Stolon Internode Research Articles

Morphological Plasticity in Clonal Plants: The Foraging Concept Reconsidered

1 Studies of morphological plasticity in clonal plants have analysed responses to habitat quality in terms of spacer (stolon or rhizome internode) length and branching intensity. The capacity for these parameters to respond to environmental quality has been interpreted as an expression of foraging behaviour, as it confers the potential to intensify the placement of ramets in the more favourable microhabitats (patches) of a heterogeneous environment. This plasticity in clonal morphology is compared with similar responses of individual shoots and roots to habitat quality that have also been accepted as manifestations of foraging behaviour. 2 The most consistent morphological response shown by clonal species is a higher branching intensity an increased propensity of lateral meristems to grow out and form lateral rhizomes and stolons under conditions of higher resource supply. In contrast, spacer lengths show a variety of responses to light and nutrient availability, and many species exhibit no significant response. Even in stoloniferous species, where stolon internodes tend to shorten under higher photon flux densities, the degree of shortening may often be insufficient to elicit a significant concentration of ramets in favourable habitat patches. 3 Many clonal and nonclonal species have however, been shown to be very efficient in placing leaves and roots in areas of high resource supply within their environment. This is achieved by a high level of morphological plasticity of the shoot and root branches. 4 We therefore suggest that it is the highly plastic changes in the morphology of individual ramets that enable effective exploitation of local concentrations of essential resources once they have been located. The unresponsive spacer lengths of many clonal species may permit a continuous search of habitat space by the plant, rather than a selective placement of ramets. 5 The foraging concept is reformulated in more general terms relating to resourceacquisition strategies, so that it is applicable to both clonal and nonclonal species of plants.

Field evidence of plastic growth responses to habitat heterogeneity in the clonal herb Ranunculus repens

AbstractStolon internode lengths were measured on plants of the clonal herb Ranunculus repens growing in a hay meadow which was subject to disturbance by mole (Talpa europaea) activity. Within the site three habitat types were recognized: closed grassland, the open ground of fresh molehills and the grass‐molehill boundary. The lengths of stolon internodes of R. repens differed significantly in each of the three habitats. The shortest internodes occurred on stolons on the open molchills. The longest occurred in the closed grassland habitat. The type of habitat in which parent ramets were rooted did not significantly influence the length of internodes on their daughter stolons. The length of a stolon internode was determined by its immediate surrounding habitat type. Consecutive internode lengths on a given stolon showed considerable plasticity, shortening significantly as stolons spread onto molehills from surrounding habitats, and increasing significantly as stolons advanced from a molehill into the surrounding closed grassland habitat. These results are consistent with the proposition that under favorable conditions (on the molehills, where resources are expected to be more abundant, and competition absent) internode lengths shorten and the plant forages intensively, whereas under conditions of low resource availability (in the closed grassland, where competition occurs) internode lengths increase, allowing R. repens to forage extensively. Such morphological plasticity may promote more efficient exploitation of resource‐rich sites and more rapid vacation of resource‐poor sites.

Consequences of Foraging in Clonal Plant Species

Some clonal plant species decrease rhizome or stolon internode lengths and/or increase the frequency of branching when they grow in favorable environments. This foraging response is thought to be beneficial since it should allow ramets to concentrate in areas of favorable habitat. However, there have been few critical tests of the effectiveness with which ramets are placed in favorable habitat as a result of the foraging response. In this paper, I use empirically calibrated stochastic simulation and diffusion models to compare the growth of clones in favorable and unfavorable habitat. I ask whether observed changes in rhizome lengths and clonal branching patterns are likely to decrease significantly the distance clones move, and thus, to enable mets to remain for longer periods of time in favorable habitat. For the empirical data used in this study, results from the models indicate that the effectiveness of the foraging response is likely to be variable. In some cases, such as results from models based on the response of Glechoma hederacea to nutrients, there is no significant difference in the distance clones move in favorable and unfavorable habitat. Thus, even through rhizome lengths may be significantly shorter in favorable patches, this does not guarantee that clones disperse significantly less far and thereby remain longer in favorable habitat. The effectiveness of the foraging response depends strongly on the distribution of clonal growth angles, the pattern of clonal branching, and the variance in rhizome or stolon internode length. These results, particularly the importance of growth angles and the relatively limited effectiveness of the foraging response, differ from those in previously published models of foraging in clonal plant species. I conclude with a discussion of reasons that may underlie these differences.

Some effects of added phosphorus on perennial ryegrass—white clover swards

AbstractThe effect of applying 100kg P ha‐1 per year in the form of triple superphosphate to mixed swards of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) was examined on a soil of low P status.The dry matter yield of total herbage was increased by 10′/i by added P. In the year of sowing the ryegrass benefited more than the clover from added P: in the subsequent four years the two species benefited equally. Both species responded 10 added P to a similar extent in terms of leaf size; the clover responded less well than the ryegrass in terms of rate of leaf emergence. However, clover responded positively to added P in terms of stolon internode length, length of stolon per m2 and number of growing points per m2. It is suggested that the application of P may promote the spread of white clover within an open sward, but that its application may not enhance the competitive power of white clover when growing with vigorous grasses.

Effects of cutting height on the growth of leaves and stolons in perennial ryegrass–white clover swards

SUMMARYAll combinations of five cutting heights (2, 4, 6, 8 and 10 cm) were compared, for 3 years (1986–88), on field plots sown with a mixture of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) in Wales. The plots were cut at 4-week intervals from April to October each year.A reduction in cutting height reduced the length, width and weight of clover leaflets, petiole length, stolon diameter, the length of stolon internodes and the height of stolons above ground level, but increased the length of clover stolon/m2, the number of stolon nodes/m2 and the proportion of stolon nodes which produced branches. The latter factors out-weighed the former, leading to the inverse relationship between cutting height and the proportion of clover in the sward previously reported (Acuñia & Wilman 1993).A reduction in cutting height reduced the following attributes of ryegrass: the height of the shoot apex, the length and width of leaf blades, sheath length, leaf extension rate, the death rate of leaves and the net gain in blade length.Cutting height had little effect on either the thickness of clover leaflets or the rates of leaf emergence in clover and ryegrass.

Morphological plasticity of the clonal herb Lamiastrum galeobdolon (L.) Ehrend. & Polatschek in response to partial shading.

The present experiment investigates the morphological plasticity of Lamiastrum galeobdolon (L.) Ehrend. & Polatschek in response to light patchiness. In a garden experiment, stolons of the plant were grown under the following six light regimes: high light, low light, from low light to high light, from high light to low light, under high light but with the apical region of the stolon under low light, and under low light but with the apical region under high light. The low-light conditions provided 50% of high light PAR and 28% of high light R/FR ratio (ratio of red to far red light), and simulated the canopy-like shade that may be relevant to this species in its natural habitat. Plants growing under different light treatments did not show significant differences in total plant dry weight, total number of ramets, number of primary ramets, number of secondary ramets, and number of branches per clone. L. galeobdolon plants growing under high-light conditions formed shorter internodes, shorter petioles and smaller laminas with lower SLA, and showed higher branching intensity than those growing under low-light conditions. Accordingly, when growing between patches of different light conditions, the primary ramets had shorter internodes, shorter petioles and smaller laminas with lower SLA in high-light than in low-light patches. Exposing the apical region, of an otherwise shaded stolon, to high light shortened its internodes, compared with those of plants growing entirely under low-light conditions. Shading the apical region, of otherwise unshaded stolon, had no effect on the lengths of these internodes. In contrast, the morphology of the leaf responded to local light conditions only. Elongation of stolon internodes was completed much faster (within 2 or 3 wk) than lamina expansion and petiole extension, which took at least 6 wk. These results are interpreted in terms of the efficiency of exploitation of high-light patches within the habitat of L. galeobdolon.

The effects of contrasting cutting regimes on the components of clover and grass growth in microswards

AbstractMicroswards of white clover and perennial ryegrass were subjected to one of four treatments: weekly cutting to 3·5 cm, weekly cutting incorporating a period of no cutting for 6 weeks starting 27 April (early rest), weekly cutting incorporating a period of no cutting for 6 weeks starting 8 June (late rest), or cutting every 3 weeks. Two sward types were used: a mixture of white clover cv. Milkanova with perennial ryegrass cv. Melle, and white clover cv. Kent with perennial ryegrass cv. Melle. Growth measurements (leaf appearance, branching/tillering and stolon internode length) were confined to the first three treatments with records collected during contiguous 21‐d measurement periods. Vertical height increments of clover and grass and red:far‐red light ratios at the sward bases were also recorded at frequent intervals. At the end of the experiment population densities and unit weights were recorded for all treatments.Significant treatment effects on the rate processes were largely confined to the 21‐d period immediately after weekly cutting of rested swards had resumed. On previously rested compared with weekly cut swards, clover leaf appearance rates were increased by 40% and branching rates by 164%. During the same period, grass leaf appearance rates were reduced by 50% and net tillering changed from positive to negative values. Though the rate responses were transient, effects were still apparent at harvest in September, when population density and content (proportion by population density and weight) of clover were significantly higher in the late rest treatment. The variety Kent showed a consistent, though usually nonsignificant, higher leaf appearance and branching rate compared with Milkanova, and in September was characterized by a higher population (7400 m−2 compared to 3200 m−2) of smaller units (27 compared to 46 mg/apical meristem) than Milkanova.The results are discussed in relation to defoliation effects and the role of light quantity and quality as they influence the component growth processes. Attention is drawn to the importance of canopy structure and the climatic and/or phenological differences in the relative seasonal behaviour of clover and grass, together with varietal variation within species in influencing responses to management manipulations.

Yield and persistency of contrasting white clover populations grown in pure swards and in mixed swards with S.23 perennial ryegrass

SummaryTen contrasting white clover populations were grown as spaced plants, in pure clover swards and in mixed swards with S.23 perennial ryegrass. Four of the populations were also tested for tolerance of low temperatures.In the establishment year, the autumn yields of populations were correlated with leaf size. However, during the severe winter which followed, populations with large leaves, particularly those of Mediterranean origin, suffered extensive stolon kill. This winter damage reduced the spring yields and total annual yields of populations with large leaves, so that leaf size and total annual yield were not correlated in the year after sowing.Stolon kill was positively correlated with autumn growth activity as measured by leaflet size and stolon internode length in October. Stolon kill during winter was correlated with cold tolerance of naturally‐hardened stolon apices, assessed in artificial cold tests.

Variation in growth of overwintered stolons of contrasting white clover populations in response to temperature, photoperiod and spring environment

SUMMARYThe response of overwintered stolons of nine contrasting white clover populations to temperature, photoperiod and natural conditions was studied in six environments during the spring. Rate of leaf appearance, leaflet length, petiole length, stolon internode length, dry matter distribution within the plant and total dry weight were measured on 15 plants of each population/environment combination.Most characters, except leaf size and proportion of dry matter allocated to leaf, responded positively to temperature in the range 10 – 20°C. A positive effect of photoperiod extension was also found for all characters except rate of leaf appearance, internode length and distribution of dry matter to leaf.Population differences in response to environment were found which were related to both leaf size classification and origin.Stolon dry weight was positively correlated with leaf length, petiole length and stolon internode length in most environments. The relationships between the eight characters were often complex and canonical variate analysis provided a convenient way to discriminate between the populations based on all eight characters.

The Effects of Light Intensity on Foraging in the Clonal Herb Glechoma Hederacea

(1) The growth form of 12-week old clones of Glechoma hederacea grown either under neutral shading (reducing light intensity by 750%), or in full daylight, was compared. All clones were genetically identical. (2) Clones grown under shading displayed a growth form characterised by a small number of slender, unbranched stolons with long internodes, and only a few small ramets. Dry weight per unit length of stolon was low. Proportional allocation of dry weight to roots was low, whereas allocation to petioles was high. Specific leaf area was nearly three times higher in the shaded clones than in the unshaded clones. Mean ramet leaf area was the same for clones in both treatments. (3) The growth form of unshaded clones was characterised by short stolon internodes, frequent stolon branching and many large ramets. The dry weight per unit length of stolon was four times greater than in the shaded clones. Proportional allocation of dry weight to roots was high, whereas allocation to petioles was lower than in the shaded clones. (4) The results of this experiment are compared with published data analysing the response of G. hederacea to different levels of nutrient availability. G. hederacea displays qualitatively similar alterations in growth form in conditions of low light and low nutrient availability. Plasticity in growth form of G. hederacea enables clones to consolidate occupation of favourable sites, through intensive foraging, and to grow through less favourable sites, concentrating resources in extensive foraging which may result in establishment of ramets in more rewarding sites.

Transformation of the forage legume Trifolium repens L. using binary Agrobacterium vectors

A system was established for introducing cloned genes into white clover (Trifolium repens L.). A high regeneration white clover genotype was transformed with binary Agrobacterium vectors containing a chimaeric gene which confers kanamycin resistance. Transformed kanamycin resistant callus was obtained by culturing Agrobacterium inoculated stolon internode segments on selective medium. The kanamycin resistance phenotype was stable in cells and in regenerated shoots. Transformation was confirmed by the expression of an unselected gene, nopaline synthase in selected cells and transgenic shoots and by the detection of neomycin phosphotransferase II enzymatic activity in kanamycin resistant cells. Integration of vector DNA sequences into plant DNA was demonstrated by Southern blot hybridisation.

The Intrusion of Clonal Growth Patterns on Plant Breeding Systems

As clonal plants get larger, individual flowers become more surrounded by other flowers borne on the same clone. With very restricted movement of pollen reported for both animal- and wind-pollinated species, the breeding system may change as a consequence of a clonal plant covering greater amounts of space. In Carex platyphylla, a wind-pollinated, self-compatible sedge, the absolute and relative amount of endogenous pollen per flowering culm increases sharply as a plant grows from one to about 10 culms, then increases at a slower rate, given a constant rain of exogenous pollen. In Trifolium repens, a bee-pollinated, self-incompatible, stoloniferous clover, a model based on known patterns of flower placement and stolon growth patterns, coupled with estimates of pollen-deposition and floret-visitation patterns by bees, suggests that the number and length of stolon internodes affect successful pollination. Also, the distribution of seeds among inflorescences is affected by clonal growth patterns and bee beha...

Effects of Temperature and Photoperiod on Stolon Characteristics, Dry Matter Partitioning, and Nonstructural Carbohydrate Concentration of Two White Clover Ecotypes1

Carbohydrate reserves are necessary for survival of perennial plants, but excess accumulation at the expense of harvestable new growth may interfere with the agronomic goal of maximizing herbage yields. The objective of this research was to examine the response of reserve carbohydrate storage and dry matter partitioning in white clover (Trifolium repens L.) to low temperature and short photoperiod. Spaced plants of two altitudinal ecotypes were grown in growth chambers at day/night temperatures and photoperiods of 18°/13°C, 16 h; 10°/7°C, 16 h; and 10°/7°C, 12 h. Plants grown under the lower temperature regime showed an increase in stolon diameter (+33% on average) and thus established a greater stolon volume (+53%) unit leaf area. Shortening the photoperiod resulted in a marked decrease in length of stolon internodes (−54%) and leaf size (−29%), but favored stolon branching (+ 23%). Low temperature depressed net assimilation rate (NAR) to a lesser extent (−38%) than rate of leaf appearance (−52%). Increased photosynthate availability under low temperature was expressed in higher total nonstructural carbohydrate (TNC) content of the leaves (+ 63%), and enabled a greater accumulation of TNC in stolons and roots (+48%). A shorter photoperiod caused a marked decrease TNC content of the leaves (−78%), but only slightly depressed reserve accumulation in stolons and roots (−14%). The alpine ecotype allocated a higher proportion of dry matter to the stolons (+ 35%) than did the valley type, which showed higher root weight proportions (+23%). It is concluded that ecotype and environment influence photosynthate partitioning by affecting plant morphology, by changing the ratio of photosynthate supply to demand, and by controlling carbohydrate use. Low temperature, short photoperiod, and alpine origin favor carbohydrate reserve storage, and thus improve prerequisites for winter survival in white clover.

Anatomy and development of the superficial layers in stolons of white clover (Trifolium repensL.)

Abstract Scanning and transrrussion electron microscopy, together with bright field and fluorescence microscopy, were used to examine the cuticle, epidermis, hypodermis, and cork cambium (phellogen) of white clover (Trifolium repens L.) stolons during growth and maturation. Development of the hypodermis included the deposition of a suberin lamella in the cell walls. No suberin was deposited in the cell walls of the epidermis. The cork cambium was formed in the third to fifth internodes basipetal to the stolon apex, and produced layers of suberised cork cells (phellem) when internodes were 11 to 14 basipetal to the apex. Breakdown of the cuticle began at a very early stage. Bacteria were seen to colonise the cuticle and apparently lyse the cuticle and epidermal cell walls and penetrate eventually into the hypodermis. The suberin lamella appeared to be more resistant to bacterial attack than the cellulose portion of the cell wall. It was concluded that variation in the degree of breakdown of the cuticle was probably responsible for the large variation in phosphate absorption of stolon internodes previously observed.

Population Dynamics and Local Specialization in a Clonal Perennial (Ranunculus Repens): II. The Dynamics of Leaves, and a Reciprocal Transplant-Replant Experiment

SUMMARY (1) Leaf demography was used as a non-destructive measure of the growth of Ranunculus repens L. in two adjacent but contrasting habitats (woodland and grassland) in North Wales. In addition, a series of destructive harvests allowed testing of the assumption that leaves could be treated as comparable modules. (2) Leaf demography was used to compare the growth of 120 plants reciprocally transplanted and replanted at the two sites. (3) At both sites, the leaf module was plastic in weight, area and unit leaf weight over the growing season. (4) Ramets replanted at their native site grew more vigorously than did their undisturbed neighbours. Ramets transplanted to an alien site did not maintain 'ecotypic' leaf birth rate, leaf death rate or stolon internode length. (5) The birth rate of leaves was always highest for plants replanted at their native site, but leaf death rates were more variable. (6) Ramets from the grassland population grew almost as well at the alien site as at their native site, but ramets from the woodland site grew relatively poorly when transplanted to the grassland site. One interpretation is that plants from the grassland population may be descended from members of the woodland population which covered both sites 12 years previously.

Growth Substances and Growth of Strawberry Stolons and Leaves1

Abstract A foliar spray of 6-(benzylamino)-9-(2-tetrahydropyranyl)-9H-purine (PBA) induced thickening and restricted elongation of strawberry (Fragaria × ananassa Duch.) petioles and stolon internodes. Leaf area of treated plants was lower than that of untreated plants. Stolon internodes of treated plants increased in cell size and number. Gibberellic acid (GA3) promoted leaf expansion and elongation of petioles and stolon internodes. A combination of PBA and GA3 induced less thickening and more elongation of petioles and stolon internodes and greater leaf area compared to treatment with PBA alone. There was a breakdown of cell walls of pith parenchyma in stolon internodes of untreated plants while the cell walls of PBA and GA3 treated plants remained intact.