Like animals, plants also have fixed life span and after completing that perioid, they die. Before death several degradation processes occur in their body like yellowing of leaves and fading of flower colour. It is due to loss in structure and function of an organ or the whole plant. The deteriorative processes which ultimately lead to complete loss of organization and functioning of the plant or its parts is known as Senescence.
Senescence occurs due to the deposition of waste material. In some plants the whole plant dies after flowering and producing seeds. This is called whole plant senescence. Example - annual plants like rice, wheat, beans, and tomato. In many other plants, parts above soil die each year and root system stays alive. This is called organ or shoot senescence.
Role of hormones in senescence: Abscissic acid and ethylene promote senescence of leaves but cytokinin delays senescence and helps leaves remain green for long period.
Whenever a leaf becomes old it separates from the plant body and falls down. Ripe fruits and older flowers also become separated from plants. This detachment of older plant parts or organs from the main plant body is called abscission.
In plants, a layer of tissue generally forms an abscission zone at the base of the petiole of a leaf or flower or fruit. The cells of this layer become soft and weak due to destruction of middle lamella and cell wall. So the organ is easily detached by wind or rain fall. Plant hormones like abscissic acid and ethylene promote leaf abscission and auxin prevents it.