Acclimation and adaptation

Senescence has ecological implications at many levels, from the physiological interactions of the structural modules that constitute the individual plant as metapopulation (White 1979, Harper 1989) through the biotic and abiotic responses of populations and communities (Chabot and Hicks 1982, Eissenstat and Yanai 1997) to the turnover behaviour of ecosystems, species and floras (Leopold 1975).

Plants are sedentary and have to accommodate their patterns of growth and survival to an environment from which they cannot physically escape. Furthermore the modular, open-ended, proliferative, fractal nature of plant development (Walbot 1985, Prusinkiewicz 2004) means that plant-environment relations can never be static, and even the maintenance of homeostasis is under constant challenge. Homeostasis is dependent on balance within a regime of flux and turnover (Leopold 1975). A homeostatic system has three modes of response to perturbation: elastic, in which the system bounces back and resumes its former state; plastic, in which it deforms and settles on a new stable configuration; and catastrophic, where the limits of elastic and plastic resilience are exceeded and the system becomes incoherent, entropic and, in the case of biology, non-viable (Thomas 1992). In engineering terms, the perturbing influence is a stress and the elastic, plastic or catastrophic response is the corresponding strain (Gordon 1978). Plants relate to stress and strain by adapting or acclimating.

	Adaptation	Acclimation
Individual or population level	Population	Individual
Caused by	Natural selection acting on allelic variation within populations.	Local environmental conditions acting on genetically-determined physiological responsiveness.
Heritability	Genotypic	Generally phenotypic (non-heritable) only, some instances of epigenetic transmission.
Reversibility	Irreversible (except by further genotypic change and selection)	Reversible
Response of homeostasis to perturbation	Mostly plastic	Mostly elastic
Timescale	From generation time of the organism up to the evolutionary.	Short-term (minutes/hours) – metabolic andphysiological adjustments of existing components without significant change in gene expression. Long-term (up to weeks or months) – altered patterns of gene expression, reallocation of resources, morphological change.
Deployment in the life-cycle	Strategic	Tactical

Senescence contributes to all aspects of plant-environment relationships by virtue of its function in the catabolic side of cell physiology, its participation in the turnover of tissues and organs, its tactical deployment in response to random exposure to non-optimal abiotic or biotic challenge and its programming within general strategies of adaptation (Thomas 1992).

References

• Chabot BF, Hicks DJ (1982) The ecology of leaf life spans. Annual Review of Ecology and Systematics 13: 229-259.
• Eissenstat DM Yanai RD (1997) The ecology of root lifespan. Advances in Ecological Research 27: 1-60.
• Gordon JE (1978) Structures. Harmondsworth: Penguin.
• Harper JL (1989) The value of a leaf. Oecologia 80: 53-58.
• Leopold AC (1975) Aging, senescence, and turnover in plants. Bioscience 25: 659-662.
• Prusinkiewicz P (2004) Modeling plant growth and development. Current Opinion in Plant Biology 7: 79-83.
• Thomas H (1992) Canopy survival. In: Crop Photosynthesis: Spatial and Temporal Determinants (eds Baker N, Thomas H) pp 11-41. Amsterdam: Elsevier .
• Walbot V (1985) On the life strategies of plants and animals. Trends in Genetics 1: 165–170.
• White J (1979) The plant as a metapopulation. Annual Review of Ecology and Systematics 10: 109-145.