Lotus stem is jain or not

Short answer: No. In Jainism, lotus stem (kamal kakdi) is treated like a root/underground part and is generally avoided because it is considered an anant-kay (root-based) item that harms many life-forms when plucked.

This view is shared by both Digambara and Śvetāmbara traditions, with slight local practice variations but the same core principle of ahimsa (non-violence) guiding the choice to avoid such parts of the plant. You can read more here