Lo was enraged by all this - called me a lousy crook and worse - and I would probably have lost my temper had I not soon discovered, to my sweetest relief, that what really angered her was my depriving her not of a specific satisfaction but of a general right. I was impinging, you see, on the conventional program, the stock pastimes, the “things that are done,” the routine of youth; for there is nothing more conservative than a child, especially a girl-child, be she the most auburn and russet, the most mythopoeic nymphet in October’s orchard-haze.
(Lolita, Vladimir Nabokov)

For what reason is there root-verb be?: It seems to denote a ‘concessive meaning,’ but I don’t get why.

Lo was enraged by all this - called me a lousy crook and worse - and I would probably have lost my temper had I not soon discovered, to my sweetest relief, that what really angered her was my depriving her not of a specific satisfaction but of a general right. I was impinging, you see, on the conventional program, the stock pastimes, the “things that are done,” the routine of youth; for there is nothing more conservative than a child, especially a girl-child, be she the most auburn and russet, the most mythopoeic nymphet in October’s orchard-haze.
(Lolita, Vladimir Nabokov)

For what reason is there root-verb be?: It seems to denote a 'concessive meaning,' but I don't get why.