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May 16, 2009 - Key words: Burial depth, early growth, Myriophyllum oguraense Miki subsp. yangtzense, immature plant emer- gence, water depth. Introduction.
Fundam. Appl. Limnol., Arch. Hydrobiol. Stuttgart, March 2010

Vol. 176/3, 263–268

Article

Effects of burial depth and water depth on sprouting of turions and early growth of Myriophyllum oguraense Miki subsp. yangtzense Wang Cun-wei Zhan 1, Dong Wang 2, *, Xiao-feng Huang 2 and Jie Zhou 2 With 4 figures and 1 table Abstract: Sedimentation and water depth are important determinants of early growth and survivorship for aquatic

macrophytes. Myriophyllum oguraense subsp. yangtzense Wang is a submerged macrophyte endemic to China and grows readily in shallow lakes of the lower Yangtze River basin. To clarify the ecology, turion emergence and biomass allocation were investigated by buring turions at 0, 2, 4, and 6 cm depths in plastic pots with unsterilized lake sediment under 0, 15, and 60 cm water depths. Emergence percentage was greatly affected by burial depth and water depth. Turion sprouting rate was highest at 0 cm burial depth, and decreased with increasing burial depth at each water depth. Total biomass, plant height, above-ground stem mass ratio and root mass ratio were significantly affected by both burial depth and water depth. With increasing water depth, total biomass, plant height, root mass ratio, above-ground stem mass ratio and leaf mass ratio decreased, while below-ground stem mass ratio generally increased with increasing burial depth. Our data suggest that both burial depth and water depth are important determinants for the sprouting of turions and early growth process of M. oguraense subsp. yangtzense. We therefore conclude that sediment accumulation and water-level fluctuation might disrupt the natural regeneration for turions of M. oguraense subsp. yangtzense, which in turn influences its abundance, distribution and population dynamics. Key words: Burial depth, early growth, Myriophyllum oguraense Miki subsp. yangtzense, immature plant emer-

gence, water depth.

Introduction Sedimentation is an important factor influencing growth, survival, and development of plant propagules, due to its negative effects on seedling emergence and biomass production. Many studies have demonstrated that burial depth is an important determinant of growth and survivorship for aquatic macrophytes (Rybicki & Carter 1986, Spencer 1987, Spencer & Ksander 1990, 2002, Bonis & Lepart 1994, Clevering 1995, Dugdale et al. 2001, Peterson & Andrew 2004, Di Carlo et al.

2007). Water depth also determines the abundance, distribution, and growth of aquatic macrophytes (Spence 1982, Grace 1989, Vretare et al. 2001, Bonis & Grillas 2002, Sorrell et al. 2002, Hayball & Pearce 2004, Xiong et al. 2004, Boar 2006, Deegan et al. 2007). Many aquatic macrophytes rely more heavily on vegetative turions and tubers for regeneration than they do on seeds (Sculthorpe 1967, Weber & Nooden 1974, Newton et al. 1978). However, most previous studies on aquatic macrophytes were grown from seeds, tubers, and oospores (Rybicki & Carter 1986,

Authors’ addresses: 1

Appraisal Center for Environment and Engineering, Ministry of Environmental Protection, Beijing 100012, P. R. China. Laboratory of Ecology and Evolutionary Biology, College of Life Sciences, HuaZhong Normal University, Wuhan 430079, P. R. China. * Corresponding author; e-mail: [email protected] 2

© 2010 E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany DOI: 10.1127/1863-9135/2010/0176-0263

www.schweizerbart.de 1863-9135/10/0176-0263 $ 1.50

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Spencer 1987, Spencer & Ksander 1990, 2002, Bonis & Lepart 1994, Dugdale et al. 2001, Ke & Li 2006). Information on submerged plants that rely predominantly on turions for regeneration is lacking. Particularly, research on the combined effects of burial depth and water depth on turion emergence and early growth is relatively sparse. Myriophyllum oguraense subsp. yangtzense, a taxon described by Wang & Yu (2007), is an endemic submerged macrophyte confined to the lower Yangtze River basin, China. Our field survey found that plants typically grew at a depth of 0.6–0.8 (–1.0) m in the littoral zone of shallow lakes. It often sparsely grew in the community of M. spicatum, a widespread congeneric species which could also grow in deeper waters of the Lake. Further observation revealed that this plant relied mainly on turions for regeneration. In their natural habitat detached turions formed in late autumn and early winter, and were buried in 0–6 cm sediment, and remained dormant until germination the following spring. Thus, it seems that water-level fluctuation and sediment burial depth are crucial to sprouting of turions and early growth, which may further influence abundance, distribution and populations dynamics of M. oguraense subsp. yangtzense. Therefore, to answer how sediment burial depth and water depth affect turion emergence and allocation strategy of this endemic species is necessary. The purpose of this study was to determine the effects of burial depth and water depth on the sprouting of turions and early growth of M. oguraense subsp. yangtzense. To this end, turions of this species were planted at different burial and water depths in outdoor ponds to answer the following questions. First, do burial depth and/or water depth affect the sprouting of turions and early growth of plant seedlings? Second, does this species adjust its biomass allocation during the early growth period as a response to the different conditions?

Material and methods Plant material Turions of Myriophyllum oguraense subsp. yangtzense were collected from Liangzi Lake (30° 6′ – 30° 18′ N, 114° 24′ – 114° 36′ E), a large shallow lake in the Yangtze River basin, China, at the end of December 2006. The turions are usually present in lake sediment at burial depths of 0–6 cm. Experimental turion samples were collected from these layers of sediment. The turions averaged 2.8 ± 0.7 cm (mean ± SE) in length. Liangzi Lake has been separated from the Yangtze River by several dams. The water surface of the lake is 20.0 m above the sea level and average water depth is about 4.16 m. The lake-

shore water depth usually ranges between 0.5–2.0 m, but varies a lot and sometimes is completely dry in the winter, which caused by the drawdown of the Yangtze River in the winter and spring and flooding in the summer and autumn.

Experimental design The experimental design was a split-plot design, with water depth as main plot and sediment depth as sub-plot, and 4 pots × 6 blocs × 4 sediment depths × 3 water depths set-up was applied. Turion samples were buried in plastic pots (14.0 cm in diameter, 10.5 cm in depth) with unsterilized lake sediment collected from Liangzi Lake. The total nitrogen and phosphorus of the sediment was 2.9 mg g–1 and 0.13 mg g–1, respectively. Turions were randomly assigned to the pots with 0, 2, 4, and 6 cm depths, respectively. All the turions were placed horizontally and had the same orientation toward the sun. There were 24 pots per treatment and each pot contained only 1 turion. The pots were placed in three outdoor experimental ponds (2.0 m ×1.5 m), located at the South-Lake Campus of HuaZhong Normal University, each pond having 96 pots. Each pond was evenly divided into six plots, and four pots per sediment depth were included in each plot. The ponds were designated to receive a water depth of 0, 15, or 60 cm, respectively and tap water was used to maintain water depth. The water depths were manipulated and restored every three days. The noon photon irradiance was 1800–2300 µmol m–2 s–1 during the experiment.

Measurements Plant emergence was defined as the first appearance of a plant at the sediment surface. Immature plant sprouts were counted daily. After about 8 weeks (from January 2007 to March 2007) of growth the experiment was terminated. All plants were first measured for height above the sediment surface and then harvested. At harvest, care was taken to collect unemerged turions. Plants were separated into four parts: above-ground stem, below-ground stem, root, and leaf. After drying at 70° C for 24 h, each segment was weighed to measure biomass production. Plant size was measured in terms of total biomass and height. Root mass ratio was defined as the ratio of root dry weight to total biomass. Above-ground stem mass ratio was defined as above-ground stem dry weight to total biomass. Below-ground stem ratio was defined as below-ground stem dry weight to total biomass, and leaf mass ratio was defined as leaf dry weight to total biomass (expressed as %). All data in each plot were averaged for statistical analysis.

Statistical analysis Our experiment was a split-plot design, so split-plot ANOVAs, with water depth as main plot and sediment depth as sub-plot, were applied to test for the combined effects on emergence percentage, plant height, total biomass, below-ground stem mass ratio, above-ground stem mass ratio, leaf mass ratio, and root mass ratio as dependent variables. Multiple comparisons of means were performed by Duncan’s test at the 0.05 significance level. Data were log10-transformed if necessary to meet with the assumptions of normality and heterogeneity. Normality was assessed by Kurtosis test and homogeneity by Levene’s test. The statistical package DPS 3.0 was employed for all analyses.

Sprouting of Myriophyllum turions

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Plant size

Results Sprouting of turions and emergence percentage

The percentage of plant emergence was highest (100 %) at 0 cm burial depth under all water depths (Fig. 1). With increased burial depth the plant emergence percentage decreased linearly, with emergence at 4 and 6 cm burial depths being much lower than that at 0 and 2 cm. At 6 cm burial depth only 1 or 2 plants emerged, and turions, which did not emerge had died (Fig. 1). The statistical analysis showed that emergence percentage was greatly affected by burial depth (p < 0.001) and water depth (p < 0.01), and the effect of water depth was greatly dependent on burial depth (with significant burial depth × water depth interaction, p < 0.01, Table 1).

Total biomass was greatly affected by water depth and burial depth, respectively (p < 0.01, Table 1). Plant height showed the same tendency as total biomass, and was also significantly affected by burial depth and water depth, respectively (p < 0.01, Table 1). There was no burial depth × water depth interaction on biomass and plant height (Table 1). Both biomass and plant height generally decreased with increasing water depth and burial depth (Figs 2 and 3). At each water depth, total biomass and plant height were lowest for 6 cm burial depth and highest for 0 cm burial depth (Figs 2 and 3). Biomass allocation

Above-ground stem mass ratio and root mass ratio were significantly affected by both burial depth and water depth (p < 0.001, Table 1), while below-ground

Fig. 1. Turion emergence percentage (%) of M. oguraense

Fig. 2. Plant biomass of M. oguraense subsp. yangtzense at four

subsp. yangtzense at four burial depths and three water depths. Values are means ± SE (n = 6).

burial depths and three water depths. Values are means ± SE (n = 6).

Table 1. F-values and significances of split-plot ANOVAs of the effects of burial depth and water depth on emergence percentage, plant height, total dry weight and biomass allocation ratio of M. oguraense subsp. yangtzense (F-values).

Dependent variable Emergence percentage (%) Total biomass (g) Plant height (cm) Below-ground stem mass ratio Above-ground stem mass ratio Root mass ratio Leaf mass ratio d.f.

Burial depth (BD)

Water depth (WD)

BD × WD

452.129 *** 19.614 *** 94.102 *** 8.491 *** 60.950 *** 47.633 *** 147.559 *** 3

9.544 ** 13.091 ** 21.366 *** 2.114 ns 68.657 *** 14.961 *** 1.961 ns 2

4.541 ** 0.110 ns 1.853 ns 0.307 ns 6.634 *** 3.895 ** 2.751 * 6

***, p < 0.001; **, p < 0.01; *, p < 0.05; ns, p > 0.05

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Cun-wei Zhan, Dong Wang, Xiao-feng Huang and Jie Zhou

stem mass ratio and leaf mass ratio were only affected by burial depth, rather than by water depth (Table 1). In addition, burial depth and water depth had a significant interactive effect on all dependent variables with the exception of below-ground stem mass ratio (Table 1). In each water depth, root mass ratio, aboveground stem mass ratio and leaf mass ratio decreased with increasing burial depth. Below-ground stem mass ratio generally increased with increasing burial depth (Fig. 4).

Fig. 3. Plant height of M. oguraense subsp. yangtzense at four

burial depths and three water depths. Values are means ± SE (n = 6).

Discussion Effects of burial depth and water depth on sprouting of turions

Both burial depth and water depth depressed the sprouting of turions in M. oguraense subsp. yangtzense, while the effect of water depth was greatly dependent on burial depth. Reduced seedling emergence with increasing burial depth has also been noted in other aquatic macrophytes grown from seeds, tubers, and oospores (Rybicki & Carter 1986, Spencer 1987, Spencer & Ksander 1990, 2002, Chen & Maun 1999, Dugdale et al. 2001, Ke & Li 2006). Turions of M. oguraense subsp. yangtzense can sprout at all burial depths and water depths in the experiment, but emergence percentage decreased as both burial depth and water depth increased. Plant survival rate declined from 100 % to less than 5 % with increased burial depth. Only a few turions emerged at burial depths of 4 and 6 cm, the remainder of material being dead black tissue. The examination of all unemerged turions showed that a proportion of the turions had germinated, but the immature plants failed to emerge above the sediment surface. This suggested that turion reserves may be exhausted before the immature plant can reach the sediment surface. Failure of the plant to emerge from the sediment will cause plant death and therefore burial depth is an important factor controlling the abundance of M. oguraense subsp. yangtzense. Furthermore, it is the upper 2 cm of sediment that contains most of the

Fig. 4. Biomass allocation of immature

plant of M. oguraense subsp. yangtzense at four burial depths and three water depths. Values are means ± SE (n = 6).

Sprouting of Myriophyllum turions

“ecologically active” turions, which may play a key role in determining population dynamics of M. oguraense subsp. yangtzense. Our results imply that sediment accumulation might influence the distribution of M. oguraense subsp. yangtzense, as the species prefers lentic waters to flowing waters, which are easily subject to sediment deposition and accumulation (Wang & Yu 2007). Effects of burial depth and water depth on plant height

In this study, increased burial depth and water depth depressed immature plant height of M. oguraense subsp. yangtzense, an occurrence that coincides with previous findings that burial depth and water depth depress the vertical growth of submerged plants grown from seeds, tubers, and oospores (Rybicki & Carter 1986, Spencer 1987, Spencer & Ksander 1990, 2002, Bonis & Lepart 1994, Dugdale et al. 2001, Ke & Li 2006). This may indicate that both burial depth and water depth are also key determinants of early growth for submerged plants grown from turions. The reasons for a decrease in M. oguraense subsp. yangtzense height with increasing burial depth and water depth may be that: (1) turions from greater burial depths usually need more time to penetrate the sediment, and thus emerge later in the growing season than those from shallower burial depths, vertical growth will then be less than that of shallower turions, (2) initial propagules allocate resources less to vertical growth and more to below-ground stem near the sediment surface, which is a typical growth response of shallow-rooted aquatic species to sediment accumulation, as reported for Potamogeton gramineus L. and Zannichellia palustris L. (Haslam 1978, Spencer & Ksander 1990, 2002), and (3) deeper water may constrain plant growth by limiting the availability of resources such as light, atmospheric carbon, and oxygen, all important determinants for growth of submerged macrophytes (Crawford 1992, Jian et al. 2003, Going, et al. 2008). Effects of burial depth and water depth on biomass production and allocation

Biomass production decreased with increasing burial depth in M. oguraense subsp. yangtzense, which is consistent with other submerged species, such as Potamogeton pectinatus (Spencer 1987) and P. gramineus (Spencer & Ksander 1990). Additionally, water depth also has a significant impact on total biomass pro-

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duction in this species, this in contrast to Deegan et al. (2007) who reported that biomass production in emergent macrophyte species did not depend heavily on water depth. Our results imply that differences in growth form of aquatic macrophytes may contribute to the variation in biomass production at different water depths. Biomass allocation pattern of the species M. oguraense subsp. yangtzense was significantly affected by burial depth. Root mass ratio, above-ground stem mass ratio and leaf mass ratio decreased, but below-ground stem mass ratio increased with increasing burial depth. Adjustment of biomass allocation pattern suggests that the immature plant allocates lower energy reserves to the roots, diverting the rest to the below-ground stem in order to better facilitate sprouting from deep sediment. Such behavior is a typical growth response of shallowrooted species to sediment accumulation (Spencer & Ksander 1990, 2002). As a result, shallow-rooted species may be more susceptible to uprooting. Interestingly, there is a similar biomass allocation pattern in some grass species. In sand dunes, seedlings that grew from deeper location of soil had a longer first internode and shorter roots than those seedlings growing from shallower location of soil (Maun & Riach 1981, Redmann & Qi 1992).

Conclusion Myriophyllum oguraense subsp. yangtzense is an endemic submerged macrophyte confined to the lower Yangtze River basin, China. The plants rely heavily on turions for regeneration. Thus, the factors affecting sprouting of turions and their early growth are crucial to the survival and growth of the species. The results of this study indicate that burial of 4 cm or more will hinder the sprouting of turions. Burial of turions in the upper 2 cm of sediment, which contains most of the “ecologically active” turions, may play a crucial role in facilitating growth and determining population dynamics of M. oguraense subsp. yangtzense. In addition, the results are congruent with our observations from nature that this endemic species grow readily in lentic and shallow waters, but increasing water depth and turion burial depth strongly depressed turion emergence. This implies that sediment accumulation and deeper water may disrupt the natural regeneration of M. oguraense subsp. yangtzense populations. These findings may contribute to the understanding of distributional pattern, population dynamics and conservation of this endemic species.

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Acknowledgements We are sincerely grateful to Dr. Elisabeth Gross and two anonymous referees for their critical and helpful comments of the manuscript. Dr. Michele Funston is thanked for correcting the English. This research was founded by the Grants from the key project of Chinese Ministry of Education (105110), the National Science Foundation of China (3067047), and the selfdetermined research funds of CCNU fom the colleges’ basic research and operation of MOE (CCNU09B01002).

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