why jains do not eat after sunset and before sunrise ?

during sun light the microbes in air are few and after dark they come to existence.  eating after sunset leads to also disturbing or consuming them so eating after sunset to sunrise is restricted.

why jains do not eat roots ?

Roots tend to be body with multiple lives.  Its like conjoined twins in single body.  human beings need food to survive so jainism suggests to avoid consuming too many lives and thus restricting the roots.  vegetables growing on trees are not packed with lives like roots and thus its preferred to eat vegetables growing above ground.

Groundnut is protected in its shell covering and does not accomodate multiple lives so it is exempted from this rule.