Leaf senescence is an orderly, active process in which nutrients in a leaf are reclaimed and mobilized to other parts of the plant. It is considered to be the final stage in leaf development. During senescence leaves undergo highly coordinated changes in cell structure, metabolism, and gene expression. For example, chloroplasts undergo obvious structural modifications during leaf senescence. Regarding gene expression, genes with steady-state mRNA levels that increase during senescence are often referred to as senescence-associated genes, or SAGs.

In order to study ultrastructural changes that occur during senescence, portions of fully expanded, mid-senescent, and fully senescent Arabidopsis thaliana leaves were chemically fixed using vacuum-accelerated microwave processing. Thin sections were observed for ultrastructural changes such as development of transfer cells that are characterized by the presence of cell wall ingrowths. Transfer cell development is correlated with an increased capacity of tissues to take up materials, related partly to an increase in plasma membrane surface area.