In tissue culture experiments, leaf mesophyll cells are put in a culture medium to form callus. This phenomenon may be called as

The phenomenon of leaf mesophyll cells forming callus in a culture medium is called dedifferentiation. Dedifferentiation refers to the ability of differentiated cells, such as mesophyll cells in a leaf, to revert back to meristematic cells.

(1) Senescence in plants refers to the natural process of aging or deterioration of cells, tissues and organs of a plant,
ultimately leading to death. 

(2) Differentiation in plants refers to the process by which cells become specialised and acquire distinct morphological and functional characteristics to perform specific roles in the organism. 

(4) Development is the sum of growth and differentiation.