Dormancy and foliar density regulation in Thalassia testudinum

Aquatic Botany 68 (2000) 281–295

Dormancy and foliar density regulation in Thalassia testudinum Brigitta I. van Tussenbroek a,∗ , Carlos A. Galindo b , Judith Marquez b a

b

Unidad Académica Puerto Morelos, Instituto de Ciencias del Mar y Limnolog´ıa, Universidad Nacional Autónoma de México, Apdo. Postal 1152, Cancún, 77500 Quintana Roo, Mexico Facultad de Ciencias, Universidad Nacional Autónoma de México, Dpto. Citolog´ıa, Circuito Exterior, Ciudad Universitaria, Del. Coyoacán, México, DF 04510, Mexico Received 2 November 1999; received in revised form 20 April 2000; accepted 29 August 2000

Abstract The hypothesis that the tropical seagrass, Thalassia testudinum from the Puerto Morelos reef lagoon (Mexican Caribbean) has a dormant apical meristem bank (as opposed to a dormant axillary meristem bank) was investigated by morphological and anatomical analyses of vertical shoot apices. Shoots were grouped as follows: (1) dead shoots (no foliar structures or active meristems), (2) developed shoots (bearing juvenile or mature foliar structures with active meristems), and (3) undeveloped shoots (bearing rudimentary foliar structures but with active meristems). In two beds, regressions of shoot density vs. proportional number of shoots in each of the above-mentioned categories, had negative slopes for developed shoots and positive slopes for undeveloped shoots, whereas no relationship was found between density and proportion of dead shoots. These data suggested that foliar shoot density was regulated by inhibition of foliar development, through suppression of meristem activity. Foliar shoot density increased significantly after experimental nutrient addition and density of the developed shoots was 544.3 shoots per meter square for control plots and 1044.1 shoots per meter square for fertilised plots. Density of undeveloped decreased after fertilisation and densities were 623.9 and 194.2 shoots per meter square for control and fertilised plots, respectively, indicating that nutrient addition resulted in meristem re-activation in the “undeveloped” group. These results confirm the previous proposed existence of a dormant apical meristem banks for T. testudinum. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Seagrass; Apical meristem; Dormancy; Density regulation; Clonal plants

∗ Corresponding author. Tel.: +52-987-10219; fax: +52-987-10138. E-mail address: [email protected] (B.I. van Tussenbroek).

0304-3770/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 7 7 0 ( 0 0 ) 0 0 1 3 0 - 3

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1. Introduction Clonal growth plays a major role in seagrass reproduction. Clonal growth in seagrasses proceeds by underground expansion of a horizontal rhizome, from which leaf-bearing vertical rhizomes protrude above the substratum. Although seagrasses have been extensively studied in relation to their environmental requirements for growth and productivity (McRoy and Helfferich, 1977; Larkum et al., 1989), little is known about the advantages and constraints associated with their clonal growth. Recently, Marbà and Duarte (1998), in a compilation of rhizome morphometry and branching patterns for 27 seagrass species, reported high interspecific variability in clonal growth patterns. In clonal plants, growth patterns such as distances between modules, branching patterns, and presence or absence of dormant modules, determine the way the plants exploit available resources and consequently their survival strategies (Lovett-Doust, 1981; De Kroon and Schieving, 1990). Morphological plasticity in growth patterns might indicate selective foraging of the plants (Slade and Hutchings, 1987). Whether this plasticity is an active mechanism of the plant to optimize foraging intensity (plant foraging theory), or a passive growth response is controversial (e.g., Oborny, 1990; De Kroon and Knops, 1990). According to Tomlinson (1974), seagrasses show a high degree of meristem dependence. That is, the “need for continually active shoot apical meristems to maintain populations”. Proliferation patterns in seagrasses can either be little organised (i.e. without predetermined differentiation between horizontal rhizome and vertical rhizome, and irregular branching pattern) or well programmed, with clear differentiation between the horizontal and vertical axis and a precise branching pattern. The findings of Marbà and Duarte (1998) of high interspecific variability in seagrass growth patterns, support the wide range of proliferation patterns described by Tomlinson (1974). Clonal growth of Thalassia testudinum, according to Tomlinson (1974) is extremely regular with absolute meristem dependence as this species has no axillary meristem bank. Additionally, horizontal rhizomes of T. testudinum show little morphological plasticity indicated by low intraspecific variability in internode length (Marbà and Duarte, 1998). The deterministic branching pattern and the relatively low morphological plasticity of its rhizomes, apparently does not allow T. testudinum to alter its architecture in response to environmental conditions. This species forms dense stands in many (sub-) tropical Atlantic shallow coastal areas, within which intra-species contacts and thus most likely intra-specific competition for resources are high. Increased crowding in even-aged monospecific stands of non-clonal plants results in the death of the smallest plants in a density dependent fashion (self-thinning) (Yoda et al., 1963; Weller, 1987). The ramets of many terrestrial clonal plants do not show such a decrease in biomass with increasing density (Hutchings, 1979; Pitelka, 1984), but there are exceptions (De Kroon and Kalliola, 1995). Clonal plants are thought to regulate ramet density by (1) plasticity in spacing of the ramets by flexible responses in rhizome length and branching pattern, or (2) inhibition of ramet development, i.e., formation of dormant buds (e.g., De Kroon and Schieving, 1990; Harper, 1985). The relatively low morphological plasticity, and the ontogeny of T. testudinum as described by Tomlinson (1974), exclude these mechanisms of density regulation. Van Tussenbroek (1996) observed that T. testudinum from the Puerto Morelos Lagoon, Mexican Caribbean, had healthy shoots that did not produce leaves and hypothesised that

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these shoots were dormant and collectively formed a dormant meristem bank analogous to the dormant bud banks proposed by Noble et al. (1979) for Carex arenaria. Here, we investigate the dormancy hypothesis in “undeveloped” shoots in T. testudinum and test whether these shoots play a role in the regulation of foliar density of this seagrass species. 2. Materials and methods 2.1. Shoot morphology and histology Shoots of T. testudinum were collected from the coastal fringe of the seagrass beds in front of the Research Station, Puerto Morelos lagoon, Mexican Caribbean. The shoots were divided into seven categories, based on detailed observations of morphological characteristics of their apices and foliar structures. The vertical rhizomes of all categories of shoots but one, had rounded tips (apices) and the anatomy of these apices was studied to evaluate whether they had functional meristems. Cores (20-cm diameter, 30–35-cm depth) were sampled, washed and the shoots were classified according to pre-established categories and preserved in FAA solution. The shoots were mounted in “Paraplast”, and cut in a longitudinal plane with a microtome (thickness 8 ␮m). Fuelgen tincture, which specifically stains chromosomes, was applied to distinguish the large nuclei (in comparison to the volume of the cytoplasm) typical for meristematic cells (Esau, 1953). In total, 10 shoots per category (except damaged shoots) were prepared for anatomical analysis. The terminology of the present study follows that of Tomlinson and Vargo (1966) and Tomlinson (1972). The horizontally creeping rhizome forms lateral vertical rhizomes at regular intervals by monopodial branching of its apex. Foliar tissue is formed by the meristem situated at the apex of the vertical rhizome, and a developed leaf consists of a colourless sheath and a green blade. A shoot is considered to be the conjunction of a vertical rhizome and its corresponding foliar structures. 2.2. Shoot density Twenty-five cores (20-cm diameter, 30–35-cm depth) were sampled per site at two sites (Village Station at 20◦ 510 1500 N, 86◦ 520 1300 W, and Research Station at 20◦ 520 0400 N, 86◦ 510 5400 W) in the Puerto Morelos reef lagoon. Both sites were situated at ≈250 m from the shoreline, and had a similar depth (≈2.5 m). The Research Station site, in front of the research station of the Universidad Nacional Autónoma de México, is an undisturbed site, with vegetation dominated by T. testudinum, mixed with Syringodium filiforme and rhizophytic algae, anchored in coarse calcareous sediment. The bed at the Village station, in front of the village of Puerto Morelos, has gradually changed during the last two years from an undisturbed state similar to the Research Station site, to a bed almost completely dominated by T. testudinum covered with a fine layer of silt. Core samples were taken during the month of April 1998, which is when T. testudinum shoots attain their highest seasonal leaf biomass (Van Tussenbroek, 1995). In the laboratory, T. testudinum plants were carefully removed from the sediment and a count was made of shoots in each of the above-mentioned categories. Per sample, green blades and white sheaths were separated and dried at 60◦ C for 24 h. Length and width of all green blades and

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sheaths were measured, and surface-areas calculated. Per sample, specific weight (g cm−2 surface) of the blades (sheaths) was calculated as the division of dry weight of all blades (sheaths) and the sum of the surface area of these blades (sheaths) from that sample. The foliar weight per shoot was calculated by multiplication of the total surface area of blade and sheath tissue of that shoot with their, respective, specific weights. Total number of shoots (i.e., the sum of the number of shoots belonging to each category) was used as a parameter of density. Mean foliar dry weight per shoot and the proportion of shoots belonging to the different categories were plotted against this density parameter, and linear regressions were SPSS package for Windows (Version 8.0). Proportional data were arcsine transformed to approach normality. 2.3. Resource exploitation (fertilisation) Seagrasses were fertilised in situ in 1 m2 plots, situated ≈150 m from the coastline south from the Research Station (20◦ 510 1500 N, 86◦ 520 1300 W). Commercially available slow release fertiliser tablets were used (25% total nitrogen, 12% P2 O5 , 7% K2 O, 16% carbon, 0.8% calcium, 0.7% magnesium, 1000 ppm iron, 1000 ppm zinc, 1200 ppm sulphur, and 60 ppm phytohormones). These amounts were in the same order of magnitude to those proven effective in stimulating seagrass development in similar regions (Agawin et al., 1996). The tablets were dissolved in seawater, and placed into 50 ml Millipore syringes with an attached sealed tube. A series of small holes at the end of the tube allowed injection of the fertiliser at ≈9–12 cm sediment depth, where rhizomes and roots were most abundant. In the field, the fertiliser was injected along a grid with 144 points per plot, and the nutrient treatment received ≈50 g N m−2 and ≈24 g P2 O5 m−2 . The controls did not receive any nutrients but the syringes were inserted into the sediment as was done for the nutrient treatments. There were two replicates per treatment and the treatments were assigned to the plots using a random number table. Fertiliser was applied in January and seagrasses were harvested in May 1998. At the termination of the experiment, seagrasses were harvested by haphazardly taking 5 cores (diameter 20 cm, depth 30–35 cm) per plot. In the laboratory, T. testudinum plants were carefully removed from the sediment core samples, and blade tissues were separated and their dry weights determined. Of all green blades per shoot, length and width were measured, and their surface areas calculated. For statistical procedures involving blade area, only developed foliar shoots (>0.3 cm wide) were taken into account. The active horizontal rhizome apices (horizontal rhizome apices with a juvenile shoot) were counted. A record was made of each shoot, listing its category, the number of leaf scars, and the number of foliar blades (dead shoot were counted only). Differences in the number of shoots per category and the horizontal rhizome apices between the control and nutrient treatments were tested with a t-test, using the plots as replicates. The samples were pooled when P < 0.25, and the t-test was repeated with the core samples as replicates. Pooling improved the power of the tests and allowed application of a test of homogeneity of variance (Levene’s Statistic). To approach homogeneity of variance, log (green blade area) and square root (counts of shoots and rhizome apices) transformations were applied when necessary. The G-test of independence was used to evaluate whether shoot size, expressed as number of scars plus blades, was independent of

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the category. This test was applied to shoots belonging to categories 3–6, for size-classes 20–50. 3. Results 3.1. Shoot external morphology Based on foliar development and morphology of the apices, shoots of T. testudinum were divided into three major groups, each of them having various subdivisions (categories). The first group (dead shoots) consisted of shoots without (rudimentary) foliar structures, and consisted of shoots belonging to category 0 (vertical rhizomes without an apex) and category 1, which had vertical rhizomes with domed apices (Fig. 1). The second group (developed shoots) consisted of shoots with juvenile or developed foliar leaves. Juvenile leaves were those in transition from prophyll to developed foliar leaves (Tomlinson and Vargo, 1966), and were distinct from immature developed foliar leaves. Characteristics of juvenile leaves were their small width, firm consistency, and absence of microscopic marginal teeth at the apex; the latter were typical for all developed foliar leaves independent of their stage of development (Tomlinson, 1972). Included in the developed shoots group were shoots of category 2 (juvenile shoots), 3 (foliar shoots) and 4 (shoots with old vertical rhizomes and juvenile leaves) (Fig. 1). The third group (undeveloped shoots) had shoots with a conical apex and rudimentary foliar structures. These shoots had either an immature foliar leaf between dead sheath tissue (category 5), or a white conical apex with densely packed prophylls (Fig. 1). 3.2. Meristem histology Histological analysis of the meristems showed that the apices of the shoots of category one have cellular structures similar to that of the meristems of the other shoots with apices. However, cytoplasmic content of the cells was absent and only the walls remained (Fig. 2). All other shoots had typical meristems in the form of a dome, protected by foliar primordia. The cells had large nuclei, and the exterior layer of the meristem formed the tunica, which through anticlinal divisions produced new cells (Fig. 2). Below the tunica, lied the corpus (Tomlinson, 1972). Categories 2 and 5 cannot be differentiated histologically: both have an undefined substance above the meristem, which is not found in other categories. Foliar primordia of the shoots belonging to categories 2–5 are aligned in almost vertical position with some space between them; whereas those of category 6 are more densely packed and fold over the meristem (Fig. 2). The latter arrangement of foliar primordia appears similar to that of the apices of rhizomes (Tomlinson, 1972; Tomlinson and Baily, 1972). 3.3. Density dependence At both the Research Station and Village sites, average foliar weight per shoot did not vary with shoot density (Table 1). However, when shoot density was high, a slight decrease (although not significant) in mean foliar weight was observed at the Research Station

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Fig. 1. Distinct morphological categories of shoots of T. testudinum from the Puerto Morelos reef lagoon. From all shoots, dead sheath tissue and roots were removed. Numbers are categories and the bars represent 1 cm. Category 1: shoots with a domed but dead apex (indicated by arrow), category 2: juvenile shoots with juvenile leaves (arrow a indicates juvenile leaves, and arrow b indicates horizontal rhizome apex), category 3: mature shoots with developed leaves (outer developed leaves are removed and the arrow indicates an immature leaf), category 4: mature shoots with juvenile leaves (indicated by arrow), category 5: undeveloped shoots with a small immature leaf (indicated by arrow) surrounded by death sheath tissue (removed), category 6: undeveloped shoots having a conical apex with prophylls (indicated by arrow).

(Table 1). Shoot density was higher at the Research station than at the Village site (Fig. 3). Additionally, T. testudinum at the Village site had proportionally more “developed shoots”, and fewer “undeveloped shoots” (Fig. 3). Foliar weight per shoot at the Village was also larger (mean weight 0.252 dry g ± 0.062 S.D., N 25) than at the Research Station (0.187 dry

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Fig. 2. T. testudinum. Anatomy of shoot apices and early stages of leaf development of the shoots belonging to the different morphological categories. Numbers represent categories. C: Corpus; LP: Leaf Primordium; T: Tunica. Magnification 64.

g ± 0.058 S.D., N 25). Despite these differences between the sites, similar trends in relative abundance of different types of shoots vs. density were observed. Relative proportions of “developed shoots” (active meristem, foliar development) decreased with increased density at both sites, whereas the proportion of “undeveloped shoots” (active meristem, without foliar development) showed an opposite trend (Table 1, Fig. 3). Mortality was independent of density as the number of “dead shoots” (without active meristem) did not show a trend in relation to the number of shoots per unit of rhizome weight (Table 1).

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Table 1 Effect of shoot density on T. testudinum at two stations in the Puerto Morelos reef lagoona Research station

Foliar weight (dry mg) Developed shoots (%) Undeveloped shoots (%) Dead shoots (%)

Village

Slope

F

p

r2

−1.67 −0.26 0.36 −0.15

3.049 8.187 13.866 1.870

0.094 0.009b 0.001b 0.185

0.079 0.230 0.349 0.035

Slope

F

p

r2

0.51 −0.55 0.79 0.07

0.212 5.594 7.620 0.024

0.649 0.027b 0.011b 0.879

0.034 0.161 0.216 0.042

a Data are results of linear regressions of average foliar weight per shoot, percentage of developed, undeveloped and dead shoots (of total number of shoots) vs. total number of shoots per sample. N = 25 per station. b Significant at α = 0.05.

3.4. Resource exploitation (fertilisation) Fertilisation increased the foliar area of the shoots of T. testudinum, and blade surface area per shoot was significantly higher in the nutrient (22.9 cm2 ) than the control treatment (17.4 cm2 : Fpooled = 14.547, P < 0.000, d.f. = 291). However, major effects of fertilisation were shifts in the abundance of shoots of the different categories. Table 2 shows the result of comparisons (independent samples t-test) between the control and the nutrient treatment, in the number of shoots belonging to each category. Neither the number of rhizome apices nor the number of juvenile shoots differed between the treatments. The difference in number of dead shoots between treatments was not significant suggesting that fertilisation did not affect the mortality of the shoots. Fertilisation resulted in a drastic decline in “undeveloped shoots” belonging to category 6, and an increase in “developed shoots” belonging to categories 3 and 4 (Table 2, Fig. 4). Whether a shoot belonged to a certain category was independent of its size (expressed as count of number of scars + blades on the shoots), with the exception of the first size class which is the only class containing juvenile shoots. When tested with the G-test of independence, all categories were found in equal proportion among all size classes of the

Fig. 3. Plots of the percentage of developed and undeveloped shoots (of total number of shoots) vs. total number of shoots per core sample (diameter 20 cm) of T. testudinum at two stations in the Puerto Morelos reef lagoon.

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Fig. 4. Number of shoots of T. testudinum in the categories where density differed significantly between control and nutrient treatment, as a function of their size-class (expressed as the number of scars and blade baring leaves per shoot).

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Table 2 Mean (±S.D.) number of shoots belonging to the different categories per core sample (diameter 20 cm) for the control and nutrient treatment applied to T. testudinuma Category

Number of shoots per sample

Results of t-test

Control

Nutrient treatment

t

p

d.f.

Dead shoots 0 (broken) and 1 (domed but dead apex)

16.5 ± 6.2

21.3 ± 4.6

1.877

0.079

18

Developed shoots 2 (juvenile shoot) 3 (mature foliar shoot) 4 (mature shoot with juvenile leaves)

2.3 ± 0.1 14.1 ± 3.5 2.0 ± 0.9

2.7 ± 1.3 21.9 ± 3.2 8.2 ± 3.8

0.442 5.133 5.695

0.702 0.000b 0.000b

2 18 18

Undeveloped shoots 5 (with immature scenesent leaf) 6 (domed apex with prophylls)

4.1 ± 1.4 15.5 ± 4.1

2.8 ± 2.5 3.3 ± 2.9

1.429 7.183

0.170 0.000b

18 18

2.4 ± 1.5

4.0 ± 2.3

1.789

0.900

18

Active apices of horizontal rhizomes a

Mean number of active apices of the horizontal rhizomes is also presented. Results of the t-test present differences in these parameters between the treatments. d.f. = 2 when plots were replicates, and d.f. = 18 for pooled samples. b Significant at α = 0.05.

shoots (G = 6.52, d.f. = 9, p = 0.69 for control). Although fertilisation resulted in a shift in the proportion of developed and undeveloped shoots, for this treatment, shoot size (expressed as number of scars plus blades) was also independent of the category (G = 5.39, d.f. = 9, p = 0.80) (Fig. 4).

4. Discussion The spacing of new ramets of clonal plants is generally regulated at just below the density at which fully grown modules would be expected to die, or their development inhibited (Hutchings and Barkham, 1976; Hutchings, 1979; Noble et al., 1979). The weight–density relationships as realised in the present study can give approximations of density dependence or density regulation. Mean foliar weight of the shoots of T. testudinum remained constant, independent of their density, but at increased shoot density, proportionally fewer shoots developed foliar structures and more shoots were assigned to the undeveloped group with an active meristem but without foliar development. This suggests that foliar development of many shoots was inhibited at a high degree of crowding, to avoid over-production of foliar shoots. Inhibition of development of the shoots under crowded conditions as a mechanism for density regulation of foliar shoots of T. testudinum, resembles that of inhibition of bud development in some terrestrial clonal plants (e.g., Noble et al., 1979; Hartnett and Bazzaz, 1985; De Kroon and Kwant, 1991). T. testudinum is considered to be a climax species in many Caribbean coastal seagrass beds (Zieman, 1982; Williams, 1990). To dominate the community, it must be able to exploit newly available resources (e.g. through mortality of foliar shoots by mechanical damage or by natural nutrient pulses etc.) before other seagrasses or rhizophytic algae invade the

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area. However, extension of T. testudinum is completely dependent on the growth of the subterranean horizontal rhizomes (Tomlinson, 1974), and rhizome elongation is very slow (≈10 cm a year, Gallegos et al., 1993). Thus, the horizontal rhizome cannot respond to sudden changes in resource availability. We hypothesise that the shoots with inhibited foliar development but with an active meristem play an essential role in the maintenance of dominance of T. testudinum where it is well established. When, e.g., new nutrients become available in the sediments, reactivation of foliar development of these shoots will allow T. testudinum to exploit newly available sediment nutrients before plants of other species have the opportunity to colonise the area. The results of the fertilisation experiments point to this mechanism of reactivation, as nutrient addition resulted in an increase of leaf-bearing shoots and an decrease of shoots with inhibited foliar development. The categories of shoot morphology corresponded with development stages in the shoots of T. testudinum. Dead shoots (without an active apical meristem) can either be damaged (without an rounded tip at the apex, category 0), or be intact (with a rounded apex) but without an active meristem (category 1). Developed shoots were divided into three categories, and for two of these, developmental stages can be assigned without much problem; the juvenile (category 2) and foliar shoots (category 3). The above-mentioned development stages of the developed shoots have been described for T. testudinum by Tomlinson and Vargo (1966), but the stages of shoots belonging to categories 4–6 have not been reported before. Based on the results of the fertilisation experiments, it is possible to assign developmental stages to these shoots. The virtual absence of shoots in category 6 and an increase in number of foliar shoots in the fertilised plots suggested that shoots of category 6 had transformed into foliar shoots. Thus, the undeveloped shoots of category 6 are dormant shoots, because they are alive and have the potential to develop into foliar shoots. These shoots were originally named inactive shoots by Van Tussenbroek (1996) as their developmental stage was not yet clear. Shoots in between the dormant and foliar stage had juvenile blades (category 4), and it is suggested that these shoots be named reactivated shoots. Shoots of category 5 were most likely an intermediate stage from foliar shoots to dormant shoots, as indicated by the presence of a small immature foliar leaf enveloped in decayed sheath tissue, and it is suggested that they be named intermediate shoots. Possible transformations of the above-mentioned development stages of the shoots are depicted in Fig. 5. Initiation of shoots occurs by the formation of juvenile shoots from the rhizome apex. Juvenile shoots, in general, develop into foliar shoots, but they can attain dormancy without passing through foliar maturation. Once the shoots have reached the foliar stage, they have the potential to pass several times through the cycle of foliar inhibition, dormancy and reactivation followed by foliar maturation (Fig. 5). Death may occur through damage or decay at any of the development stages or through lignification of the apical tips of dormant or intermediate shoots (shoots of category 1). All shoots have equal probability to attain dormancy or to develop into foliar shoots (or any of their intermediate stages), independent of their size, as shoots of all development stages were equally distributed over all size-classes (except for juvenile shoots). Also, transformation from one development stage to another was size independent, as the shoots belonging to the different categories remained equally distributed among all size classes after a shift in composition of the shoot population as a result of nutrient addition. Horizontal rhizome apices also have a dormant stage, and it is not uncommon to find dormant shoots which are bifurcated, the vertical axes being a vertical shoot with dormant

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Fig. 5. Transformation of development stages of shoots of T. testudinum from the Puerto Morelos reef lagoon. Damaged shoots (category 0) are formed during each development stage. Potential cycle indicates that foliar shoots may pass through the cycle of foliar scenescence, dormancy and reactivation, once or several times during their lives. The numbers in brackets represent category numbers.

apex, and the horizontal one being a horizontal rhizome with a “dormant apical meristem” (Fig. 6). Some foliar shoots also have such a horizontal offshoot with dormant apex, although more typically the apex of these horizontal rhizomes are active producing juvenile shoots (Fig. 6). The “activation” of dormant apical meristems of the horizontal rhizomes is probably less common as that of dormant shoots. However, “activation” of these horizontal rhizome apices has more far-reaching consequences than that of dormant shoots as they lead to vegetative propagation. Dormancy by “inactivation” of an apical meristem, as reported for T. testudinum in the present study is not commonly registered for plants, which generally induce dormancy by inhibition of the development of axillary buds. From the descriptions of Tomlinson (1974, and pers. com. from S. Enr´ıquez), we deduce that the seagrass species Cymodocea nodosa from the Mediterranean might have a similar mechanism as T. testudinum for dormancy. C. nodosa has monomorphic axes, i.e. there is no strict differentiation between horizontal and vertical rhizome. Thus, branch meristems at the apex of the horizontal rhizome can become new rhizome units, or vertical rhizomes with determinate or indeterminate growth

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