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Development and Morphogenesis |
Department of Environmental, Population, and Organismic Biology, University of Colorado, Boulder, Colorado 80309-0334 USA
Received for publication May 12, 2000. Accepted for publication August 31, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Acomastylis rossii • allocation • alpine • bud dormancy • development • phenotypic plasticity • preformation • Rosaceae
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Despite the fact that preformation, the initiation of organs one or more growing seasons prior to their maturation and function, is reported to be common in plants of diverse seasonal environments (Foerste, 1891
; Moore, 1909
; Mooney and Billings, 1960
; Kozlowski and Clausen, 1966
; Remphrey, 1989
; Diggle, 1997
), it has been well characterized in few species. A variety of patterns of preformation have been documented in a number of temperate trees, shrubs, and herbaceous perennials (Foerste, 1891
; Critchfield, 1960, 1970
; Remphrey and Steeves, 1984a, b
; Inouye, 1986
; Hayes, Steeves, and Neal, 1989
; Jones, Watson, and Lu, 1993
; Walton and Hufford, 1994
). These studies and others (reviewed in Diggle, 1997
) show that preformation is common in temperate plants and that the duration can range from several months to 3 yr.
Preformation is widely theorized to be important in tundra environments because the short growing seasons there are insufficient for the complete development of most plant organs from initiation to maturation and function (Sørensen, 1941
; Billings and Bliss, 1959
; Bliss, 1962
; Billings and Mooney, 1968
; Mark, 1970
; Diggle, 1997
). Few studies, however, have investigated the time interval between organ initiation and functional maturity. Furthermore, although preformation is thought to be a common and critical feature of tundra plants, complete developmental analyses are available for only two species, Polygonum viviparum (Polygonaceae; Diggle, 1997
) and Caltha leptosepala (Ranunculaceae; Aydelotte and Diggle, 1997
), and these make up a relatively small component of the tundra biomass. Characterizing preformation in a plant that dominates large areas of tundra adds substantially to understanding of tundra growth dynamics.
Preformation has also been implicated in delayed responses of plants to environmental change (Aydelotte and Diggle, 1997
; Diggle, 1997
, and references therein; Geber, De Kroon, and Watson, 1997a, b
). Preformation is predicted to place a time constraint upon shoot development. If shoot borne organs such as leaves require multiple years to complete development, then some of the responses of that shoot to a change in environmental conditions may only appear years later, when the affected structures mature. The result is a substantial lag between environmental inputs and measurable changes in plant growth or biomass such as those observed by Walker et al. (1994)
. As preformation has been so little studied in tundra environments it is unclear if this phenomenon contributes substantially to observed lags in response.
In this study, we examine patterns of development in the alpine perennial Acomastylis rossii (alpine avens). Acomastylis rossii is a ubiquitous and dominant species of the alpine tundra in the Southern Rockies and has a highly branched, complex morphology. Our objectives were to examine basic features of preformation, including: (1) the duration of preformation, i.e., the timing of organ initiation and the duration of development prior to maturation; and (2) the extent of preformation, i.e., the number of structures preformed and the amount of growth and development of structures prior to maturation. In addition, we address the intraseasonal duration of primordium growth and the potential importance of axillary bud formation for developmental flexibility.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Acomastylis rossii is a dominant component of the vegetation in both moist and dry meadow communities, which cover the majority of the vegetated terrain above timberline in the Southern Rockies.
Study site
All collection and field work took place in the alpine tundra of Niwot Ridge (elevation 3750 m) in the Front Range of the Colorado Rocky Mountains (40°03' N, 105°36' W). Detailed climatic records have been maintained for this site since 1981 (Losleben, 1983–1988
). Acomastylis rossii was sampled from a dry meadow dominated by Kobresia myosuroides (Vill.) Fiori & Paol. with significant stands of Selaginella densa (Rydb.) and A. rossii. This site is characterized by a snow-free period of 150–200 d lasting from early June to mid-November. Soil moisture is low throughout this period (May, 1973
).
Plants produce a much-branched underground rhizome system, making it difficult to identify distinct individuals (Spomer and Salisbury, 1968
). Therefore, for the purposes of this study, an "individual" is defined as the rosette of leaves generated by a single shoot apical meristem, inflorescences subtended by those leaves, and 4–6 cm of rhizome subjacent to the apical meristem. The individual rhizomes always bear roots at or near the point where the shoot emerges from the ground and each inflorescence is subtended by a functioning leaf (C. G. Meloche, personal observations). Based on these observations, it is inferred that each "individual" is an integrated physiological unit sensu Watson and Casper (1984)
and capable of affecting its own uptake, storage, growth, and sexual reproduction.
Aboveground phenology
Thirty plants were permanently marked at the study site for examination of aboveground phenology. These individuals were monitored over the course of three growing seasons (1996–1998) from the beginning of June until leaf and inflorescence senescence in early September. At 
9-d intervals the number of leaves, inflorescences, and flowers on each individual were counted and the longest leaf and longest inflorescence were measured. The phenological stage or stages of each plant were also recorded. Phenological stages consist of the following either singly or in combination: leaves expanding, inflorescences expanding, leaves mature (fully expanded), full flower, fruit set, fruit dispersal, leaves senescing, and dormancy.
Dissections and microscopy
From June through October in 1996, 15 randomly selected individuals were destructively harvested at 3-wk intervals for laboratory dissection. After harvesting, plants were preserved in formalin-acetic acid-alcohol (FAA; Berlyn and Miksche, 1976
) for a minimum of 48 h and then stored in 70% ethanol. Plants were dissected, and measurements of primordia were made using a Zeiss SV11 dissecting microscope with a calibrated ocular micrometer. The following characteristics were recorded for each plant: number of mature leaves, length of each leaf, number of mature inflorescences, length of each inflorescence, number of leaf primordia, length of each leaf primordium, number of inflorescence primordia, length of each inflorescence primordium, and the length of rhizome elongation in the current year. The amount of rhizome elongation per year can be inferred by measuring the length of rhizome along which a single year's mature leaves are attached. Plants were photographed during dissection with Kodak, T-Max 100 film. Additional specimens were prepared for scanning electron microscopy. They were dehydrated in an ethanol series, critical point dried in CO2, coated with gold, viewed with a Zeiss DSM940a scanning electron microscope at 10 kV and photographed on Polaroid 55, 9 x 12 cm black and white film.
An architectural and developmental model was constructed, and the duration of preformation was determined from plants harvested and dissected at regular intervals during the growing season. To determine the duration of preformation, the number of primordia and mature leaves and flowers per plant were averaged separately for each sampling date. Then the number of primordia present at dormancy (initiated before the season of function and therefore preformed) was compared with the number that had emerged above ground during the same season (Aydelotte and Diggle, 1997
; Diggle, 1997
). The numbers of mature and developing organs and the distribution of vegetative and floral meristems were used to describe the architecture and growth of this species. All statistical tests used for data analyses were performed with Statview 4.5 (SAS Institute, Cary, North Carolina, USA).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Duration of preformation
Leaves and inflorescences begin to emerge above ground within 1–7 d after snowmelt in June. Emergence is rapid, and leaf maturation is completed within 2–3 wk. All of the A. rossii plants in the study area had ceased leaf expansion by 21 d after snowmelt. After the emergence and maturation of the current year's leaves and inflorescences, many leaf and inflorescence primordia remain below ground within the apical bud. Because emergence of leaves and inflorescences occurs only at the beginning of the growing season, it can be concluded that primordia below ground will not emerge and mature until subsequent years. Therefore, these structures are preformed.
The duration of preformation can be inferred by comparison of the number of leaves that typically function each year with the number of leaf primordia present within the apical bud over the course of a growing season. Six to nine leaves (mean ± 1 SD, 7.2 ± 1.6, N = 135) per individual mature by the end of June (Fig. 7). The average number of mature leaves per individual does not differ significantly among the three years measured; 1996–1998 (repeated measures ANOVA, F = 0.052, df = 2, P = 0.94). As leaves mature, 6–9 (8.20 ± 1.47, N = 15) preformed leaf primordia remain in the apical bud below ground. An additional 6–9 leaf primordia are initiated by the apical meristem during the growing season, resulting in a total of 12–18 (16.1 ± 2.0, N = 15) leaf primordia present in the apical bud when the soil freezes in November (Fig. 7). Thus, the average number of leaf primordia per plant is approximately two times the average number of mature leaves. Furthermore, although the number of leaves and leaf primordia vary among individuals, the number of leaf primordia present on each individual at the end of a growing season is consistently twice the number of leaves matured by that plant (paired t test, P < 0.428, df = 134).
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), plants bear a total of three cohorts of leaf primordia and mature leaves during each growing season (Fig. 8). At the beginning of any growing season, an individual bears two cohorts of leaf primordia in the apical bud. Immediately after snowmelt in June one cohort of preformed leaves expands above ground and matures, while the second cohort remains below ground where it continues to develop (cohorts A and B in Fig. 8; Figs. 9 and 10, respectively). As the growing season advances, an additional cohort of leaf primordia is initiated (cohort C in Fig. 8, Fig. 11) resulting in two cohorts of primordia present in the apical bud at dormancy (cohorts B and C in Fig. 8). Thus, over the course of one growing season, a plant bears three cohorts of leaves, each requiring 3 yr to pass from initiation through functional maturity to senescence (Fig. 8).
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Leaf growth and morphogenesis
Throughout each growing season, there is a continual increase in size and in morphological complexity of the leaf primordia. Figure 12 shows the average increase in total length of preformed leaf primordia developing below ground in apical buds over the course of one growing season (numbering from the most basal, LP1 to the most distal LP13). These leaf primordia comprise two cohorts. Because there is a continuum of sizes and developmental stages among the primordia, however, the assignment of a primordium to a particular cohort cannot be absolute. Nevertheless, the cohort to which a primordium belongs (and thus the age of that primordium) can be estimated by its position. There are, on average, seven or eight leaves per cohort. Assuming a cohort size of seven, then leaf primordia 1–7 of Fig. 12 can be assigned to the older cohort. They are in year 2 of development and will mature and function in the following year (see cohort B in Figs. 8, 12). Leaf primordia 8–13 of Fig. 12 comprise the younger cohort (leaf primordium 14 is too small to appear in Fig. 12). They were initiated in the current year and will remain below ground continuing to develop during the following year with maturation and function in the third year (see cohort C in Figs. 8, 12).
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The number of leaf primordia present in the apical bud increases throughout the traditionally recognized growing season from June through August and beyond (Fig. 7). There are (N = 15) 8.20 ± 1.47 (mean ± 1 SD) leaf primordia present in June, 11.40 ± 1.35 (N = 15) in July, 12.93 ± 2.43 (N = 15) in August, 14.20 ± 1.86 (N = 15) in September, and 16.1 ± 2.0 (N = 15) in October. Therefore, leaf primordia are not only initiated throughout the growing season but initiation extends well into the period of time between the senescence of aerial organs in August (75 d after snowmelt) and freezing of the soil in October (150 d after snowmelt).
The pinnately compound shape of leaves is established in the first year of development for all but the youngest four leaf primordia of a cohort and early in the second year of development for those youngest leaves. Following initiation of a leaf primordium (Fig. 13), the leaf base expands to partially encircle the younger leaf primordia and the apical meristem (e.g., LP14–15, Fig. 14). The upper leaf zone of the primordium elongates and marginal growth in this region forms the lamina (e.g., LP12, Fig. 15). Growth of the lamina is unequal, resulting in the bi-directional formation of leaflets (Fig. 16). Differential growth of leaflet primordium margins subsequently results in leaflets with three prominent lobes (Fig. 17). As the leaflets are forming lobes, the leaf base elongates and continues to expand laterally, ultimately encircling the younger leaf primordia (Figs. 10, 11, 17).
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In the second year of development, all leaf primordia in a cohort increase in size (Fig. 12). The four youngest leaf primordia initiate characteristic three-lobed leaflets at this time. By 45 d after snowmelt, all second-year leaf primordia are once pinnate with three-lobed pinnae. Late in August of the second year, the two oldest leaf primordia often become green while still below ground. Due to loss of pigment during the fixation process specific data on the frequency of this phenomenon are not available. In the third year of development, leaves expand above ground and mature.
Axillary shoot growth and morphogenesis
Axillary shoot primordia (inflorescences and vegetative branches) are initiated in the same year and belong to the same cohort as the leaves that subtend them. These shoot primordia first become visible in the axil of the fourth or fifth youngest leaf primordium on the main axis (Fig. 15). In the first year of development, axillary meristems initiate 0–3 leaf primordia; it is not apparent at this stage whether these axillary shoot primordia will develop as vegetative branches or as inflorescences.
During the second year of development, axillary shoot primordia become distinctly floral or vegetative. The timing of this commitment is highly variable and appears to depend on position (age) within the cohort. Inflorescence primordia are recognizable when the first internode elongates (Fig. 18). By the end of the second year inflorescences bear 1–3 flower primordia with clearly distinguishable sepals, petals, stamens, and carpels (Figs. 19–21). In the third year nearly all inflorescences either mature or abort. Very rarely (<0.4% of observed cases) an inflorescence will continue to develop in the terminal bud during the third year, and mature and function in the fourth year.
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The likelihood of a particular axillary meristem developing into an inflorescence or a vegetative branch bud is variable among positions within a cohort. Overall, 40–75% of axillary meristems differentiate into inflorescence primordia and the remaining 25–60% are vegetative. Inflorescences are more common in the axils of leaves in the central region of a cohort, whereas vegetative axes are more common in the axils of the proximal and distalmost leaves of each cohort (Fig. 22A).
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28% of plants, while shoots in the central portion of a cohort (axils of LP3, 4, 5, and 6) are aborted in 83% of the plants examined (Fig. 22B). Because the regions with highest abortion rates are those most likely to bear inflorescences, the rate of inflorescence abortion is greater than that for vegetative shoots. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Because it is such a dominant component of the Rocky Mountain tundra, understanding the development of A. rossii provides insight into the developmental and phenological dynamics of a large portion of the Southern Rockies tundra. Acomastylis rossii displays an extended period of leaf and inflorescence development. Each of these structures develops in the apical bud below ground for 2 yr and only functions above ground in the third and final year. The belowground development of shoots begins at or prior to snowmelt and continues for 2.5 mo after senescence of aerial organs. Thus, the developmental season of A. rossii is significantly longer than the "growing season" of aboveground structures. In addition to preformed apical buds, plants produce abundant axillary meristems that are capable of developing as either inflorescences or vegetative buds. The presence of these preformed axillary buds confers multiple options for developmental flexibility. Buds may either continue development or abort at any time after initiation. Developmental flexibility is further enhanced by the ability of vegetative buds to become dormant and persist on the rhizome for many years. The establishment of a large bud bank may provide a mechanism for response to either favorable environmental change or damage to mature rosettes.
Extent and duration of preformation
Each mature leaf and inflorescence of A. rossii is the culmination of a 3-yr period of development. After initiation, these organs develop below ground as primordia for two growing seasons and finally mature and function in the third year. Because each structure develops over such a lengthy period of time, individual plants may simultaneously bear up to three cohorts of aerial organs in various stages of development. Unlike leaves, which do not abort prior to the year of maturation, mortality of inflorescences prior to maturation is high and only 38 ± 20% complete the full 3 yr of development.
The 3-yr duration of leaf and inflorescence development documented for A. rossii is intermediate among alpine tundra species for which data are available. Polygonum viviparum (Polygonaceae) has leaves and inflorescences with a 4-yr period of development, and leaves of Caltha leptosepala (Ranunculaceae) require 2–3 yr to develop from initiation to maturity (Aydelotte and Diggle, 1997
; Diggle, 1997
). These observations support a hypothesis of ubiquity of developmental preformation among tundra species but also indicate that the duration of preformation is not uniform. Acomastylis rossii can commonly be found growing in the same communities as other species with both longer and shorter periods of preformation.
Whereas leaves of A. rossii appear invariably to complete development in 3 yr, the duration of development and ultimate fate of axillary structures is much more variable. A single meristem is initiated in the axil of each leaf primordium in the first year of development. Axillary meristems can develop into inflorescences, vegetative branches, dormant vegetative buds, or they can abort. The ultimate fate of an axillary meristem can be viewed as the result of a series of developmental decisions or options available over the multiyear period of ontogeny (Fig. 23). The identity of an axillary meristem as vegetative or floral does not appear upon visual inspection to be fixed until the second year of development. Meristems that become committed as inflorescences (year 2 in Fig. 23) can continue development to maturation in year 3 or abort; nearly two-thirds of all inflorescences abort during this period. Inflorescences do not remain viable after the third year. In contrast, meristems that become committed as vegetative buds may remain viable for decades and retain multiple developmental options. Vegetative buds in the third year may grow as lateral branches bearing mature leaves, remain dormant, or abort (year 3 in Fig. 23). Each year dormant buds remaining from previous years have the option of growth, abortion, or continued dormancy for additional years (years 4–n in Fig. 23). Such multiple developmental options may be an important avenue for phenotypic plasticity in the variable alpine environment (discussed further below).
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; Shaver and Kummerow, 1992
). Simultaneous maturation of organs and development of organ primordia of different ages has also been reported in Polygonum viviparum (Diggle, 1997
) and Caltha leptosepala (Aydelotte and Diggle, 1997
), revealing that a pattern of simultaneous photosynthetic acquisition and growth in the apical bud is a common feature of alpine tundra forbs.
Our observations show that A. rossii uses a larger portion of the year for the growth and development of primordia than has been appreciated. Previous studies of A. rossii have recognized the end of the growing season as the time when aboveground foliage senesces in August or September (Mooney and Billings, 1960; Holway and Ward, 1965
; Spomer and Salisbury, 1968
). Although senescence terminates the photosynthetic season, it clearly does not signify the end of growth and development. In A. rossii, leaf primordium initiation, morphogenesis, and expansion occur continuously from before the time of snowmelt until well into October; this was 65 d after the senescence of mature leaves in 1996. Approximately 40% of the yearly total of leaf initiation (Fig. 7) and up to 21% of leaf primordium expansion (Fig. 12) occur after the senescence of mature leaves. Thus, this late portion of the growing season is likely quite important to development in A. rossii. These observations of belowground developmental processes expand the recognized alpine growing season from 3 mo to 5 mo. Moreover, records of soil temperatures collected near our study site show that underground portions of plants are regularly exposed to conditions above freezing, and therefore potentially suitable for active growth, well into November (Losleben, 1997
). Thus the occurrence of significant development below ground after senescence of aerial plant parts may be widespread.
Consequences of preformation in apical buds
Acomastylis rossii experiences large variation from year to year in growing season length, depth of snowpack, date of release from snowpack, soil moisture during the growing season, and growing season temperature (Walker et al., 1993
). Preformation may constrain the morphological responses of A. rossii to such environmental variation. For example, if leaf cohort membership is not flexible, i.e., the timing of leaf maturation is determined absolutely by the time of initiation, then the only window of opportunity for increase or decrease in the number of leaves per shoot is during initiation. Because aerial organs develop over a period of 3 yr changes in initiation will not affect mature, functioning leaf number until 2 yr after the stimulus that elicited the change.
Leaf primordia of A. rossii do not appear to change cohort membership. That is, all leaves have a fixed 3-yr period of development. This conclusion is based on several observations. First, mature leaf number is extremely stable from year to year. Plants did not show variation in the number of leaves matured per individual from 1996 to 1998. Second, the number of primordia borne by each individual is reliably twice the number of mature leaves. Finally, when plants lost leaves to herbivores they were never observed to mature additional leaves during that same year (C. G. Meloche, personal observation). The consistency of relationship between leaf number and leaf primordium number and the lack of response to herbivory by individual rosettes suggest that each leaf primordium becomes part of a cohort when it is initiated and invariably matures with that cohort 2 yr later. Preformation results in an apparent inability to shift developmental timing in the apical buds of A. rossii. Delays in response to environmental change have been recognized as a general feature of tundra plants in both arctic and alpine environments (Shaver, Chapin, and Gartner, 1986
; Parsons et al., 1994
; Walker, Ingersoll, and Webber, 1995
). Preformation combined with inflexible developmental rates is a likely explanation for such delays.
Developmental options of axillary buds
The presence of developing and dormant axillary buds may confer the potential for more developmental flexibility despite the constraints imposed by preformation. Plants can effect response to changing conditions by altering the development of these meristems at several critical stages. For example, the identity of axillary meristems as inflorescences or as vegetative buds is not determined until their second year of development. Thus, the number of buds committed to sexual reproduction vs. vegetative spread may remain flexible until this time (Fig. 23). Large variation in the relative proportions of inflorescences and vegetative branches occurs among individuals of A. rossii at the study site (C. G. Meloche, personal observation) suggesting some plasticity in the commitment of axillary meristems to these alternative fates.
Once the identity of an axillary shoot has been determined, substantial opportunity for variation in response remains. Inflorescences may either mature or abort, and vegetative buds may either mature, abort, or enter a dormant state (Fig. 23). When an axillary meristem becomes an inflorescence primordium, the plant must either commit resources to sexual reproduction within the next 2 yr or abort the inflorescence. Adjustment of allocation by the abortion of flowers is widespread (Lloyd, 1980
; Stephenson, 1981) and accounts for a substantial degree of plasticity in a diverse array of plants. Inflorescence abortion has been reported in other preforming tundra plants (Clarke, 1968; Aydelotte and Diggle, 1997
; Diggle, 1997
) and in A. rossii >76% of the axillary meristems that develop inflorescence morphology abort by the end of the second year (N = 574). Inflorescence abortion, like flower abortion, likely provides options for the regulation of reproductive investment.
In contrast to inflorescences, developmental decisions regarding allocation to vegetative branches can occur over many years. The resulting large numbers of dormant vegetative buds, each containing several preformed leaf primordia, may provide additional mechanisms for response to variation in resource availability (Cook, 1983
; Hutchings and Mogie, 1990; Cain, 1994
). If resources become temporarily abundant due to disturbance or nutrient inputs, these buds, containing preformed leaves that have already undergone 2 yr of development, could mature in the subsequent spring. In comparison, variation in leaf production at the apex of the main shoot is predicted to require 2 yr. Thus, while preformation alone in apical buds will likely impose a substantial delay in the timing of morphological response to changes in resource availability, preformation coupled with late stage dormancy in axillary buds can enable more rapid developmental responses to the same conditions.
Sherrod (1999)
found that A. rossii was better able to respond to gopher disturbance than other plants occupying the same community. While other plant species were absent from new gopher mounds, A. rossii was capable of colonizing the disturbed soil both by growth up through the soil covering shallowly buried plants and by growth into bare areas from adjacent rhizomes. The complex architecture of A. rossii and the resulting developmental flexibility that it confers compared to unbranched preforming plants may explain, in part, the ubiquity and relative dominance of this plant in multiple community types in the tundra of the Southern Rockies.
Conclusions
Development of A. rossii is characterized by extreme preformation of all parts of the plant shoot. The 3-yr period required for leaf and inflorescence development can have a significant impact on the timing of responses to environmental change. Walker et al. (1994)
observed a 1-yr lag in the response of plant productivity to precipitation input in several communities at Niwot Ridge. Given the ubiquity of A. rossii in each of these communities, the presence of numerous dormant buds arrested in the second year of development may be a causal factor in the timing of observed delays. Observations of development in A. rossii, as well as P. viviparum, C. leptosepala, and other tundra species (Sørensen, 1941
; Mark, 1970
; Aydelotte and Diggle, 1997
; Diggle, 1997
), indicate that extended preformation of both vegetative and reproductive structures is a ubiquitous feature of tundra plants. With the mature forms of organs buffered by more than one season of development, the morphological responses to an environmental input in one year will not be fully manifest until those organs have matured in subsequent years. The presence of several developmental options for axillary meristems, including identity determination, dormancy, and abortion, may mitigate the lag in timing to a degree not possible in plants with a less complex morphology. By adjusting the development of existing axillary structures A. rossii could adjust biomass much more rapidly than the 3-yr period of preformation would suggest.
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FOOTNOTES |
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2 Author for correspondence (phone: 303-449-8159; e-mail: christopher.meloche{at}colorado.edu
).
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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