What is the main difference between mature xylem cells and mature ploloem cells?

Xylem cells die as they mature while phloem cells are alive at maturity.

Both xylem and phloem are vascular tissues. Xylem is a tubular structure which is responsible for water transport from the roots towards all of the parts of the plant. Phloem is also a tubular structure but is responsible for the transportation of food and other nutrients needed by plant. Xylem exists as non-living tissue at maturity, but phloem is living cells.