Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

7. Plant Growth and Development
7.3 Senescence and Death

The stage of senescence and death encompasses the period from the complete cessation of fruiting to the natural death of the Organism. This is a phase of progressive decline in vital activity, the duration of which is largely determined by 121

the overall lifespan of the plants. This varies significantly across different taxonomic groups:

· ephemerals live for 2-4 weeks;

· annual and biennial plants last 1-2 years;

· certain perennial grasses persist for 10-30 years;

· grapevine and tau-saghyz reach up to 100 years;

· trees (such as linden, spruce, oak, and sequoia) exceed 1,000 years.

This indicates that the lifespan of each species is genetically determined. Senescence and death are the final Phases of plant ontogeny, though the term "senescence" can also be applied to individual plant Organs.

Plants exhibit various types of senescence:

· annual plants die off entirely;

· in perennial grasses, the aerial parts die back completely every year;

· in many plants, lower leaves age and die off progressively during growth;

· in deciduous trees, all leaves age and fall simultaneously in autumn.

Prior to leaf or fruit abscission, a Separation layer consisting of tightly packed, transversely oriented Cells forms at the Base of the petiole or pedicel. The Cell walls and middle lamellae within this layer undergo partial dissolution. This entire process is induced by Ethylene, which is synthesized by senescing leaves. The senescence of detached leaves can be delayed by Cytokinins, and in some species, by Auxins and Gibberellins, whereas ABA and ethylene accelerate Aging.

Senescence holds profound biological significance. It serves as a mechanism of plant adaptation to adverse environmental conditions. Furthermore, senescence drives faster evolution by accelerating generational turnover, thereby facilitating the "turnover" of genetic material.

Unlike animals, plants continue to grow throughout their entire lives by forming new Tissues and organs initiated in embryonic zones known as Meristems. The existence of meristems is sustained by initial cells (initials) capable of undergoing division indefinitely. Apical meristems are located at the tips of shoots and roots. Lateral meristems extend along the length of shoots and roots, comprising primary lateral meristems (procambium and pericycle) and secondary ones (cambium and phellogen). Intercalary meristems function at the bases of young internodes and leaves. Following The formation of tissues and organs within meristems, growth proceeds via Cell Division and subsequent elongation. Unlike animal cells, plant cells—surrounded by cell walls and interconnected by plasmodesmata—do not migrate relative to one another during growth.

Thus, morphogenesis encompasses the initiation, growth, and development of cells (cytogenesis), tissues (histogenesis), and organs (Organogenesis), which are genetically programmed and mutually coordinated. Furthermore, apical meristems act as formative tissues and primary coordinating centers.