I only have Medicago experience, but Medicago does not need Rhizobium bacteria for germination, at least not in the greenhouse. It cannot hurt to add the bacteria later on, but both M. truncatula and M. sativa grow well in the greenhouse if you give them inorganic N. I always followed the Medicago Handbook's instructions to grow them, and they worked well for me. This is for truncatula, but I used the same germination protocol for sativa, too.

http://www.noble.org/Global/medicagohandbook/pdf/SeedStorage_Germination.pdf

I always used the sulfuric acid scarification, as it results in a great uniform germination. If you want to, you can put out scarified seed directly into soil, too, barely buried.

It depends on the used strain of bacteria. many of Rhizobium produce auxin-like hormones, and can stimulate seed germination especially in hard conditions (too cold, dry, wet, pathogens in soil, etc.).  

That is good to know.  Can they therefore also inhibit germination or early seedling growth? I have found some strange results from a trial that I have running. I have used 10 cultivars of annual Medicago and Trifolium species and planted them at different burial depths. This was replicated but seeds were also inoculated with their respective root nodule bacteria. From the initial results it seems that those seeds planted at deeper burial depths that were inoculated had a lower germination percentage than those seeds that were not inoculated.

The dark side of biomethod: symbiotic/antagonistic bacteria can be close to plant pathogens and in certain conditions (lack of O2 in deep soil, etc) can turn on synthesis of some enzymes/ plant toxic  compaunds that will kill germinating seeds.  For instance: we have isolated many strains of Rhizobium/Agrobacterium from rotted potato tubers. I cannot say that they were the main reason for rot but  were present in many samples. 

 In my opinion the Rhizobia inoculation help in formation of nodules to fix BNF. The low germination at lower depths could be due to small seed size of Medicago and Trfolium spp.. As per Agronomic principles, Bold seeds like beans, chickpea, pea and maize etc even if sown deep, when less moisture in top soil comes out because the seeds have enough energy and food reserves to push the coleoptile above soil surface.

hi francoise, i doubt whether the bacterial symbionts would assist in seed germination, since the symbiosis only gets initiated when the radicle emerges from the seed. in order to encourage seed germination, you may consider to scarify the seeds with sulphuric acid or something similar. furthermore, you should find out whether these seeds are sensitive to red or far-red light, because these wavelengths change with soil depth and may have an influence on seed germination. i see that you are at stellenbosch university... come to our lab, we are currently working on medicago truncatula in the glasshouse.