Senile (senescent) stage

In woody plants: The annual shoot elongation and trunk thickening (cambial activity) become negligible (Gatsuk et al., 1980). Whereas mature generative trees (g2–g3) still show noticeable growth, in senile individuals it is practically zero. The crown tops die off, resulting in "top-dying."

In herbaceous polycarpics: The formation of new vegetative and generative shoots completely ceases. For example, in dense-tussock grasses like Deschampsia cespitosa, no new tillering shoots are formed in the senile state, and the existing living shoots sharply reduce their linear growth (Gatsuk et al., 1980).

Physiological basis: This growth reduction is associated with the exhaustion of meristematic activity and disruption of the synthesis and transport of phytohormones, especially auxins and cytokinins, which are key stimulators of cell division and elongation (Bidlack & Jansky, 2021). Apical dominance is disrupted, which can lead to the chaotic growth of "water sprouts" as a last attempt at rejuvenation, but these shoots themselves quickly degenerate.

Generalized chlorosis: Leaves lose their species-specific green color, becoming yellowish or pale green. This is a consequence of chloroplast degradation and the breakdown of chlorophyll-protein complexes (Avila-Ospina et al., 2014). Unlike natural autumnal leaf fall, this chlorosis is not seasonal or adaptive – it is irreversible and progressive.

Necrosis: Patches of dead tissue (necrosis) appear on leaves, bark, and other organs. This results from the accumulation of toxic metabolic products and the activation of programmed cell death (PCD) processes (Avila-Ospina et al., 2014; Miryeganeh, 2022).

Epigenetic regulation: Interestingly, color changes during aging may be linked to changes in DNA methylation patterns and histone modifications. For example, in Arabidopsis, it has been shown that with age, the expression of methyltransferase genes (MET1, CMT3) decreases, leading to overall genome hypomethylation and derepression of transposons, which in turn can cause genomic instability and accelerate aging (Miryeganeh, 2022; Ogneva et al., 2016, cited in Miryeganeh).

In dense-tussock grasses (Deschampsia, Stipa): Aging begins in the center of the tussock. Here, old shoots die, and the tussock breaks up into separate particles – small tussocks, each consisting of several vegetative shoots connected by short rhizomes. In the senile state, the particles become completely isolated from each other (Gatsuk et al., 1980).

In long-rhizome plants (Aegopodium, Convallaria): Aging manifests in the death (destruction) of old sections of the rhizome. The clone breaks up into genetically identical but physiologically independent individuals. In the senile period, such particles become small, have few leaves, and are incapable of further vegetative spread.

Biological significance: Disintegration is not just destruction, but an evolutionarily fixed mechanism of "payback" for vegetative mobility. At the same time, it is a way of returning occupied resources to the ecosystem.

External manifestations: Woody plants sometimes exhibit very abundant, but late flowering with small, malformed, or sterile flowers. In herbaceous polycarpics, senile individuals may form single, underdeveloped inflorescences.

Difference from normal generation: Unlike old generative plants (g3), which produce full-fledged, albeit few, seeds, a senile plant, if it forms flowers at all, generally does not set viable seeds. The germination capacity of seeds, if they are nonetheless formed, is sharply reduced (Yakovlev et al., 2008).

Possible explanation: This phenomenon can be seen as an "agonal" (pre‑mortem) activation of the organism’s last reserves. Apparently, against a background of general regulatory decline, derepression (removal of blockage) of flowering genes occurs, but the resource and hormonal support for the proper development of generative organs is lacking (Miryeganeh, 2022). This phenomenon is akin to "rejuvenating" flowering, but in this case it only underscores the depth of catabolic processes.

Organelle degradation: The initial trigger is membrane damage, especially the membranes of chloroplasts and mitochondria, by reactive oxygen species (ROS) that accumulate during aging (Avila‑Ospina et al., 2014). The process of autophagy is initiated – the cell begins to "digest" its own damaged components. Genes of the AuTophaGy (ATG) family play a key role here; their products form autophagosomes that capture fragments of chloroplasts (Rubisco‑containing bodies, RCBs) and deliver them to the vacuole for degradation (Avila‑Ospina et al., 2014; Ishida et al., 2008, cited in Avila‑Ospina).

Formation of air spaces: After protoplast death, the cell is no longer sealed. Air‑filled spaces arise in the cortex and xylem parenchyma. In the heartwood of trees, this phenomenon is called "dry rot" – wood sections lose moisture and mechanical strength (Strasburger, 1971). The formation of air spaces is one of the early microscopic signs of senility.

Epigenetic control: The intensity of symplastic senescence is regulated by epigenetic mechanisms. In aging leaves of Arabidopsis, a decrease in the level of repressive histone marks (H3K27me3) on the promoters of genes related to cell wall degradation and programmed cell death is observed, leading to their activation (Miryeganeh, 2022; Brusslan et al., 2012, cited in Miryeganeh).

Direction of dieback: In most woody and herbaceous plants, leaf senescence proceeds from the periphery to the center (in rosette plants) or from the bottom up (in plants with a shoot). The oldest, lower leaves turn yellow and fall first, then the process moves upward. In trees, desiccation begins with the apical (top‑dying) and peripheral branches of the crown (Gatsuk et al., 1980; Raven et al., 2013).

Resource mobilization: Leaf dieback is not a passive decay but an active process of mobilizing nutrients. Before falling, nitrogen, phosphorus, potassium, and other elements are withdrawn from the leaf and transported to storage tissues or growing points. This process provides up to 70‑80% of nitrogen reutilization (Avila‑Ospina et al., 2014).

Agronomic aspect: In agricultural crops (cereals, vegetables), premature dieback of lower leaves is a signal of nutrient deficiency or aging. In perennial grasses (cereals, legumes) at the senile stage, a rapid thinning of the stand is observed due to the death of old leaves while few new ones are formed.

Mechanism: Once xylem cells die, their lumens become accessible to saprotrophic fungi and bacteria. In the absence of living defense mechanisms (e.g., production of phytoalexins), fungi quickly colonize the heartwood, causing brown or white rot (Strasburger, 1971). The process begins with heart (core) rot, then spreads, and the trunk becomes hollow.

Biological norm: Despite its seemingly pathological nature, hollow formation is a natural stage of aging for many long‑lived trees (oak, linden, willow). Hollow trees remain viable because the cambium and young sapwood (peripheral layers) remain alive and continue to conduct water.

Ecological role: Hollow trees acquire enormous ecological significance as nesting sites for birds, shelters for mammals and insects. From this perspective, the senile stage increases biodiversity in forest ecosystems.

Manifestation: On old trees (e.g., apple, linden, oak), vertical, fast‑growing shoots with large leaves suddenly begin to grow actively on the trunk or at its base from dormant buds (latent meristems). Such shoots are called water sprouts or epicormic shoots. In herbaceous polycarpics (e.g., Asarum europaeum), senile particles sometimes form abnormally large leaves not typical of the species (Gatsuk et al., 1980).

Physiological meaning: Water sprouts are the plant’s attempt to "restart" ontogenesis. Apparently, with the weakening of apical dominance (due to reduced auxin synthesis), dormant buds receive a signal to grow. These shoots often have signs of juvenility: large leaves, high growth rate. However, they are typically functionally incomplete (weakly lignified, often affected by diseases) and quickly age themselves.

Evolutionary analogy: Some researchers (Gatsuk et al., 1980) draw a parallel between the formation of water sprouts on senile plants and the formation of particles in long‑rhizome species. In both cases, the organism tries to survive by forming new "young" modules against the background of the dying mother part. However, this mechanism rarely allows a significant extension of the individual’s life.

Brevity of the stage: In typical annuals (e.g., Arabidopsis thaliana, Pisum sativum), the senile stage is extremely compressed and almost coincides with the completion of mass fruiting. As soon as the last seeds mature, a rapid aging program is triggered in the vegetative organs, and the plant dies within a few days or weeks (Raven et al., 2013).

Mechanism: A key role is played by monocentric aging – dieback begins in the growing points and spreads throughout the organism. At the molecular level, this is associated with a sharp activation of senescence genes (SAG12, etc.) and chloroplast degradation, a process largely independent of external conditions and rigidly genetically determined (Avila‑Ospina et al., 2014).

Agronomic nuance: In grain crops (wheat, barley), the senile stage occurs after grain filling. Yellowing and dieback of leaves ("ripening") is normal, but if it occurs too quickly (under the influence of drought or disease), grain filling is disrupted, and yield decreases (Bidlack & Jansky, 2021).

Long‑rhizome plants (Aegopodium podagraria, Convallaria majalis): In the senile state, their ability to form new "stolons" (plagiotropic shoots) sharply decreases. Particles become solitary or consist of 1‑2 shoots, leaves become smaller, the root system is reduced. Such a particle does not last long and does not give rise to a new clone.

Dense‑tussock grasses (Deschampsia cespitosa, Stipa pennata): In the senile period, the tussock breaks down into many micro‑tussocks, in which living shoots account for less than 10% of the total biomass (Gatsuk et al., 1980). Tillering completely stops.

Tuber‑forming and bulbous plants: In tulips, potatoes, the senile stage occurs after the mother plant has formed a replacement bulb or tuber. The mother part dies, while the daughter part can enter a new generative phase. However, as the clone ages, bulbs become smaller and stop flowering.

The "old age" stage in trees: Gatsuk and co‑authors (1980) using the example of ash (Fraxinus excelsior) showed that trees clearly exhibit a period of old generative individuals (g3), which still bear fruit, and then the actual senile individuals, which no longer bear fruit and live out their days.

Key signs of senility in trees (according to Gatsuk et al., 1980; Strasburger, 1971):

Top‑dying: Death of the apical part of the crown. Water and nutrients cease to reach the upper part of the trunk, leading to its desiccation.

Hollowness: Complete destruction of the heartwood (heart rot). The trunk becomes hollow, but the tree continues to live thanks to the sapwood and cambium.

Bark exfoliation: Bark peels off in large blocks or strips (e.g., in old pines, plane trees). This results from the cessation of phellogen (cork cambium) activity and phloem death.

Water sprouts (epicormic shoots): Active growth of dormant buds on the trunk and thick branches. This is the tree’s attempt to restore the crown. However, water sprouts are usually weakly attached, have fragile wood, and often break off (Strasburger, 1971).

Exceptions to the rule: Some tree species (e.g., yew – Taxus baccata) can reach advanced old age with almost no hollow formation and retain the ability to regenerate. In others (e.g., birch), aging occurs relatively early (80‑100 years), and the tree quickly decays. The duration of the senile stage is determined by species specificity and growing conditions.

Reduction in fruit and seed yield: In fruit trees (apple, pear, cherry) and seed crops (sunflower, grain legumes), the setting of full‑fledged fruits and seeds ceases in the senile stage. Whereas old generative trees (g3) may still produce a small but marketable yield, senile individuals either do not fruit at all or produce small, shriveled seeds with low germination (Gatsuk et al., 1980; Yakovlev et al., 2008).

Decline in vegetative biomass: In forage perennial grasses (alfalfa, clover, timothy), the yield of green mass drops sharply in the senile stage. This is due to the cessation of tillering, leaf dieback, and the general diminution of particles (Gatsuk et al., 1980). The protein content in leaves also decreases due to degradation of the chlorophyll‑protein complex and disruption of nitrogen metabolism (Avila‑Ospina et al., 2014).

Economic threshold: The profitability of cultivating perennial crops is determined by the time when the increase in productivity from agronomic interventions (fertilization, irrigation) ceases to cover the costs. For an orchard, this is the moment when the costs of maintenance and harvesting exceed the value of the produce obtained. For meadows and pastures – when the projective cover by senile plants falls below 30‑40%, and active weed infestation begins.

Weakened immunity: In the senile stage, the synthesis of defense compounds – phytoalexins, protease inhibitors – sharply decreases, and the ability to heal wounds (formation of callus and periderm) weakens. Epigenetic changes, including transposon derepression, may also make the genome more vulnerable (Miryeganeh, 2022).

Bark beetles and longhorn beetles: Weakened senile trees emit specific volatile substances (acids, alcohols) that serve as primary attractants for stem pests – bark beetles (Ipidae) and longhorn beetles (Cerambycidae). Infestation of healthy trees by these insects is unlikely; they prefer old and diseased specimens (Strasburger, 1971).

Necrotrophic fungi: Senile plants are massively infected by necrotrophic fungi (causal agents of root, foot, and stem rots), such as Armillaria mellea (honey fungus), Heterobasidion annosum (root rot fungus), Fusarium spp., and others. Dead tissues serve as a substrate for these saprotroph‑parasites.

Agroecosystem rule: In intensive agriculture, senile and over‑mature plants must be removed and destroyed (e.g., burned or deeply incorporated into the soil). Leaving them in the field or orchard creates a reservoir of pathogens and pests, which then spread to healthy plantings of the next crop rotation. This is especially important for controlling cereal root rots and fruit tree canker.