Partially correct. [Source : I'm part of the Jain community]
There's sects that enforce not eating underground veg (potato/onion/ginger/etc) at anytime. Others avoid eating these only during 4 months of monsoon. Others maintain no restrictions.
One of the the core tenets is non-violence, and the Jain ideology provides levels of life, from animals, to trees, to wind and fire. Violence to sensory animals is seen as abhorrent, whereas violence to plants is unavoidable. Underground plants seem to have a higher level of life as many animals are part of their ecosystem. (One hypothesis I have is that these rules actually came about due to the seasonality of these vegetables during the inception of the ideology -- we no longer are similarly restricted in our farming.)
Lapsed members such as myself form a large part of the community -- hence the above claim is only partially true.
On a sidenote, more and more science fiction I've been reading seem to have elements (like Gaia) that provide a higher level of respect to vegetation than non-life, which has provided for some stimulating ideas.
