Annuality (living and reproducing in a single year) and perenniality (living more than two years) represent major life history strategies within plant lineages. [1] These traits can shift from one to another over both macroevolutionary and microevolutionary timescales. [2] While perenniality and annuality are often described as discrete either-or traits, they often occur in a continuous spectrum. [2] The complex history of switches between annual and perennial habit involve both natural and artificial causes, and studies of this fluctuation have importance to sustainable agriculture. (Note that "perennial" here refers to both woody and herbaceous perennial species.)
According to some studies, either the trait of annuality or perenniality may be ancestral. [3] [4] This contradicts the commonly held belief that annuality is a derived trait from an ancestral perennial life form, as is suggested by a regarded plant population biology text. [5]
Above the species level, plant lineages clearly vary in their tendency for annuality or perenniality (e.g., wheat vs. oaks). On a microevolutionary timescale, a single plant species may show different annual or perennial ecotypes (e.g., adapted to dry or tropical range), as in the case of the wild progenitor of rice ( Oryza rufipogon ). [6] [7] Indeed, ability to perennate (live more than one year) may vary within a single population of a species. [2] [5]
Annuality and perenniality are complex traits involving many underlying, often quantitative, genotypic and phenotypic factors. [5] [8] They are often determined by a trade-off between allocation to sexual (flower) structures and asexual (vegetative) structures. [9] Switches between the annual and perennial habit are known to be common among herbaceous angiosperms. [9]
Increased allocation to reproduction early in life generally leads to a decrease in survival later in life (senescence); this occurs in both annual and perennial semelparous plants. [5] Exceptions to this pattern include long-lived clonal (see ramets section below) and long-lived non-clonal perennial species (e.g., bristlecone pine). [5]
Many traits involving mating patterns (e.g., outcrossing or selfing) and life history strategies (e.g., annual or perennial) are inherently linked.
Self-fertilization (selfing, or autogamy) is more common in annual compared to perennial herbs. [10] Since annuals typically have only one opportunity for reproduction, selfing provides a reliable source of fertilization. [10] However, switches to selfing in annuals may result in an "evolutionary dead end," in the sense that it is probably unlikely to return to an outcrossing (allogamous) state. [10] Selfing and inbreeding can also result in the accumulation of deleterious alleles, resulting in inbreeding depression. [5]
All annual plants are considered semelparous (a.k.a., monocarpy or big-bang reproduction [11] ), i.e., they reproduce once before death. [12] Even semelparity exerts some plasticity in terms of seed-production timing over the year (see "Anomalies" section). That is, it is uncommon for all offspring to be generated at exactly the same time, which would be considered the extreme end of semelparity. [13] Instead offspring are usually generated in discrete packages (as a sort of micro-iteroparous strategy), and the temporal spacing of these reproductive events varies by organism. [13] This is attributed to phenotypic plasticity. [13]
Biennial plants (living two years and reproducing in the second) are also considered semelparous. [12] [14]
Although annuals have no vegetative regrowth from year to year, many retain a dormant population back-up underground in the form of a seed bank. [5] The seed bank serves as an annual's source of age structure in the sense that often not all seeds will germinate each year. [5] Thus, each year's population will consist of individuals of different ages in terms of seed dormancy times. The seed bank also helps to ensure the annual's survival and genetic integrity in variable or disturbed habitats (e.g., a desert), where good growing conditions are not guaranteed every year. [5] Not all annuals, however, retain a seed bank. [5] As far as population density, annuals with seed banks are predicted to be more temporally variable yet more spatially constant over time, while plants with no seed bank would be expected to be patchy (spatially variable). [5]
Certain non-selfing reproductive adaptations, such as dioecy (obligate outcrossing via separate male and female individuals), may have arisen in long lived herbaceous and woody species due to negative side effects of selfing in these species, notably genetic load and inbreeding depression. [10] Among angiosperms, dioecy is known to be substantially more common than pure self-incompatibility. [5] Dioecy is also more typical of trees and shrubs compared to annual species. [5]
Most perennials are iteroparous (or polycarpic), which means they reproduce multiple times during their lifespan. [5]
Ramets are vegetative, clonal extensions of a central genet. Common examples are rhizomes (modified stem), tillers, and stolons. A plant is perennial if the birth rate of ramets exceeds their death rate. [5] Several of the oldest known plants are clonal. Some genets have been reported to be many thousands of years old, and a steady rate of branching likely aids in avoiding senescence. [5] [15] [16] The oldest reported minimum age of a single genet is 43,600 years, for Lomatia tasmanica W.M.Curtis. [17] It is hypothesized that some perennial plants even display negative senescence, in which their fecundity and survival increase with age. [16]
Examples of plants with rhizomatous growth include perennial Sorghum and rice, which likely share similar underlying genes controlling rhizome growth. [18] [19] [20] In wheat ( Thinopyrum ), perenniality is associated with production of a secondary set of tillers (stems arising from the crown's apical meristem) following the reproductive phase. This is called post-sexual cycle regrowth (PSCR). [8] Such long-lived genets in a population may provide a buffer against random environmental fluctuations. [5]
There is a possible connection between polyploidy (having more than two copies of one's chromosomes) and perenniality. [3] One potential explanation is that both polyploids (larger in size) and asexual reproduction (common in perennials) tend to be selected for in inhospitable extremes of a species' distribution. [5] One example could be the intricate polyploidy of native Australian perennial Glycine species. [21] [22]
Woody species have been found to occupy fewer climatic niches than herbaceous species, which was suggested to be a result of their slower generation time; such differences in adaptation may result in niche conservatism among perennial species, in the sense that their climatic niche has not changed much over evolutionary time. [23] [24]
Semelparity in perennials is rare but occurs in several types of plants, likely due to adaptive changes for greater seed allocation in response to seed predation (although other drivers, such as biased pollination, have been proposed). [5]
List of semelparous perennials: [5]
The Polemoniaceae (phlox) family shows considerable flexibility in both life history and mating system, showing combinations of annual / selfing, annual / outcrossing, perennial / selfing, and perennial / outcrossing lineages. These switches indicate a more ecologically determined, rather than a phylogenetically fixed, change in habit. [10]
High environmental stochasticity, i.e., random fluctuations in climate or disturbance regime, can be buffered by both the annual and perennial habit. [5] However, the annual habit is more closely associated with a stochastic environment, whether that is naturally or artificially induced. [5] [25] This is due to higher seedling compared to adult survival in such stochastic environments; common examples are arid environments such as deserts as well as frequently disturbed habitats (e.g., cropland). [5] Iteroparous perennial species are more likely to persist in habitats where adult survival is favored over seedling survival (e.g., canopied, moist). [5] This adult/juvenile trade-off can be described succinctly in the following equations:
λa = cma
λp = cmp+p
ma > (or <) mp + (p/c) [5]
(Silvertown & Charlesworth, 2001, p. 296)
Where: λa = rate of growth of annual population. λp = rate of growth of perennial population. c = survival to reproductive age (flowering). ma = seeds produced for each annual individual (average). mp = seeds produced for each perennial individual. p = adult survival. [5]
If ma > mp + (p/c), the annual habit has greater fitness. If ma < mp + (p/c), the perennial habit has greater fitness. [5] Thus a great deal of the fitness balance depends on the reproductive allocation to seeds, which is why annuals are known for greater reproductive effort than perennials. [5] [26]
Different climate and disturbance patterns may also cause demographic changes in populations. [5] [27] [28]
The annual vs. perennial trait has been empirically associated with differing subsequent rates of molecular evolution within multiple plant lineages. The perennial trait is generally associated with a slower rate of evolution than annual species when looking at both non-coding [29] [30] [31] and coding DNA. [32] Generation time is often implicated as one of the major factors contributing to this disparity, with perennials having longer generation times and likewise an overall slower mutation and adaptation rate. [29] This may result in higher genetic diversity in annual lineages. [33]
Plant taxon groups that have evolved both annual and perennial life forms.
Taxon group | Shift | Reported Cause | Sequence-type | Geographic Region | Literature |
---|---|---|---|---|---|
Bellis (daisies) | perennial→annual | aridity | nrDNA (ITS) | Western Mediterranean | [34] |
Castilleja (Indian paintbrush) | annual→perennial | cpDNA (trnL-F, rps16); nrDNA (ITS, ETS) | Western North America | [3] | |
Ehrharta (veldtgrass) | perennial→annual | aridity | cpDNA (trnL-F); nrDNA (ITS1) | South African Cape | [35] |
Houstonia | perennial→annual | cpDNA (trnL-F intron); nrDNA (ITS) | [36] | ||
Medicago (alfalfa) | annual→perennial (possibly perennial→annual instead) | nrDNA (ITS, ETS) | [4] | ||
Nemesia | perennial→annual | change in precipitation | cpDNA (trnL); nrDNA (ITS, ETS) | South African Cape | [37] |
Oryza (rice) | perennial→annual | artificial selection for loss of rhizomes | coding nuclear DNA | Asia | [18] |
Polemoniaceae (phlox) | annual→perennial; perennial→annual | ann.→peren.: ecology peren.→ann.: ecology (desert climate) | cpDNA (matK) | [10] | |
Sidalcea (checker mallow) | perennial→annual | aridity | nrDNA (ITS, ETS) | Western North America | [29] [30] |
Artificial selection seems to have favored the annual habit, at least in the case of herbaceous species, likely due to fast generation time and therefore a quick response to domestication and improvement efforts. [38] However, woody perennials also exemplify a major group of crops, especially fruit trees and nuts. High yield herbaceous perennial grain or seed crops, however, are virtually nonexistent, despite potential agronomic benefits. [39] [40] Several common herbaceous perennial fruit, herbs, and vegetables exist, however; see perennial plants for a list.
Annual and perennial species are known to respond to selection in different ways. For instance, annual domesticates tend to experience more severe genetic bottlenecks than perennial species, which, at least in those clonally propagated, are more prone to continuation of somatic mutations. [1] [41] Cultivated woody perennials are also known for their longer generation time, outcrossing with wild species (introducing new genetic variation), and variety of geographic origin. [1] Some woody perennials (e.g., grapes or fruit trees) also have a secondary source of genetic variation within their rootstock (base to which the above-ground portion, the scion, is grafted). [41]
Compared to annual monocultures (which occupy c. 2/3 of the world's agricultural land), perennial crops provide protection against soil erosion, better conserve water and nutrients, and undergo a longer growing season. [42] [43] Wild perennial species are often more resistant to pests than annual cultivars, and many perennial crop wild relatives have already been hybridized with annual crops to confer this resistance. [42] Perennial species also typically store more atmospheric carbon than annual crops, which can help to mitigate climate change. [42] [43] Unfavorable characteristics of such herbaceous perennials include energetically unfavorable trade-offs and long periods of juvenile non-productivity. [41] Some institutions, such as The Land Institute, have begun to develop perennial grains, such as Kernza (perennial wheat), as potential crops. [42] Some traits underlying perenniality may involve relatively simple networks of traits, which can be conferred through hybrid crosses, as in the case of perennial wheat crossed with annual wheat. [8]
The Saxifragales (saxifrages) are an order of flowering plants (Angiosperms). They are an extremely diverse group of plants which include trees, shrubs, perennial herbs, succulent and aquatic plants. The degree of diversity in terms of vegetative and floral features makes it difficult to define common features that unify the order.
Paulowniaceae are a family of flowering plants within the Lamiales. They are a monophyletic and monogeneric family of trees with currently 7 confirmed species. They were formerly placed within Scrophulariaceae sensu lato, or as a segregate of the Bignoniaceae.
Orobanchaceae, the broomrapes, is a family of mostly parasitic plants of the order Lamiales, with about 90 genera and more than 2000 species. Many of these genera were formerly included in the family Scrophulariaceae sensu lato. With its new circumscription, Orobanchaceae forms a distinct, monophyletic family. From a phylogenetic perspective, it is defined as the largest crown clade containing Orobanche major and relatives, but neither Paulownia tomentosa nor Phryma leptostachya nor Mazus japonicus.
Phrymaceae, also known as the lopseed family, is a small family of flowering plants in the order Lamiales. It has a nearly cosmopolitan distribution, but is concentrated in two centers of diversity, one in Australia, the other in western North America. Members of this family occur in diverse habitats, including deserts, river banks and mountains.
Erythranthe guttata, with the common names seep monkeyflower and common yellow monkeyflower, is a yellow bee-pollinated annual or perennial plant. It was formerly known as Mimulus guttatus.
Hydatellaceae are a family of small, aquatic flowering plants. The family consists of tiny, relatively simple plants occurring in Australasia and India. It was formerly considered to be related to the grasses and sedges, but has been reassigned to the order Nymphaeales as a result of DNA and morphological analyses showing that it represents one of the earliest groups to split off in flowering-plant phylogeny, rather than having a close relationship to monocots, which it bears a superficial resemblance to due to convergent evolution. The family includes only the genus Trithuria, which has at least 13 species, although species diversity in the family has probably been substantially underestimated.
Haloragaceae is a eudicot flowering plant family in the order Saxifragales, based on the phylogenetic APG system. In the Cronquist system, it was included in the order Haloragales.
Sidalcea is a genus of the botanical family Malvaceae. It contains several species of flowering plants known generally as checkerblooms or checkermallows, or prairie mallows in the United Kingdom. They can be annuals or perennials, some rhizomatous. They are native to West and Central North America.
Opuntioideae is a subfamily of the cactus family, Cactaceae. It contains 15 genera divided into five tribes. The subfamily encompasses roughly 220–250 species, and is geographically distributed throughout the New World from Canada, to Argentina. Members of this subfamily have diverse habits, including small geophytes, hemispherical cushions, shrubs, trees, and columnar cacti consisting of indeterminate branches or determinate terete or spherical segments.
Haloragis is a genus of flowering plants in the family Haloragaceae. Some species are known commonly as seaberry and most are native to the southern hemisphere. They are annual or perennial herbs to small shrubs, and many are terrestrial wetland plants.
Semelparity and iteroparity are two contrasting reproductive strategies available to living organisms. A species is considered semelparous if it is characterized by a single reproductive episode before death, and iteroparous if it is characterized by multiple reproductive cycles over the course of its lifetime. Iteroparity can be further divided into continuous iteroparity and seasonal iteroparity Some botanists use the parallel terms monocarpy and polycarpy.
Strophostyles is monophyletic three-species genus of flowering plants in the family Fabaceae, subfamily Faboideae. Common names for the genus include wild bean and fuzzybean. It consists of annual and perennial herbaceous vines, ranging in their native distribution from Nevada, east to Florida, and north to the Great Lakes and eastern Canada. The etymology of the name is strophe (turning) + stylos (style), referring to the curve of the style within the keel petal.
Flowering synchrony is the amount of overlap between flowering periods of plants in their mating season compared to what would be expected to occur randomly under given environmental conditions. A population which is flowering synchronously has more plants flowering at the same time than would be expected to occur randomly. A population which is flowering asynchronously has fewer plants flowering at the same time than would be expected randomly. Flowering synchrony can describe synchrony of flowering periods within a year, across years, and across species in a community. There are fitness benefits and disadvantages to synchronized flowering, and it is a widespread phenomenon across pollination syndromes.
Rhinantheae is a tribe with less than 20 genera of herbaceous plants in the family Orobanchaceae.
A mixed mating system, also known as “variable inbreeding” a characteristic of many hermaphroditic seed plants, where more than one means of mating is used. Mixed mating usually refers to the production of a mixture of self-fertilized (selfed) and outbred (outcrossed) seeds. Plant mating systems influence the distribution of genetic variation within and among populations, by affecting the propensity of individuals to self-fertilize or cross-fertilize . Mixed mating systems are generally characterized by the frequency of selfing vs. outcrossing, but may include the production of asexual seeds through agamospermy. The trade offs for each strategy depend on ecological conditions, pollinator abundance and herbivory and parasite load. Mating systems are not permanent within species; they can vary with environmental factors, and through domestication when plants are bred for commercial agriculture.
Cryptic self-incompatibility (CSI) is the botanical expression that's used to describe a weakened self-incompatibility (SI) system. CSI is one expression of a mixed mating system in flowering plants. Both SI and CSI are traits that increase the frequency of fertilization of ovules by outcross pollen, as opposed to self-pollen.
Alpinia rafflesiana, commonly known in Malaysia as Tepus Telor, is a perennial herb belonging to the family Zingiberaceae. It is native to peninsular Malaysia.
Silphium integrifolium is a species of flowering plant in the family Asteraceae. Its common names include rosinweed, whole-leaf rosinweed, entire-leaf rosinweed, prairie rosinweed, and silflower. It is native to eastern North America, including Ontario in Canada and the eastern and central United States as far west as New Mexico.
Hybridization, when new offspring arise from crosses between individuals of the same or different species, results in the assemblage of diverse genetic material and can act as a stimulus for evolution. Hybrid species are often more vigorous and genetically differed than their ancestors. There are primarily two different forms of hybridization: natural hybridization in an uncontrolled environment, whereas artificial hybridization occurs primarily for the agricultural purposes.