Pollinators are important for many crop types (e.g. wikipedia list), but I am wondering how important the crops are to feed the pollinator. Obviously, some of the crop plants provide much more food than others, and the plants that depend on pollinators for their reproduction will be likely to attract the pollinators with pollen, nectar, or both. But I am not sure that I can use the dependance on pollinator as an accurate estmator of the quantity of nectar that is actually available for pollinator.
I can think of the rapeseed as being very rich in nectar, but are there other "nectar rich" flowers ? For instance, what about sunflower, potatoes, peas ,sugar beet, mustard, flax, lucern, clover, maize, wheat...
To sum up, I would like to have a list of the most common European cultivated plants that produce a reasonalble amount of nectar accessible to insect pollinators. (rem : I've asked a similar question about pollen, and maybe the answer is the same, but as far as I understood some plants have more pollen and some plants have more nectar.)