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Rhizome growth of Menyanthes trifoliata L. in a population on a floating peat mat in Mizorogaike Pond, central Japan
Aquatic Botany 53 (1996) 163-173
Rhizome growth of Menyanthes trifoliata L. in a population on a floating peat mat in Mizorogaike Pond, central Japan Akira Haraguchi
*
Hokkaido University Uryu Experimental Forest, Nayoro 096. Japan
Accepted 9 January 1996
Abstract The growth of rhizomes of Menyanthes trifiliatu L. was investigated in a floating mat in central Japan in order to examine the phenotypic properties of the rhizomes and hence to clarify the role of these organs. The part of the rhizome which elongated in autumn had high specific dry weight during autumn and winter, and the specific dry weight decreased in early spring when generative buds and leaves started to develop. This part of the rhizome has a function for dry matter storage. The part of the rhizome which elongated in summer had a low specific dry weight. Rhizomes with low specific dry weight are thin and long and are useful for occupation of the peat surface, hence playing a role in vegetative expansion. The percentage of generative buds and lateral buds formed was related to the size of rhizomes formed in the preceding year. The percentage of generative bud formation increased with increasing rhizome size. In contrast to generative buds, lateral bud formation was restricted to rhizomes 2-32 cm in length: rhizomes longer than 32 cm never formed lateral buds. Keywords; Clonal plants; Dry
matter storage; Nitrogen dynamics; Vegetative expansion; Menyanthes trififiatu
1. Introduction Clonal growth is widely observed in vascular plants (reviewed by Mogie and Hutchings, 1990). For most aquatic angiosperm taxa, clonal reproduction predominates over sexual reproduction, because the inhibition of sexual reproduction by the aquatic
* Present address: Faculty of Agriculture, Hokkaido University, N9 W9, Sapporo 060, Japan. Tel.: + 8 l- 11-71X5-2427; fax: + 8 I - 1I-706-4960; e-mail: akhgc@al .hines.hokudai.ac.jp. 0304-3770/96/$15.00
8 1996 Elsevier Science B.V. All rights reserved
PII SO304-3770(96)01022-4
164
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environment acts to favor a reliance on asexual reproduction (Grace, 1993). There are various types of organs associated with clonal reproduction, e.g. rhizomes, corms, tubers, bulbs, stolons and runners. Environmental conditions affect the resource allocation to these organs (e.g. Ogden, 1974; Abrahamson, 1975; Hartnett, 1990; Nishitani and Kimura, 1993) and the consequent growth form of clonal plants. The length of stolons, for example, changes according to light or nutritional conditions (Hutchings and Mogie, 1990). An examination of variation within a single species under various environmental conditions can help clarify not only phenotypic plasticity in relation to environmental gradients, but also the significance of the growth form or ecological function of individual organs. Menyunrhes trifoliuta L. is an example of a clonal plant with creeping rhizomes. The present paper describes a case study on a population in Mizorogaike Pond, central Japan. In the population, M. trifoliatu dominates the whole area of the floating mat and its periphery except for the hummocks. The species occurs on peat, in open water and in nutritionally rich and poor sites. The associated species are also. variable (Haraguchi, 1993). Thus the habitat of the species in the pond varies considerably and the plants show a wide size range over the study area. This facilitates examination of phenotypic plasticity with reference to habitat conditions and interspecific relationships within a population without having to consider climatic conditions. The first objective was then made to investigate seasonal variation in rhizome growth over a period of 1 year. An attempt was then made to examine how the formation of generative buds, lateral buds of the rhizome and rhizome growth are related to shoot growth during the preceding growing season. Grace (1993) classified the functions of plant tissues that participate in clonal reproduction as numerical increase, dispersal, resource acquisition, storage, protection and anchorage. He also made a functional comparison of different methods of clonal reproduction based on specific example species over a range of heterogeneous taxonomic groups. Here, I attempted to make a functional comparison of rhizomes based on habitat differences in a single species, M. trifoliatu, with special reference to two functions, dispersal and storage. Such an autecological approach would clarify the function of rhizomes excluding the effects of species differences that had evolved through certain factors unrelated to rhizome evolution. To clarify the variation and plasticity of the habit in a community would serve as an aid to understanding the ecological role of rhizomes of M. triifoliutu and their adaptive significance.
2. Materials and methods 2.1. Plant material and study area M. trifoliuta has 4O”Nand the Arctic the cool temperate southwestern Japan
a circumboreal distribution and it is common in wetlands between Circle (Hewett, 1964). In Japan, it is distributed rather commonly in zone (from ca. 38”N), while in the warm temperate zone of it shows localized, discontinuous distribution (Kokawa, 1961).
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The population of M. trifoliata chosen for study is at Mizorogaike Pond (9 ha in area, maximum depth ca. 2 ml, situated north of Kyoto City (35”03’N, 135”46’E, 75 m a.s.1.). The climate of the area is warm temperate. An extensive floating mat of peat (ca. 240 m X 140 ml covers the pond. The pond is one of the areas where M. trifoliatu grows as a relic species (Kokawa. 1961). M. trifoliutu has a horizontal creeping sympodial rhizome with adventitious roots and leaves at the apex. Leaf blades project above the surface of the water or peat. The species is the dominant one in the mat at this pond. In the population, inflorescences are formed in September and remain dormant over winter. The inflorescences and petioles show gradual growth from February. The inflorescences elongate and expand rapidly in late March, and the average time of first flowering is early April. Expansion of leaves begins immediately after the start of flowering. Most of the seeds begin to ripen in mid-May. Live rhizomes of M. trifoliata cover almost all of the mat at a depth of ca. 5 cm, and shoots of M. trifoliatu emerge from the accumulated peat. On the mat surface, there are about 130 hummocks varying from 0.2 to 200 m2 in size (Shimizu, 19861. They support Sphagnum palustre L. and some shrubs e.g. Rex crenata Thunb. and Pinus densiflora Sieb. et Zucc. M. trifoliutu also occurs in the margins of the hummocks, and grows even on the surface of open water, e.g. pools in the mat, and the peripheral area of the mat. M. trifoliutu forms both pure stands and mixed stands with other species. Other species present include Rhynchospora fauriei Franch., Eriocaulon sikokianum Maxim., E. hondoense Satake f. pilosum Satake, Phragmites australis (Cav.) Trin. ex Steud., Sphagnum cuspidatum Hoffm., Carex thunbergii Steud. and Isachne globosa 0. Kuntze. 2.2. Growth analysis of rhizomes Ten shoots (ramets) of M. tr$oliatu were collected about once a month from September 1986 to September 1987 at four sites: (1) pure M. trifoliatu community on peat; (2) mixed stand of M. trifoliata and P. australis on peat; (3) pure M. trifoliata community in ambient open water and (4) pure M. trifoliuta community in pools. Samples included leaf blade, petiole, youngest rhizome and 5 cm of l-year-old rhizome just behind the youngest part (Fig. 11. The age of the rhizome was established by counting the inflorescence scars produced per year. When necessary, the node (leaf scar) on the rhizome just behind the section with the shortest internode length was used as a substitute for the inflorescence scar. Nishimura (1983) has shown that this is a reliable estimate of age because inflorescences are formed during the preceding winter when the rate of extension of the rhizomes has diminished. The length, diameter and node number of the youngest rhizomes, and the diameter of l-year-old rhizomes were measured immediately after sampling. The dry weights of youngest and l-year-old rhizomes were measured after drying at 75-80°C for a minimum of 48 h. The volumes of the rhizomes were estimated by measuring length and diameter. The specific dry weight of a rhizome was calculated from its fresh volume and dry weight. A single-factor ANOVA for length, dry weight, node number of youngest rhizomes and specific dry weight of
166
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Fig. I. Structure of a shoot of Menyanrhes trifoliara.
youngest and l-year-old rhizomes was carried out. Data were analyzed for nine levels of month and four levels of sites. 2.3. Nitrogen and carbon contents of rhizomes A shoot of M. trijioliata was collected once a month from May 1985 to May 1986 at four sites: hummock, pure M. trzyoliata community on peat, open water, pool. Samples included youngest, l-year-old and 2-year-old rhizomes. After drying the rhizomes as described above, they were ground into a powder with a mill. Total nitrogen content was determined using a N,C analyzer (Sumitomo-Shimadzu GCT-13N). Data were pooled by sampling month (n = 4) and analyzed. A single-factor ANOVA for nitrogen content of youngest, l-year-old and 2-year-old rhizomes taken over nine levels of month was carried out. 2.4. Formation of generative
buds and lateral buds
To establish the relationship between rhizome size and the formation of generative and/or lateral buds, rhizome length, rhizome age, rhizome diameter and presence or absence of generative buds and lateral buds were determined in February 1989 in a site of open water and three pools in the mat. Measurements were made for each rhizome going back from the youngest to the older part.
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0 ONDJFMAMJJAS
ONDJFMAMJJAS 1986
1986
1987
1987
ONDJFMAMJJAS 1986
1987 Year,
Calendar
Fig. 2. Growth in (a) length, (b) dry weight and (c) number trifoliata in a population in Mizorogaike Pond. Symbols are, water (open circle), pools (closed circle), peat mat (open Phragmites australis on peat mat (closed square). Means and
month
of nodes of youngest rhizomes of Menyanthcs pure M. trifdiata community on ambient open square) and mixed stand of M. trifoliata and SE are presented (n = IO).
3. Results 3.1. Seasonal pattern of rhizome growth After the initiation of generative buds in September, the length and dry weight of the youngest rhizomes (from the generative buds to terminal) changed little until March, but then increased rapidly after leaf extension in April (Fig. 2a, Fig. 2b). The marked differences observed in length and dry weight after May were probably due to differences between sites. Despite the low growth in length and dry weight of rhizomes from October to December, the node number increased during this period with a rate comparable to that from April to September (Fig. 2~). Variation in node number among sites after May was less than that for length or dry weight. Length, dry weight and node number showed significant differences between monthly mean values. Length and dry weight also showed significant differences between the site mean values, but node number did not (Table 1). The specific dry weight of youngest rhizomes increased from October to January and decreased rapidly from January to March, the season when the petioles and inflorescences started to elongate (Fig. 3a). After April the increment in specific dry weight of
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A. Haraguchi/Aquatic
Table 1 The single-factor ANOVA of the length, Menyanthes trifidiata in Mizorogaike Pond
Botany 53 (1996) 163-173
dry weight
and node
number
of the youngest
Variables
Source of variation
Sum of squares
d.f.
Mean square
Length (cm)
Month Residual Site Residual Month Residual Site Residual Month Residual Site Residual
4141 3117 232 7026 94.76 196.6 34.96 256.4 6936 2232 71.9 9095
8 355 3 360 8 355 3 360 8 355 3 360
517.6 8.8 77.3 19.5 11.84 0.5538 11.65 0.7122 867.0 6.3 24.0 25.3
Length (cm) Dry weight(g) Dry weight (9, Node number Node number
Probability:
?? ?? ??
P < 0.001,
rhizomes
of
F 58.9 * * * 3.96 * ’ 21.4 * * * 16.4 * 137.9
??
??
*
**
0.95 NS
* * P < 0.01, * P < 0.05, NS P > 0.05.
youngest rhizomes was small. The specific dry weight of l-year-old rhizomes showed little change throughout the year (Fig. 3b). The specific dry weight of youngest and 1-year-old rhizomes showed significant seasonal differences, but less difference between the site mean values (Table 2). The rhizomes collected from 1985 to 1986 showed similar seasonal changes in length, dry weight, node number and specific dry weight, as presented in Figs. 2 and 3 (data not shown). 3.2. Seasonal change in nitrogen content of rhizome The nitrogen content of the youngest rhizome increased from September, reached a maximum value in December and subsequently decreased until June (Fig. 41, before
ONDJFMAMJJAS 1986
1981
ONDJFMAMJJAS 1986
1987
Year, Calendar
month
Fig. 3. Seasonal changes in specific dry weight (dry weight per unit fresh volume) of (a) youngest and l-year-old rhizomes of Menyanthes trifoliata in a population in Mizorogaike Pond. Symbols are, pure trifoliata community on ambient open water (open circle), pools (closed circle), peat mat (open square) mixed stand of M. trifoliata and Phragmites australis on peat mat (closed square). Means and SE presented (n = 10).
(b) M. and are
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Table 2 The single-factor ANOVA of the specific dry weight (g) of youngest trifoliata in Mizorogaike Pond
169
and 1-year-old
rhizomes
of Menyanthes
Variables
Source of variation
Sum of squares
d.f.
Mean square
F
Youngest
Month Residual Site Residual Month Residual Site Residual
0.8692 2.1052 0.07785 2.896 0.08772 0.1946 0.0045 0.277 1
8 329 3 333 8 351 3 355
0.10865 0.0064 0.2595 0.0087 0.10965 0.00055 0.00150 0.00078
17.0
Youngest 1-year-old 1-year-old
Probability:
***
??
**
2.98 * 19.8 * * *
I .93 NS
P < 0.001, * P < 0.01, * P < 0.05, NS P > 0.05. ??
Table 3 The single-factor ANOVA of the nitrogen Menyanthes trifoliata in Mizorogaike Pond
content
in youngest,
l-year-old
and 2-year-old
rhizomes
Variables
Source of variation
Sum of squares
d.f.
Mean square
F
Probability
Youngest
Month Residual Month Residual Month Residual
2.841 4.787 1.263 4.798 0.418 1.973
10 32 11 36 8 27
0.284 0.15 0.155 0.133 0.052 0.073
1.899
0.083
0.862
0.784
0.715
0.677
l-year-old 2-year-old
of
increasing again until August. The nitrogen content of l- and 2-year-old rhizomes increased from October, reached a maximum in January and subsequently decreased until May. The seasonal differences in the nitrogen content of youngest, l- and 2-year-old rhizomes were not significant at P < 0.05, although the F-value for the youngest rhizome was much higher than that for older rhizomes (Table 3).
o.ol. SONDJ
FMAMJ
1985
J
A
1986 Year,
Calendar month
Fig. 4. Seasonal changes in total nitrogen contents in youngest (open circle), l-year-old (closed circle) and 2-year-old (open square) rhizomes of Menyanthes trifofiata in a population in Mizorogaike Pond. Means and SE are presented (n = 4).
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0
20
Botany 53 (1996) 163-173
40
60
80
100
120
140
Rhizome length (yearn; cm)
Fig. 5. Length of rhizomes Pond (n = 331).
of Menyanthes trijbliata of two successive
years in a population
in Mizorogaike
3.3. Year to year variation in rhizome growth The relationship between the increment in rhizome length during 1 year (Year = n) and that in the following year (Year = n + 1) was plotted for all the samples with data for the two successive years (Fig. 5). There was a significant positive correlation between the rhizome length for Year n and that for Year (n + 1). 3.4. Relationship between rhizome growth and formation of generative buds or lateral buds Conditions for the formation of generative buds and lateral vegetative buds (branching) from the rhizomes were estimated relative to rhizome size. Rhizomes were divided by age, and the presence or absence of generative buds and lateral buds at the apex of each were examined. The percentage of generative buds increased with increasing length and volume of rhizome. All rhizomes exceeding 32 cm in length or 64 cm3 in volume had only generative buds (Fig. 6). Rhizomes measuring 2-32 cm in length and 4-64
2-4
4-8
8-16
Len@h class
16-32
32-
O-2
2-4
48
8-16
16-3232-64
64-
Volume class
Fig. 6. Percentage of generative bud and lateral bud (branch) formation of Menyanthes irifoliata in a population in Mizorogaike Pond (n= 219). Percentage of shoots which have only generative buds (black), both generative buds and lateral buds (hatched) and only lateral buds (dotted) arc presented.
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171
cm3 in volume had also lateral vegetative buds, though the proportion of lateral buds was less than 20%. In contrast to generative bud formation, rhizomes exceeding 32 cm in length or 64 cm3 in volume never formed lateral buds.
4. Discussion The creeping growth of a rhizome enables a plant to colonize its adjacent area, and hence it is considered an important aspect of vegetative expansion. The growth form of M. trifoliatu resembles that of plants with primitive monopodial rhizomes of uniform diameter. Rhizomes of M. trifoliatu differ from such primitive rhizomes in that the size and specific weight of rhizomes of the former show marked changes according to the season. From October to December (end of the growth season), the specific dry weight (Fig. 3a) and nitrogen content (Fig. 4) of youngest rhizomes increase, but the length of youngest rhizomes shows little change (Fig. 2a). These facts imply that organic matter accumulates at the apex of rhizomes to form short and heavy rhizomes in the season. There was a rapid decrease in specific dry weight and nitrogen content from January to February due to the gradual growth of petioles and generative buds. Hence the rhizomes formed from September to December play a role in storage. After extension of the leaves, the rhizomes grew rapidly using the dry matter stored in youngest rhizomes. The specific dry weight and nitrogen content of rhizomes which elongated during the growth season (April to September) did not increase, despite the presence of dense foliage. The rhizomes formed in this season were not used as storage organs. Thin and long rhizomes with low density are advantageous when there is a need for maximum elongation. Hence the rhizomes formed in this season have a role in dispersing the shoot apex as far as possible. In contrast with the youngest rhizomes, l- and 2-year-old rhizomes made little contribution to dry matter storage, because seasonal changes in specific dry weight and nitrogen content presented less fluctuation than in the youngest ones. This implies that the contribution of the older parts of rhizomes to dry matter storage is low and that distribution or transfer of materials does not occur through the rhizomes. The role of the rhizomes of M. trz$oliatu changes according to the season: thick and heavy rhizomes (pachymorphic) formed in the autumn have roles as storage organs and thin and light rhizomes (leptomorphic) formed in summer are used for vegetative expansion. The growth forms of rhizomes of M. trifoliatu are an advance over the simple type of rhizome with constant diameter. Rhizomes of Reineckia carnea Kunth also present a habit similar to that of M. trifooliata (Yamamura, 1984). Among the six functions of plant organs that serve as means of clonal reproduction, as summarized by Grace (19931, dispersal and storage are important when discussing the structure of the M. trifoliutu rhizome. It appears that there is a certain degree of trade-off between them. Rhizomes can function both as storage and dispersal organs. Pachymorphic rhizomes have a tendency to act as storage organs and leptomorphic rhizomes have a tendency to be dispersal organs. Rhizomes of M. trifoliatu with remarkable seasonal variation in diameter have both these functions. Stolons are very thin and highly suitable for vegetative expansion, with a special role of carrying shoots
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as far as possible from the parent plant. Glechoma hederacea L., for example, produces primary stolons with longer internodes under low light or low nutrient conditions @lade and Hutchings, 1987). This implies that the stolons of this species control the extent of shoot length according to environmental conditions. In this sense, the rhizomes of M. trifoliata formed in summer function as stolons. Rhizome size is a determining factor for bud initiation. The greater the rhizome weight, the greater the number of generative buds (Fig. 6). However, the condition of formation of rhizome lateral buds was restricted to medium-sized rhizomes, although the percentage of lateral bud formation was low (Fig. 6). Large shoots never formed lateral buds. Hutchings and Slade (1988) concluded that plants under low resource supply have fewer stolon branches per clone. However M. trifoliata with a high nutrient supply has long rhizomes without branches. This relates to the fact that elongation of rhizomes in a year is correlated significantly with elongation in the previous year; shoots which elongated well in a year also grew well the following year. These shoots were observed in the ambient open water area where nutrient concentration was higher than in the mat (Haraguchi and M atsui, 1990). Rhizomes of M. trifoliata tend to elongate by monopodial growth when conditions allow. Under conditions where elongation was slightly restricted by some factor, e.g. intraspecific or interspecific competition, the shoot tended to form lateral buds; however, the probability of this is low. With respect to this feature, the species shows a guerrilla type of growth, switching to phalanx-type growth under conditions where rhizome growth is restricted. This switching ability and the seasonal plasticity of rhizome growth, together with the small number of competing species, has allowed M. trzyoliata to become the predominant population in the floating peat mat in Mizorogaike Pond.
Acknowledgements
I thank members of the Center for Ecological Research, Kyoto University, for their helpful suggestions about this work. Parts of the calculations were carried out at the Data Processing Center, Hokkaido University.
References Abmhamson, W.G., 1975. Reproductive strategies in dewbenies. Ecology, 56: 721-726. Grace, J.B., 1993. The adaptive significance of clonal reproduction in angiosperms: an aquatic perspective. Aquat. Bot., 44: 159-180. Haraguchi, A., 1993. Phenotypic and phenological plasticity of an aquatic macrophyte Menyanthes trifofiata L. J. Plant Res., 106: 31-35. Haraguchi, A. and Matsui, K., 1990. Nutrient dynamics in a floating mat and pond system with special reference to its vegetation. Rcol. Res., 5: 63-79. Hartnett, D.C., 1990. Size-dependent allocation to sexual and vegetative reproduction in four clonal composites. Gecologia, 84: 254-259. Hewett, D.G., 1964. Menyanthes trijbliata L. J. Ecol., 52: 723-735. Hutchings, M.J. and Mogie, M., 1990. The spatial structure of clonal plants: control and consequences. In: J.
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173
van Groenendael and H. de Kroon (Editors), Clonal Growth in Plants: Regulation and Function. SPB Academic Publishing, The Hague, pp. 57-76. Hutchings, M.J. and Slade, A.J., 1988. Morphological plasticity, foraging and integration in clonal perennial herbs. In: A.J. Davy, M.J. Hutchings and A.R. Watkinson (Editors), Plant Population Ecology. Blackwell, Oxford, pp. 83-109. Kokawa. S., 1961. Distribution and phytostratigraphy of Menyanthes remains in Japan. J. Osaka City University, 12: 123- 15 1. Mogie, M. and Hutchings, M.J., 1990. Phylogeny, ontogeny and clonal growth in vascular plants. In: J. van Groenendael and H. de Kroon (Editors), Clonal Growth in Plants: Regulation and Function. SPB Academic Publishing, The Hague, pp. 3-22. Nishimura, M., 1983. Rhizome growth and matter production of the common buckbean. Menyunthes trijidiutu L. Physiol. Ecol. Jpn., 20: 115-128. Nishitani, S. and Kimura, M., 1993. Resource allocation to sexual and vegetative reproduction in a forest herb Syneilesis palmata (Compositae). Ecol. Res., 8: 173- 183. Ogden, J., 1974. The reproductive strategy of higher plants. 11. The reproductive strategy of Tussilago furfirru L. J. Ecol., 62: 291-324. Shimizu, Y., 1986. Species numbers, area, and habitat diversity on the habitat-islands of Mizorogaike Pond, Japan. Ecol. Res., I: 185-194. Slade, A.J. and Hutchings, M.J., 1987. Clonal integration and plasticity in foraging behaviour in Glechomu hederacea. J. Ecol., 75: 1023- 1036. Yamamura, Y., 1984. Matter production processes of Reineckiu carneu Kunth, an evergreen forest floor herb in the warm-temperate region of Japan. Bot. Mag. Tokyo, 97: 179- 191.
Rhizome growth of Menyanthes trifoliata L. in a population on a floating peat mat in Mizorogaike Pond, central Japan Akira Haraguchi
*
Hokkaido University Uryu Experimental Forest, Nayoro 096. Japan
Accepted 9 January 1996
Abstract The growth of rhizomes of Menyanthes trifiliatu L. was investigated in a floating mat in central Japan in order to examine the phenotypic properties of the rhizomes and hence to clarify the role of these organs. The part of the rhizome which elongated in autumn had high specific dry weight during autumn and winter, and the specific dry weight decreased in early spring when generative buds and leaves started to develop. This part of the rhizome has a function for dry matter storage. The part of the rhizome which elongated in summer had a low specific dry weight. Rhizomes with low specific dry weight are thin and long and are useful for occupation of the peat surface, hence playing a role in vegetative expansion. The percentage of generative buds and lateral buds formed was related to the size of rhizomes formed in the preceding year. The percentage of generative bud formation increased with increasing rhizome size. In contrast to generative buds, lateral bud formation was restricted to rhizomes 2-32 cm in length: rhizomes longer than 32 cm never formed lateral buds. Keywords; Clonal plants; Dry
matter storage; Nitrogen dynamics; Vegetative expansion; Menyanthes trififiatu
1. Introduction Clonal growth is widely observed in vascular plants (reviewed by Mogie and Hutchings, 1990). For most aquatic angiosperm taxa, clonal reproduction predominates over sexual reproduction, because the inhibition of sexual reproduction by the aquatic
* Present address: Faculty of Agriculture, Hokkaido University, N9 W9, Sapporo 060, Japan. Tel.: + 8 l- 11-71X5-2427; fax: + 8 I - 1I-706-4960; e-mail: akhgc@al .hines.hokudai.ac.jp. 0304-3770/96/$15.00
8 1996 Elsevier Science B.V. All rights reserved
PII SO304-3770(96)01022-4
164
A. Haraguchi/Aquatic
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environment acts to favor a reliance on asexual reproduction (Grace, 1993). There are various types of organs associated with clonal reproduction, e.g. rhizomes, corms, tubers, bulbs, stolons and runners. Environmental conditions affect the resource allocation to these organs (e.g. Ogden, 1974; Abrahamson, 1975; Hartnett, 1990; Nishitani and Kimura, 1993) and the consequent growth form of clonal plants. The length of stolons, for example, changes according to light or nutritional conditions (Hutchings and Mogie, 1990). An examination of variation within a single species under various environmental conditions can help clarify not only phenotypic plasticity in relation to environmental gradients, but also the significance of the growth form or ecological function of individual organs. Menyunrhes trifoliuta L. is an example of a clonal plant with creeping rhizomes. The present paper describes a case study on a population in Mizorogaike Pond, central Japan. In the population, M. trifoliatu dominates the whole area of the floating mat and its periphery except for the hummocks. The species occurs on peat, in open water and in nutritionally rich and poor sites. The associated species are also. variable (Haraguchi, 1993). Thus the habitat of the species in the pond varies considerably and the plants show a wide size range over the study area. This facilitates examination of phenotypic plasticity with reference to habitat conditions and interspecific relationships within a population without having to consider climatic conditions. The first objective was then made to investigate seasonal variation in rhizome growth over a period of 1 year. An attempt was then made to examine how the formation of generative buds, lateral buds of the rhizome and rhizome growth are related to shoot growth during the preceding growing season. Grace (1993) classified the functions of plant tissues that participate in clonal reproduction as numerical increase, dispersal, resource acquisition, storage, protection and anchorage. He also made a functional comparison of different methods of clonal reproduction based on specific example species over a range of heterogeneous taxonomic groups. Here, I attempted to make a functional comparison of rhizomes based on habitat differences in a single species, M. trifoliatu, with special reference to two functions, dispersal and storage. Such an autecological approach would clarify the function of rhizomes excluding the effects of species differences that had evolved through certain factors unrelated to rhizome evolution. To clarify the variation and plasticity of the habit in a community would serve as an aid to understanding the ecological role of rhizomes of M. triifoliutu and their adaptive significance.
2. Materials and methods 2.1. Plant material and study area M. trifoliuta has 4O”Nand the Arctic the cool temperate southwestern Japan
a circumboreal distribution and it is common in wetlands between Circle (Hewett, 1964). In Japan, it is distributed rather commonly in zone (from ca. 38”N), while in the warm temperate zone of it shows localized, discontinuous distribution (Kokawa, 1961).
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165
The population of M. trifoliata chosen for study is at Mizorogaike Pond (9 ha in area, maximum depth ca. 2 ml, situated north of Kyoto City (35”03’N, 135”46’E, 75 m a.s.1.). The climate of the area is warm temperate. An extensive floating mat of peat (ca. 240 m X 140 ml covers the pond. The pond is one of the areas where M. trifoliatu grows as a relic species (Kokawa. 1961). M. trifoliutu has a horizontal creeping sympodial rhizome with adventitious roots and leaves at the apex. Leaf blades project above the surface of the water or peat. The species is the dominant one in the mat at this pond. In the population, inflorescences are formed in September and remain dormant over winter. The inflorescences and petioles show gradual growth from February. The inflorescences elongate and expand rapidly in late March, and the average time of first flowering is early April. Expansion of leaves begins immediately after the start of flowering. Most of the seeds begin to ripen in mid-May. Live rhizomes of M. trifoliata cover almost all of the mat at a depth of ca. 5 cm, and shoots of M. trifoliatu emerge from the accumulated peat. On the mat surface, there are about 130 hummocks varying from 0.2 to 200 m2 in size (Shimizu, 19861. They support Sphagnum palustre L. and some shrubs e.g. Rex crenata Thunb. and Pinus densiflora Sieb. et Zucc. M. trifoliutu also occurs in the margins of the hummocks, and grows even on the surface of open water, e.g. pools in the mat, and the peripheral area of the mat. M. trifoliutu forms both pure stands and mixed stands with other species. Other species present include Rhynchospora fauriei Franch., Eriocaulon sikokianum Maxim., E. hondoense Satake f. pilosum Satake, Phragmites australis (Cav.) Trin. ex Steud., Sphagnum cuspidatum Hoffm., Carex thunbergii Steud. and Isachne globosa 0. Kuntze. 2.2. Growth analysis of rhizomes Ten shoots (ramets) of M. tr$oliatu were collected about once a month from September 1986 to September 1987 at four sites: (1) pure M. trifoliatu community on peat; (2) mixed stand of M. trifoliata and P. australis on peat; (3) pure M. trifoliata community in ambient open water and (4) pure M. trifoliuta community in pools. Samples included leaf blade, petiole, youngest rhizome and 5 cm of l-year-old rhizome just behind the youngest part (Fig. 11. The age of the rhizome was established by counting the inflorescence scars produced per year. When necessary, the node (leaf scar) on the rhizome just behind the section with the shortest internode length was used as a substitute for the inflorescence scar. Nishimura (1983) has shown that this is a reliable estimate of age because inflorescences are formed during the preceding winter when the rate of extension of the rhizomes has diminished. The length, diameter and node number of the youngest rhizomes, and the diameter of l-year-old rhizomes were measured immediately after sampling. The dry weights of youngest and l-year-old rhizomes were measured after drying at 75-80°C for a minimum of 48 h. The volumes of the rhizomes were estimated by measuring length and diameter. The specific dry weight of a rhizome was calculated from its fresh volume and dry weight. A single-factor ANOVA for length, dry weight, node number of youngest rhizomes and specific dry weight of
166
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Fig. I. Structure of a shoot of Menyanrhes trifoliara.
youngest and l-year-old rhizomes was carried out. Data were analyzed for nine levels of month and four levels of sites. 2.3. Nitrogen and carbon contents of rhizomes A shoot of M. trijioliata was collected once a month from May 1985 to May 1986 at four sites: hummock, pure M. trzyoliata community on peat, open water, pool. Samples included youngest, l-year-old and 2-year-old rhizomes. After drying the rhizomes as described above, they were ground into a powder with a mill. Total nitrogen content was determined using a N,C analyzer (Sumitomo-Shimadzu GCT-13N). Data were pooled by sampling month (n = 4) and analyzed. A single-factor ANOVA for nitrogen content of youngest, l-year-old and 2-year-old rhizomes taken over nine levels of month was carried out. 2.4. Formation of generative
buds and lateral buds
To establish the relationship between rhizome size and the formation of generative and/or lateral buds, rhizome length, rhizome age, rhizome diameter and presence or absence of generative buds and lateral buds were determined in February 1989 in a site of open water and three pools in the mat. Measurements were made for each rhizome going back from the youngest to the older part.
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167
0 ONDJFMAMJJAS
ONDJFMAMJJAS 1986
1986
1987
1987
ONDJFMAMJJAS 1986
1987 Year,
Calendar
Fig. 2. Growth in (a) length, (b) dry weight and (c) number trifoliata in a population in Mizorogaike Pond. Symbols are, water (open circle), pools (closed circle), peat mat (open Phragmites australis on peat mat (closed square). Means and
month
of nodes of youngest rhizomes of Menyanthcs pure M. trifdiata community on ambient open square) and mixed stand of M. trifoliata and SE are presented (n = IO).
3. Results 3.1. Seasonal pattern of rhizome growth After the initiation of generative buds in September, the length and dry weight of the youngest rhizomes (from the generative buds to terminal) changed little until March, but then increased rapidly after leaf extension in April (Fig. 2a, Fig. 2b). The marked differences observed in length and dry weight after May were probably due to differences between sites. Despite the low growth in length and dry weight of rhizomes from October to December, the node number increased during this period with a rate comparable to that from April to September (Fig. 2~). Variation in node number among sites after May was less than that for length or dry weight. Length, dry weight and node number showed significant differences between monthly mean values. Length and dry weight also showed significant differences between the site mean values, but node number did not (Table 1). The specific dry weight of youngest rhizomes increased from October to January and decreased rapidly from January to March, the season when the petioles and inflorescences started to elongate (Fig. 3a). After April the increment in specific dry weight of
168
A. Haraguchi/Aquatic
Table 1 The single-factor ANOVA of the length, Menyanthes trifidiata in Mizorogaike Pond
Botany 53 (1996) 163-173
dry weight
and node
number
of the youngest
Variables
Source of variation
Sum of squares
d.f.
Mean square
Length (cm)
Month Residual Site Residual Month Residual Site Residual Month Residual Site Residual
4141 3117 232 7026 94.76 196.6 34.96 256.4 6936 2232 71.9 9095
8 355 3 360 8 355 3 360 8 355 3 360
517.6 8.8 77.3 19.5 11.84 0.5538 11.65 0.7122 867.0 6.3 24.0 25.3
Length (cm) Dry weight(g) Dry weight (9, Node number Node number
Probability:
?? ?? ??
P < 0.001,
rhizomes
of
F 58.9 * * * 3.96 * ’ 21.4 * * * 16.4 * 137.9
??
??
*
**
0.95 NS
* * P < 0.01, * P < 0.05, NS P > 0.05.
youngest rhizomes was small. The specific dry weight of l-year-old rhizomes showed little change throughout the year (Fig. 3b). The specific dry weight of youngest and 1-year-old rhizomes showed significant seasonal differences, but less difference between the site mean values (Table 2). The rhizomes collected from 1985 to 1986 showed similar seasonal changes in length, dry weight, node number and specific dry weight, as presented in Figs. 2 and 3 (data not shown). 3.2. Seasonal change in nitrogen content of rhizome The nitrogen content of the youngest rhizome increased from September, reached a maximum value in December and subsequently decreased until June (Fig. 41, before
ONDJFMAMJJAS 1986
1981
ONDJFMAMJJAS 1986
1987
Year, Calendar
month
Fig. 3. Seasonal changes in specific dry weight (dry weight per unit fresh volume) of (a) youngest and l-year-old rhizomes of Menyanthes trifoliata in a population in Mizorogaike Pond. Symbols are, pure trifoliata community on ambient open water (open circle), pools (closed circle), peat mat (open square) mixed stand of M. trifoliata and Phragmites australis on peat mat (closed square). Means and SE presented (n = 10).
(b) M. and are
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Table 2 The single-factor ANOVA of the specific dry weight (g) of youngest trifoliata in Mizorogaike Pond
169
and 1-year-old
rhizomes
of Menyanthes
Variables
Source of variation
Sum of squares
d.f.
Mean square
F
Youngest
Month Residual Site Residual Month Residual Site Residual
0.8692 2.1052 0.07785 2.896 0.08772 0.1946 0.0045 0.277 1
8 329 3 333 8 351 3 355
0.10865 0.0064 0.2595 0.0087 0.10965 0.00055 0.00150 0.00078
17.0
Youngest 1-year-old 1-year-old
Probability:
***
??
**
2.98 * 19.8 * * *
I .93 NS
P < 0.001, * P < 0.01, * P < 0.05, NS P > 0.05. ??
Table 3 The single-factor ANOVA of the nitrogen Menyanthes trifoliata in Mizorogaike Pond
content
in youngest,
l-year-old
and 2-year-old
rhizomes
Variables
Source of variation
Sum of squares
d.f.
Mean square
F
Probability
Youngest
Month Residual Month Residual Month Residual
2.841 4.787 1.263 4.798 0.418 1.973
10 32 11 36 8 27
0.284 0.15 0.155 0.133 0.052 0.073
1.899
0.083
0.862
0.784
0.715
0.677
l-year-old 2-year-old
of
increasing again until August. The nitrogen content of l- and 2-year-old rhizomes increased from October, reached a maximum in January and subsequently decreased until May. The seasonal differences in the nitrogen content of youngest, l- and 2-year-old rhizomes were not significant at P < 0.05, although the F-value for the youngest rhizome was much higher than that for older rhizomes (Table 3).
o.ol. SONDJ
FMAMJ
1985
J
A
1986 Year,
Calendar month
Fig. 4. Seasonal changes in total nitrogen contents in youngest (open circle), l-year-old (closed circle) and 2-year-old (open square) rhizomes of Menyanthes trifofiata in a population in Mizorogaike Pond. Means and SE are presented (n = 4).
A. Haraguchi/Aquatic
170
0
20
Botany 53 (1996) 163-173
40
60
80
100
120
140
Rhizome length (yearn; cm)
Fig. 5. Length of rhizomes Pond (n = 331).
of Menyanthes trijbliata of two successive
years in a population
in Mizorogaike
3.3. Year to year variation in rhizome growth The relationship between the increment in rhizome length during 1 year (Year = n) and that in the following year (Year = n + 1) was plotted for all the samples with data for the two successive years (Fig. 5). There was a significant positive correlation between the rhizome length for Year n and that for Year (n + 1). 3.4. Relationship between rhizome growth and formation of generative buds or lateral buds Conditions for the formation of generative buds and lateral vegetative buds (branching) from the rhizomes were estimated relative to rhizome size. Rhizomes were divided by age, and the presence or absence of generative buds and lateral buds at the apex of each were examined. The percentage of generative buds increased with increasing length and volume of rhizome. All rhizomes exceeding 32 cm in length or 64 cm3 in volume had only generative buds (Fig. 6). Rhizomes measuring 2-32 cm in length and 4-64
2-4
4-8
8-16
Len@h class
16-32
32-
O-2
2-4
48
8-16
16-3232-64
64-
Volume class
Fig. 6. Percentage of generative bud and lateral bud (branch) formation of Menyanthes irifoliata in a population in Mizorogaike Pond (n= 219). Percentage of shoots which have only generative buds (black), both generative buds and lateral buds (hatched) and only lateral buds (dotted) arc presented.
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Botany 53 (1996) 163-173
171
cm3 in volume had also lateral vegetative buds, though the proportion of lateral buds was less than 20%. In contrast to generative bud formation, rhizomes exceeding 32 cm in length or 64 cm3 in volume never formed lateral buds.
4. Discussion The creeping growth of a rhizome enables a plant to colonize its adjacent area, and hence it is considered an important aspect of vegetative expansion. The growth form of M. trifoliatu resembles that of plants with primitive monopodial rhizomes of uniform diameter. Rhizomes of M. trifoliatu differ from such primitive rhizomes in that the size and specific weight of rhizomes of the former show marked changes according to the season. From October to December (end of the growth season), the specific dry weight (Fig. 3a) and nitrogen content (Fig. 4) of youngest rhizomes increase, but the length of youngest rhizomes shows little change (Fig. 2a). These facts imply that organic matter accumulates at the apex of rhizomes to form short and heavy rhizomes in the season. There was a rapid decrease in specific dry weight and nitrogen content from January to February due to the gradual growth of petioles and generative buds. Hence the rhizomes formed from September to December play a role in storage. After extension of the leaves, the rhizomes grew rapidly using the dry matter stored in youngest rhizomes. The specific dry weight and nitrogen content of rhizomes which elongated during the growth season (April to September) did not increase, despite the presence of dense foliage. The rhizomes formed in this season were not used as storage organs. Thin and long rhizomes with low density are advantageous when there is a need for maximum elongation. Hence the rhizomes formed in this season have a role in dispersing the shoot apex as far as possible. In contrast with the youngest rhizomes, l- and 2-year-old rhizomes made little contribution to dry matter storage, because seasonal changes in specific dry weight and nitrogen content presented less fluctuation than in the youngest ones. This implies that the contribution of the older parts of rhizomes to dry matter storage is low and that distribution or transfer of materials does not occur through the rhizomes. The role of the rhizomes of M. trz$oliatu changes according to the season: thick and heavy rhizomes (pachymorphic) formed in the autumn have roles as storage organs and thin and light rhizomes (leptomorphic) formed in summer are used for vegetative expansion. The growth forms of rhizomes of M. trifoliatu are an advance over the simple type of rhizome with constant diameter. Rhizomes of Reineckia carnea Kunth also present a habit similar to that of M. trifooliata (Yamamura, 1984). Among the six functions of plant organs that serve as means of clonal reproduction, as summarized by Grace (19931, dispersal and storage are important when discussing the structure of the M. trifoliutu rhizome. It appears that there is a certain degree of trade-off between them. Rhizomes can function both as storage and dispersal organs. Pachymorphic rhizomes have a tendency to act as storage organs and leptomorphic rhizomes have a tendency to be dispersal organs. Rhizomes of M. trifoliatu with remarkable seasonal variation in diameter have both these functions. Stolons are very thin and highly suitable for vegetative expansion, with a special role of carrying shoots
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as far as possible from the parent plant. Glechoma hederacea L., for example, produces primary stolons with longer internodes under low light or low nutrient conditions @lade and Hutchings, 1987). This implies that the stolons of this species control the extent of shoot length according to environmental conditions. In this sense, the rhizomes of M. trifoliata formed in summer function as stolons. Rhizome size is a determining factor for bud initiation. The greater the rhizome weight, the greater the number of generative buds (Fig. 6). However, the condition of formation of rhizome lateral buds was restricted to medium-sized rhizomes, although the percentage of lateral bud formation was low (Fig. 6). Large shoots never formed lateral buds. Hutchings and Slade (1988) concluded that plants under low resource supply have fewer stolon branches per clone. However M. trifoliata with a high nutrient supply has long rhizomes without branches. This relates to the fact that elongation of rhizomes in a year is correlated significantly with elongation in the previous year; shoots which elongated well in a year also grew well the following year. These shoots were observed in the ambient open water area where nutrient concentration was higher than in the mat (Haraguchi and M atsui, 1990). Rhizomes of M. trifoliata tend to elongate by monopodial growth when conditions allow. Under conditions where elongation was slightly restricted by some factor, e.g. intraspecific or interspecific competition, the shoot tended to form lateral buds; however, the probability of this is low. With respect to this feature, the species shows a guerrilla type of growth, switching to phalanx-type growth under conditions where rhizome growth is restricted. This switching ability and the seasonal plasticity of rhizome growth, together with the small number of competing species, has allowed M. trzyoliata to become the predominant population in the floating peat mat in Mizorogaike Pond.
Acknowledgements
I thank members of the Center for Ecological Research, Kyoto University, for their helpful suggestions about this work. Parts of the calculations were carried out at the Data Processing Center, Hokkaido University.
References Abmhamson, W.G., 1975. Reproductive strategies in dewbenies. Ecology, 56: 721-726. Grace, J.B., 1993. The adaptive significance of clonal reproduction in angiosperms: an aquatic perspective. Aquat. Bot., 44: 159-180. Haraguchi, A., 1993. Phenotypic and phenological plasticity of an aquatic macrophyte Menyanthes trifofiata L. J. Plant Res., 106: 31-35. Haraguchi, A. and Matsui, K., 1990. Nutrient dynamics in a floating mat and pond system with special reference to its vegetation. Rcol. Res., 5: 63-79. Hartnett, D.C., 1990. Size-dependent allocation to sexual and vegetative reproduction in four clonal composites. Gecologia, 84: 254-259. Hewett, D.G., 1964. Menyanthes trijbliata L. J. Ecol., 52: 723-735. Hutchings, M.J. and Mogie, M., 1990. The spatial structure of clonal plants: control and consequences. In: J.
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van Groenendael and H. de Kroon (Editors), Clonal Growth in Plants: Regulation and Function. SPB Academic Publishing, The Hague, pp. 57-76. Hutchings, M.J. and Slade, A.J., 1988. Morphological plasticity, foraging and integration in clonal perennial herbs. In: A.J. Davy, M.J. Hutchings and A.R. Watkinson (Editors), Plant Population Ecology. Blackwell, Oxford, pp. 83-109. Kokawa. S., 1961. Distribution and phytostratigraphy of Menyanthes remains in Japan. J. Osaka City University, 12: 123- 15 1. Mogie, M. and Hutchings, M.J., 1990. Phylogeny, ontogeny and clonal growth in vascular plants. In: J. van Groenendael and H. de Kroon (Editors), Clonal Growth in Plants: Regulation and Function. SPB Academic Publishing, The Hague, pp. 3-22. Nishimura, M., 1983. Rhizome growth and matter production of the common buckbean. Menyunthes trijidiutu L. Physiol. Ecol. Jpn., 20: 115-128. Nishitani, S. and Kimura, M., 1993. Resource allocation to sexual and vegetative reproduction in a forest herb Syneilesis palmata (Compositae). Ecol. Res., 8: 173- 183. Ogden, J., 1974. The reproductive strategy of higher plants. 11. The reproductive strategy of Tussilago furfirru L. J. Ecol., 62: 291-324. Shimizu, Y., 1986. Species numbers, area, and habitat diversity on the habitat-islands of Mizorogaike Pond, Japan. Ecol. Res., I: 185-194. Slade, A.J. and Hutchings, M.J., 1987. Clonal integration and plasticity in foraging behaviour in Glechomu hederacea. J. Ecol., 75: 1023- 1036. Yamamura, Y., 1984. Matter production processes of Reineckiu carneu Kunth, an evergreen forest floor herb in the warm-temperate region of Japan. Bot. Mag. Tokyo, 97: 179- 191.