Senescence in Polycarpic Plants and Clones

Although polycarpic plants and clones may decline in their vigor, e.g., growth rates, etc., and they may even have characteristic longevities, they may not experience whole plant senescence in the sense of endogenously programmed degeneration (physiologists' sense), and usually, this decline is gradual (Nooden, 1988). It seems necessary to differentiate here between unitary organisms and clones. A unitary organism would be a single individual (integrated physiological unit) with a single root-shoot complex even if the shoot is branched. A clone, on the other hand, would be a genet, all the products of a single zygote independent of their size or degree of subdivision/separation. Where clonal organisms form multiple shoots (ramets), these can become separated into individual organisms, each with its own roots, i.e., unitary organisms. For example, all the Cabernet Sauvignon grape vines in the world belong to one genet, even though they have been carried far from each other (Penning-Roswell, 1971). It will only cause confusion if these clones consisting of disparate parts are not distinguished from individual organisms. Thus, senescence in clones may not be the same as that of individuals (e.g., a unitary individual). Likewise, longevity of clones is a different matter from longevity of unitary organisms.

Do clones or populations senesce? Some clonal populations do lose their vigor with age, even though they continue to produce new ramets, and sometimes, but probably not always, these declines may be attributed to accrual of pathogens, e.g., viruses (see Sax, 1962; Nooden, 1980, 1988) or genetic load (Kleckowski, 1997). Even whole communities may decline in their biomass growth and productivity as they age (Leopold, 1980; Huettl and Mueller-Dombois, 1993; Ryan etal., 1997), and they may be reinvigorated after destruction by burning, etc. (Hilbert and Larigauderie, 1990; Ryan et al., 1997). Sometimes, pollution is a good suspect for causing the degeneration of plant clones or communities, but other times, as in the case of ohi'a trees on Hawaii, the cause remains obscure and it could even be endogenous (Mueller-Dombois, 1992; Huettl and Mueller-Dombois, 1993). Some large populations such as bamboo species are clonal and also monocarpic perennials whose distantly disconnected individuals may die off synchronously (Janzen, 1976). While some of these declines may be endogenous (Nooden, 1988), the cause is unclear for most, and therefore whether or not they senesce in the physiologists' sense is very uncertain.

The question of senescence in polycarpic plants, especially trees, is also a complex matter with uncertain conclusions, and these have been addressed more completely elsewhere (Nooden, 1988; Watkinson, 1992; Nooden and Guiamet, 1996; Silvertown et al., 2001; Roach, Chapter 23). While plants, even unitary plants, owing to their special growth patterns and regenerative abilities, can be viewed as a sort of clonal system or a metapopulation of cells, they still form a distinctive organism and that poses some limits for the community of cells. In other words, they are defined organisms, and they do have a finite, species-characteristic longevity (Nooden, 1988; Nooden and Guiamet, 1996).

In polycarpic perennials, whole organism senescence is not as conspicuous as that of monocarpic plants, yet a degeneration phenomenon can be demonstrated in longevity patterns and the different species have characteristic longevities which does suggest that there may be some sort of genetic (endogenous) control even if it affects mainly regenerative capacity and environmental resistance (Roach, Chapter 23).

Numerous and diverse ideas have been set forth to explain what limits the life of trees and causes their decline (see Nooden, 1988; Watkinson, 1992; Huettl and Mueller-Dombois, 1993; Larson, 2001; Lanner and Connor, 2001). Most are concerned with declining growth rate and diminished regenerative capacity in older organisms. These declines may be due to various factors or combinations of factors, none of which are clearly implicated, but some of these may be endogenous. Being part of an old organism with its accumulated bulk (necromass and unproductive living tissues) could become an increasing liability with age. In general, excising a part with growth potential (i.e., a shoot cutting) often can produce a vigorous new plant flourishing after its "parent" has died, and this process can be repeated indefinitely as witnessed by the many perennial cultivars (Sax, 1962; Nooden, 1988) that have been propagated vegetatively, some for hundreds of years, e.g., Cabernet Sauvignon grape (Penning-Roswell, 1971). On the other hand, accumulated genetic load could also be a factor in the decline of some polycarpic plants (Kleckowski, 1997), and that may be why vegetative regenerative capacity diminishes with age in some species (see Nooden,

1980, 1988). Here, a periodic sexual cycle seems to be required to restore vigor of the plant.

One particularly remarkable aspect of longevity in polycarpic plants is the ability of environmental stress to prolong it (Nooden, 1988). For example, a more stressful high, dry alpine environment greatly increases the longevity of bristlecone pines (Schulman, 1954), mild mineral nutrient deficiency extends the life of duckweed plants (Wangerman, 1965), and living in the stark environment on a cliff face similarly prolongs white cedar longevity (Larson, 2001). The underlying causes of these life extensions are unknown; however, acute stress can, of course, overwhelm and kill an organism.

Polycarpic (usually perennial) plants have evolved into monocarpic (mostly annual and biennial) plants, and this process has apparently occurred independently in several different taxonomic groups (Section IH above). It is also important to note the environmental factors or forces that drive the selection of an annual life cycle over a perennial life cycle. Ultimately, the pattern that yields the best reproductive output for a given environment is the one that evolves (Fox, 1990; Begon et al, 1996; Nooden and Guiamet, 1996; Barbour et al., 1999). Annuals (monocarpy) tend to evolve in environments where the survival of mature or established plants is low, that is where small, short-lived plants reproduce more successfully than large, long-lived plants. Thus, environments that are highly variable (e.g., deserts or disturbed areas) or habitats subject to heavy predation tend to favor annuals, but many crop plants have been selected for a clear, synchronous annual life cycle. By contrast, annuals may be disfavored (perennials favored) in stable environments such as mature forests. Perennials are also favored (annuals disfavored) in environments with very short growing seasons (McKenna and Houle, 2000).

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  • iggi
    What is monocarpic nd polycarpic senescence?
    2 years ago

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