of legume, it can be years before they flower, but most will have formed nodules full of rhizobia prior to that.
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