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Effects of simulated submergence on survival and recovery growth of three species in water fluctuation zone of the Three Gorges reservoir
Acta Ecologica Sinica 30 (2010) 216–220
Contents lists available at ScienceDirect
Acta Ecologica Sinica journal homepage: www.elsevier.com/locate/chnaes
Effects of simulated submergence on survival and recovery growth of three species in water fluctuation zone of the Three Gorges reservoir Liao Jianxiong, Jiang Mingxi *, Li Lianfa Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden, Chinese Academy of Sciences, Wuhan 430074, China
a r t i c l e
i n f o
Keywords: Paspalum distichum Cynodon dactylon Hemarthria altissima Water fluctuation zone of the Three Gorges reservoir Survival Recovery growth
a b s t r a c t Paspalum distichum, Cynodon dactylon and Hemarthria altissima distribute widely in natural water fluctuation zone of the Three Gorges region. To investigate whether they are suitable for growing in the artificial water fluctuation zone, which has longer submergence time and different submergence rhythm, of the Three Gorges reservoir, three complete submergence depths (0.5, 1 and 2 m) were conducted for about 6 months (from 12 November 2007 to 30 April 2008), and the survival and recovery growth of the three species were recorded after re-emergence for two weeks. The three species could start recovery growth within one week and more than 50% plants could survive. Among the three species, P. distichum had the largest increments in branch number and maximum stem length, and the smallest root shoot ratio. C. dactylon, however, had the smallest maximum stem length increment, and its survival and branch number increment were both larger than those of H. altissima. For C. dactylon and H. altissima, the survival and branch number increment significantly increased, while maximum stem length increment tended to decrease when submergence depth went higher. For P. distichum, the survival and the shoot mass were the lowest after 2 m submergence depth, but the other parameters were not different among different submergence treatments. Compared with control plants, submergence increased root shoot ratio of C. dactylon and H. altissima, but did not affect that of P. distichum. These results demonstrated that the three species are submergence-tolerant and can be applied in vegetation reconstruction in water fluctuation zone of the Three Gorges reservoir. Meanwhile, the results also suggested that the three species developed different survival tactics during the long-term submergence. Ó 2010 Ecological Society of China. Published by Elsevier B.V. All rights reserved.
1. Introduction After the completion of the Three Gorges Project, an artificial water fluctuation zone with a maximum upright fall of 30 m (145–175 m) appears along the two banks. Comparing it with the natural water fluctuation zone before the construction of Three Gorges Project, which forms due to precipitation, snowmelt and so on, the main differences are as the follows: Newly formed water fluctuation zone prolongs submergence duration, which is at least 6 months below 155 m, and changes submergence time from summer into winter and spring. Specifically, from June to September, the natural flood season, water level is maintained at flood control restriction, 145 m. Shortly after the end of flooded season (about October), water begin to be impounded and the water level gradually rises to 175 m and maintains it till December. The following year, January–April is water supply period and the water level reduces gradually to 155 m. These great changes may cause many of existing plants difficult to survive in the new water fluctuation zone, and thus a ‘‘yellow” bare area will appear after water level * Corresponding author. E-mail address: [email protected] (M. Jiang).
declines, which greatly affects the ecological landscape, reservoir bank stability and service life of the Three Gorges reservoir [1,2]. Therefore, selection of promising plant species to adapt to the fluctuation rhythm, which can survive during long-term submergence and rapidly grow during soil emergence, is crucial for vegetation reconstruction in water fluctuation zone of the Three Gorges reservoir [3,4]. In general, submergence-tolerant plants develop persistence tactics to adapt to their environments of long-term submergence [5,6]. Different species, however, have different ways. For example, some species can maintain low levels of submerged net photosynthesis [7], some species can remain dormant through periods of submergence or anoxia [3,7], and some species reduce maintenance cost by slowing growth or aboveground loss or death [8]. After submergence, the remaining carbohydrate status is the key factor that determines the ability of plants to withstand submergence stress and regenerate in subsequent re-emergence [9–11]. Therefore, measuring the changes of growth during submergence and after re-emergence, will contribute to analyze adaptation strategies of plants to long-term submergence. In the natural water fluctuation zone of the Three Gorges, there exist many submergence-tolerant plants, which grow in autumn
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J. Liao et al. / Acta Ecologica Sinica 30 (2010) 216–220
and winter and remain dormant in summer [12,13]. They may disappear in the new water fluctuation zone due to the great changes such as submergence rhythm. However, some species such as P. distichum, C. dactylon and H. altissima, which can grow all year round or remain dormant only in low temperature of winter, may adapt to the natural as well as artificial water fluctuation zone. Wang et al. found more than 90% submerged plants of C. dactylon and H. altissima survived after 180 d submergence [14]. Their submergence time (from July to January of the following year), however, did not follow the rhythm of the artificial water fluctuation zone. Thus, further studies are still necessary to investigate whether P. distichum, C. dactylon and H. altissima can tolerate long-term submergence and hence become candidate species for vegetation reconstruction in water fluctuation zone of the Three Gorges reservoir. In this study, three complete submergence depths were conducted according to the submergence rhythm below 155 m of the Three Gorges reservoir, and the recovery growth abilities, the changes of branch number and maximum stem length, and the root and shoot mass of survival plants of P. distichum, C. dactylon and H. altissima were compared after re-emergence. The aims were to address the following specific questions: (1) How are the abilities of the three species to withstand long-term submergence stress? (2) How are the growth changes of the three species during the long-term submergence and post-submergence? Moreover, the adaptation strategies of the three species to long-term submergence and their applied potentialities in the water fluctuation zone of the Three Gorges reservoir were analyzed. 2. Materials and methods 2.1. Plants and treatments P. distichum, C. dactylon and H. altissima are all perennial herbs in the family Gramineae. P. distichum, which is drought-, moistureand shade-tolerant, can sprout all year round and maintain a certain growth in winter. C. dactylon favors moist and has strong drought and salt tolerance but poor shade and cold tolerance. The suitable growth temperature of C. dactylon is 20–32 °C and its growth will stop at temperature 6–9 °C and its aboveground parts will die after frost. H. altissima, which prefers heat but has strong tolerance to low temperature, can reproduce asexually throughout the year and keep green but grow slowly in subtropical winter. For each species, 8–10 cm long shoot cuttings were picked on September 24, 2007. After sprouting and growing for one month, 60 healthy cuttings of approximately equal height for each species were selected and replanted into 21 cm (diameter) 15 cm (depth) pots (five plants per pot), filled with a mixture of leaf mold, peat soil and sand (2:1:1, v/v/v). On November 12, submergence treatments were started at Aquarium of Submerged Plants of Wuhan Botanical Garden, Chinese Academy of Sciences. Three pots per treatment and per species were randomly selected and immersed in 0.5 m, 1 m, 2 m deep pools, which are always filled with clean and fresh water by keeping tap open, the remaining three pots as control were placed in the open space beside the pools and were weeded and watered regularly. 2.2. Measurements About 6 months later (30 April 2008), all submerged pots were re-exposed to air and the recovery growth abilities and survival were estimated by observing the emergence of new leaves in an individual plant every other day [11]. After 2 weeks, the increments in branch number (including all tillers emerge from a rhizome and secondary branches from a node) and maximum stem length (the length from stem base to the top of the plant) of all survival plants were calculated by comparing with their initial values
217
measured at the beginning of submergence (12 November 2007). After the measurements were made, all survival plants were dug up, washed carefully in a nylon bag, oven-dried at 80 °C for 24 h and weighted. 2.3. Statistical analysis Statistical analysis was conducted using SPSS 15.0 for windows (SPSS Inc., Chicago, USA). Differences of the increments in branch number and maximum stem length, root shoot ratio among species and treatments were tested by two-way ANOVA. Differences of root mass and shoot mass among treatments of the same species were tested by one-way ANOVA. When main effects were significant, significant differences between species in the same treatment and those between treatments in the same species were tested by Duncan’s multiple range test. 3. Results 3.1. Survival and recovery growth ability After submergence for about 6 months, most of P. distichum, C. dactylon and H. altissima could survive (Fig. 1). The survival of C. dactylon and H. altissima increased when submergence depth went higher. For P. distichum, however, the survival was the lowest after 2 m submergence depth. Among the three species, the survival of P. distichum was the highest after 0.5 m and 1 m submergence depths but the lowest after 2 m submergence depth. For any submergence depth treatments, the survival of C. dactylon was higher than H. altissima. After the long-term submergence, the recovery growth of C. dactylon first started and quickened with submergence depths (Fig. 1B). The recovery growth rate of H. altissima also increased with the increase of submergence depths, but it required the longest time, 6–8 d, to emerge new leaves and had the lowest recovery growth rate among the three species (Fig. 1C). P. distichum could start recovery growth after 4 days and its recovery growth rate decreased with the increase of submergence depths (Fig. 1A). 3.2. Increments in branch number and maximum stem length For survival P. distichum, branch number increment was not different among different treatments (Fig. 2A). For C. dactylon and H. altissima, branch number increment was tended to increase when submergence depth went higher. Compared with the control plants, branch number increment of C. dactylon decreased significantly only after 0.5 m submergence depth, while that of H. altissima decreased significantly after all submergence treatments. Among the three species, branch number increment of P. distichum was largest, intermediate for C. dactylon and smallest for H. altissima. In the order of control, 0.5 m, 1 m and 2 m submergence treatment, maximum stem length increment of the three species declined successively, but the difference in C. dactylon was most significant, followed by P. distichum, H. altissima was not significant (Fig. 2B). Among the three species, maximum stem length increment of C. dactylon was significantly lower than P. distichum and H. altissima, but the difference between the latter two species was not significant. 3.3. Root mass, shoot mass and root shoot ratio For any one species, the difference in root mass among different submergence treatments was not significant (Fig. 3A). Compared with the control plants, root mass of P. distichum did not differ after
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100
Percentage of plants with new emerging leaves (%)
(B)
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20 0
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0
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14
16
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12
14
16
(C)
80
H. altissima 60 40 20 0
0
2
4
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Days after re-emergence (d) Fig. 1. Percentage of plants with new emerging leaves of P. distichum (A), C. dactylon (B) and H. altissima (C) during re-emergence after different submergence depths.
5
B a b ab a
CK 0.5 m 1 m C (B) a c bc b
4 3 2 1 0
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Maximum stem length increment (cm)
Branch number increment
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A a a a a
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7.5
A a ab ab b
B a b b c
A a a a a
6.0 4.5 3.0 1.5 0.0
P. distichum C. dactylon H. altissima
Fig. 2. Increments in branch number (A) and maximum stem length (B) of P. distichum, C. dactylon and H. altissima subjected to different submergence depths. Different capital letters indicate significant differences among three species (p < 0.05). Different small letters indicate significant differences among four treatments of the same species (p < 0.05).
0.5 m submergence depth but decreased significantly after the other two submergence depths. In contrast, root mass of C. dactylon decreased significantly after 0.5 m and 1 m submergence treatments while no difference was found after 2 m submergence treatment. In H. altissima, root mass for all submergence treatments were significantly lower than that for the control treatment. For C. dactylon and H. altissima, shoot mass did not differ among different submergence treatments, and they were significantly lower than the control treatments (Fig. 3B). In P. distichum, shoot mass for the control and 0.5 m submergence treatments were significantly higher than that for 1 m and 2 m submergence treatments, but the difference between the control and 0.5 m submergence treatments was not significant. Root shoot ratio of the three species was not different among different submergence treatments (Fig. 3C). Compared with the
control plants, root shoot ratio of H. altissima was significantly higher while that of P. distichum remained almost unaffected after all submergence treatments. For C. dactylon, submergence treatments, except 0.5 m depth, increased significantly root shoot ratio. Among the three species, root shoot ratio of C. dactylon was the largest, and that of P. distichum was the smallest.
4. Discussion Survival and recovery growth after submergence are important factors to evaluate submergence-tolerance of plants [15,16]. After about 6 months of submergence (from 12 November 2007 to 30 April 2008) and subsequent re-emergence for 2 weeks, P. distichum, C. dactylon and H. altissima had different survival and recovery
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CK
(A)
0.15
a ab b b
a b b ab
a b b b
0.5 m
(B) 0.75
2m a a b b
a b b b
a b b b
0.60
Shoot mass (g)
0.12
Root mass (g)
1m
0.09 0.06
0.45 0.30
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0.15
0.00
0.00
P. distichum C. dactylon H. altissima
Root shoot ratio
(C)
0.4
B a a a a
P. distichum C. dactylon H. altissima A b ab a a
AB b a a a
0.3
0.2
0.1
0.0
P. distichum C. dactylon H. altissima Fig. 3. Root mass (A), shoot mass (B) and root shoot ratio (C) of P. distichum, C. dactylon and H. altissima subjected to different submergence depths. Different capital letters indicate significant differences among three species (p < 0.05). Different small letters indicate significant differences among four treatments of the same species (p < 0.05).
growth ability. The survival was the highest for C. dactylon and the lowest for P. distichum after 2 m submergence treatment, while the highest for P. distichum and the lowest for H. altissima after 0.5 m and 1 m submergence treatments. The survival of the three species was lower than that reported in Wang et al. [14] that C. dactylon and H. altissima were subjected 6 months submergence of 2.5 m depth (from July 2006 to January 2007), but the minimum value was above 50% and they could start recovery growth within one week. Therefore, the three species are submergence-tolerant and can be applied in vegetation reconstruction in water fluctuation zone of the Three Gorges reservoir. Comparing their growth changes during submergence and re-emergence, however, the three species could develop different survival tactics during the long-term submergence. Due to reflection and absorption of water, and absorption and scattering of dissolved substances, suspended soil particles, debris particles and plankton in water, light intensity decrease significantly with the increase of submergence depths. Reduced light availability greatly limit photosynthesis of submerged plants [17,18]. In order to escape from low light and hypoxia environments as soon as possible, some species can increase shoot elongation in the early submergence [18–20], while some species reduce consumption of reserves and survive prolonged submergence through growth retardation [3,5,20]. After long-term complete submergence, initiation of new leaves and their subsequent growth require carbohydrate reserves and too low remaining reserves will lead to poor recovery growth and even death of plants [9–11,21,22]. Therefore, if shoot elongation depletes the existing reserves during long-term complete submergence, it will hasten death of submerged plants [10]. If the increases in elongation growth are very closely correlated with net photosynthesis,
however, it may increase survival [7]. In this study, P. distichum had the largest increments in branch number and maximum stem length, and the smallest root shoot ratio, suggesting it had more shoot growth than the other two species during submergence. Its higher survival after 0.5 m and 1 m submergence, however, indicated it may maintain a certain net photosynthesis during submergence, at least in the early stage. When submergence depth increased to 2 m, net photosynthesis of P. distichum may reduce to negative, resulting in decrease of shoot mass and consequently poor survival percentage. In contrast, survival of C. dactylon and H. altissima increased with the increase of submergence depths, which might attribute to less elongation growth and root mass changes during deeper submergences because less shoot consumption can result in submerged plants survival longer time when reserves are limited [8,14]. The submergence time in water fluctuation zone of the Three Gorges reservoir is artificially regulated in winter and spring. In the early stage, non-submerged P. distichum, C. dactylon and H. altissima were still green, but their growth had slowed or stopped and photosynthates began to transport to belowground organs. For the complete submerged plants, however, the rapid drop in photosynthetic rate hastened aging and death of aboveground parts and reduced photosynthate accumulation in belowground parts, resulting in their lower maximum stem length increment, shoot mass and root mass than non-submerged plants. In general, longterm complete submergence has a greater effect on aboveground than on belowground parts [8,14]. In this study, similar results were also found in C. dactylon and H. altissima, which led to their higher root shoot ratio than control plants. However, all submergence treatments did not change root shoot ratio of P. distichum, indicating that more shoot growth, at least in the early submer-
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gence, partially offset the different effects of submergence on aboveground and belowground parts. In conclusion, P. distichum, C. dactylon and H. altissima could start recovery growth within 1 week and most plants could survive after about 6 months of submergence. However, the three species developed different survival tactics during the long-term submergence. P. distichum could maintain a certain net photosynthesis during submergence and hence more shoot growth did not decrease its survival. C. dactylon reduced consumption of reserves and survived prolonged submergence through growth retardation. Shoot growth of H. altissima was intermediate between the former two species, and its survival was lower but more than 50%, indicating it is also submergence-tolerant. Since the simulated submergence was carried out in pools, in which water is relatively static, less silt and the maximum depth is only 2 m, the survival of the three species will be far lower than the results of this study when they apply in vegetation reconstruction in water fluctuation zone of the Three Gorges reservoir, which wave erosion and silt covering appear frequently. For P. distichum, its survival will be lower than H. altissima because its adaptation strategy to long-term submergence is unique and muddy river water will inhibit its submerged net photosynthesis. Therefore, to determine which plants are more suitable for the vegetation recovery in water fluctuation zone of the Three Gorges reservoir, more field submergence experiments are urgently required. Acknowledgments This project was supported by the National Natural Science Foundation of China (30670368), the National Key Technology R&D Program of China (2006BAC10B01), Western China Action Programme of Chinese Academy of Sciences (KZCX2-XB2-07) and Wuhan Garden & Forest Bureau Project (200817). References [1] Y. Wang, Y.F. Liu, S.B. Liu, et al., Vegetation reconstruction in the water-levelfluctuation zone of the Three Gorges reservoir, Chinese Bulletin of Botany 22 (5) (2005) 513–522. [2] H. Yuan, L.A. Wang, Y.H. Zhang, et al., Health evaluation system of the waterlevel-fluctuation zone in the Three Gorges area, Resources and Environment in the Yangtze Basin 15 (2) (2006) 249–253. [3] F.L. Luo, B. Zeng, T. Chen, et al., Response to simulated flooding of photosynthesis and growth of riparian plant Salix variegata in the Three Gorges reservoir region of China, Journal of Plant Ecology 31 (5) (2007) 910– 918.
[4] L.M. Ma, Y.P. Tang, M. Zhang, et al., Evaluation of adaptability of plants in Water-Fluctuation-Zone of the Three Gorges Reservoir, Acta Ecologica Sinica 29 (4) (2009) 1885–1892. [5] J.B. He, G.M. Bögemann, H.M. van de Steeg, et al., Survival tactics of Ranunculus species in river floodplains, Oecologia 118 (1) (1999) 1–8. [6] W.Q. Wang, F.S. Zhang, The physiological and molecular mechanism of adaptation to anaerobiosis in higher plants, Plant Physiology Communications 37 (1) (2001) 63–70. [7] D.E. Lynn, S. Waldren, Survival of Ranunculus repens L. (Creeping Buttercup) in an amphibious habitat, Annals of Botany 91 (1) (2003) 75–84. [8] H. Cooling, G.G. Ganf, K.F. Walker, Leaf recruitment and elongation: an adaptive response to flooding in Villarsia reniformis, Aquatic Botany 70 (4) (2001) 281–294. [9] H.P. Singh, B.B. Singh, P.C. Ram, Submergence tolerance of rainfed lowland rice. Search for physiological marker traits, Journal of Plant Physiology 158 (7) (2001) 883–889. [10] R.K. Sarkar, J.N. Reddy, S.G. Sharma, et al., Physiological basis of submergence tolerance in rice and implications for crop improvement, Current Science 91 (7) (2006) 899–906. [11] D. Panda, S.G. Sharma, R.K. Sarkar, Chlorophyll fluorescence parameters, CO2 photosynthetic rate and regeneration capacity as a result of complete submergence and subsequent re-emergence in rice (Oryza sativa L.), Aquatic Botany 88 (2) (2008) 127–133. [12] Y. Wang, J.Q. Wu, Y. Tao, et al., Natural distribution and ex situ conservation of endemic species Myricaria laxiflora in water-level-fluctuation Zone within Three-Gorges reservoir area of Changjiang River, Journal of Wuhan Botanical Research 21 (5) (2003) 415–422. [13] Y. Wang, J.Q. Wu, H.W. Huang, et al., Quantitative analysis of plant communities in water-level-fluctuation zone within Three Gorges reservoir area of Changjiang River, Journal of Wuhan Botanical Research 22 (4) (2004) 307–314. [14] H.F. Wang, B. Zeng, Y. Li, et al., Effects of long-term submergence on survival and recovery growth of four riparian plant species in Three Gorges reservoir region, China, Journal of Plant Ecology 32 (5) (2008) 977–984. [15] L. Mommer, J.P.M. Lenssen, H. Huber, et al., Ecophysiological determinants of plant performance under flooding: a comparative study of seven plant families, Journal of Ecology 94 (6) (2006) 1117–1129. [16] Y. Li, B. Zeng, X.Q. Ye, et al., The effects of flooding on survival and recovery growth of the riparian plant Salix variegate Franch in Three Gorges reservoir region, Acta Ecologica Sinica 28 (5) (2008) 1923–1930. [17] L. Mommer, K.I.W. Visser, Underwater photosynthesis in flooded terrestrial plants: a matter of leaf plasticity, Annals of Botany 96 (4) (2005) 581–589. [18] L.A.C.J. Voesenek, T.D. Clomer, R. Pierik, et al., How plants cope with complete submergence, New Phytologist l70 (2) (2006) 213–226. [19] H.W. Groeneveld, L.A.C.J. Voesenek, Submergence-induced petiole elongation in Rumex palustris is controlled by developmental stage and storage compounds, Plant and Soil 253 (1) (2003) 115–123. [20] L.A.C.J. Voesenek, J.H.G.M. Rijnders, A.J.M. Peeters, et al., Plant hormones regulate fast shot elongation under water: from genes to communities, Ecology 85 (1) (2004) 16–27. [21] K.K. Das, R.K. Sarkar, A.M. Ismail, Elongation ability and non-structural carbohydrate levels in relation to submergence tolerance in rice, Plant Science 168 (1) (2005) 131–136. [22] H.F. Wang, B. Zeng, P. Qiao, et al., Survival and growth response of Vetiveria zizanioides, Acorus calamus and Alternanthera philoxeroides to long-term submergence, Acta Ecologica Sinica 28 (6) (2008) 2571–2580.
Contents lists available at ScienceDirect
Acta Ecologica Sinica journal homepage: www.elsevier.com/locate/chnaes
Effects of simulated submergence on survival and recovery growth of three species in water fluctuation zone of the Three Gorges reservoir Liao Jianxiong, Jiang Mingxi *, Li Lianfa Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden, Chinese Academy of Sciences, Wuhan 430074, China
a r t i c l e
i n f o
Keywords: Paspalum distichum Cynodon dactylon Hemarthria altissima Water fluctuation zone of the Three Gorges reservoir Survival Recovery growth
a b s t r a c t Paspalum distichum, Cynodon dactylon and Hemarthria altissima distribute widely in natural water fluctuation zone of the Three Gorges region. To investigate whether they are suitable for growing in the artificial water fluctuation zone, which has longer submergence time and different submergence rhythm, of the Three Gorges reservoir, three complete submergence depths (0.5, 1 and 2 m) were conducted for about 6 months (from 12 November 2007 to 30 April 2008), and the survival and recovery growth of the three species were recorded after re-emergence for two weeks. The three species could start recovery growth within one week and more than 50% plants could survive. Among the three species, P. distichum had the largest increments in branch number and maximum stem length, and the smallest root shoot ratio. C. dactylon, however, had the smallest maximum stem length increment, and its survival and branch number increment were both larger than those of H. altissima. For C. dactylon and H. altissima, the survival and branch number increment significantly increased, while maximum stem length increment tended to decrease when submergence depth went higher. For P. distichum, the survival and the shoot mass were the lowest after 2 m submergence depth, but the other parameters were not different among different submergence treatments. Compared with control plants, submergence increased root shoot ratio of C. dactylon and H. altissima, but did not affect that of P. distichum. These results demonstrated that the three species are submergence-tolerant and can be applied in vegetation reconstruction in water fluctuation zone of the Three Gorges reservoir. Meanwhile, the results also suggested that the three species developed different survival tactics during the long-term submergence. Ó 2010 Ecological Society of China. Published by Elsevier B.V. All rights reserved.
1. Introduction After the completion of the Three Gorges Project, an artificial water fluctuation zone with a maximum upright fall of 30 m (145–175 m) appears along the two banks. Comparing it with the natural water fluctuation zone before the construction of Three Gorges Project, which forms due to precipitation, snowmelt and so on, the main differences are as the follows: Newly formed water fluctuation zone prolongs submergence duration, which is at least 6 months below 155 m, and changes submergence time from summer into winter and spring. Specifically, from June to September, the natural flood season, water level is maintained at flood control restriction, 145 m. Shortly after the end of flooded season (about October), water begin to be impounded and the water level gradually rises to 175 m and maintains it till December. The following year, January–April is water supply period and the water level reduces gradually to 155 m. These great changes may cause many of existing plants difficult to survive in the new water fluctuation zone, and thus a ‘‘yellow” bare area will appear after water level * Corresponding author. E-mail address: [email protected] (M. Jiang).
declines, which greatly affects the ecological landscape, reservoir bank stability and service life of the Three Gorges reservoir [1,2]. Therefore, selection of promising plant species to adapt to the fluctuation rhythm, which can survive during long-term submergence and rapidly grow during soil emergence, is crucial for vegetation reconstruction in water fluctuation zone of the Three Gorges reservoir [3,4]. In general, submergence-tolerant plants develop persistence tactics to adapt to their environments of long-term submergence [5,6]. Different species, however, have different ways. For example, some species can maintain low levels of submerged net photosynthesis [7], some species can remain dormant through periods of submergence or anoxia [3,7], and some species reduce maintenance cost by slowing growth or aboveground loss or death [8]. After submergence, the remaining carbohydrate status is the key factor that determines the ability of plants to withstand submergence stress and regenerate in subsequent re-emergence [9–11]. Therefore, measuring the changes of growth during submergence and after re-emergence, will contribute to analyze adaptation strategies of plants to long-term submergence. In the natural water fluctuation zone of the Three Gorges, there exist many submergence-tolerant plants, which grow in autumn
1872-2032/$ - see front matter Ó 2010 Ecological Society of China. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.chnaes.2010.06.005
J. Liao et al. / Acta Ecologica Sinica 30 (2010) 216–220
and winter and remain dormant in summer [12,13]. They may disappear in the new water fluctuation zone due to the great changes such as submergence rhythm. However, some species such as P. distichum, C. dactylon and H. altissima, which can grow all year round or remain dormant only in low temperature of winter, may adapt to the natural as well as artificial water fluctuation zone. Wang et al. found more than 90% submerged plants of C. dactylon and H. altissima survived after 180 d submergence [14]. Their submergence time (from July to January of the following year), however, did not follow the rhythm of the artificial water fluctuation zone. Thus, further studies are still necessary to investigate whether P. distichum, C. dactylon and H. altissima can tolerate long-term submergence and hence become candidate species for vegetation reconstruction in water fluctuation zone of the Three Gorges reservoir. In this study, three complete submergence depths were conducted according to the submergence rhythm below 155 m of the Three Gorges reservoir, and the recovery growth abilities, the changes of branch number and maximum stem length, and the root and shoot mass of survival plants of P. distichum, C. dactylon and H. altissima were compared after re-emergence. The aims were to address the following specific questions: (1) How are the abilities of the three species to withstand long-term submergence stress? (2) How are the growth changes of the three species during the long-term submergence and post-submergence? Moreover, the adaptation strategies of the three species to long-term submergence and their applied potentialities in the water fluctuation zone of the Three Gorges reservoir were analyzed. 2. Materials and methods 2.1. Plants and treatments P. distichum, C. dactylon and H. altissima are all perennial herbs in the family Gramineae. P. distichum, which is drought-, moistureand shade-tolerant, can sprout all year round and maintain a certain growth in winter. C. dactylon favors moist and has strong drought and salt tolerance but poor shade and cold tolerance. The suitable growth temperature of C. dactylon is 20–32 °C and its growth will stop at temperature 6–9 °C and its aboveground parts will die after frost. H. altissima, which prefers heat but has strong tolerance to low temperature, can reproduce asexually throughout the year and keep green but grow slowly in subtropical winter. For each species, 8–10 cm long shoot cuttings were picked on September 24, 2007. After sprouting and growing for one month, 60 healthy cuttings of approximately equal height for each species were selected and replanted into 21 cm (diameter) 15 cm (depth) pots (five plants per pot), filled with a mixture of leaf mold, peat soil and sand (2:1:1, v/v/v). On November 12, submergence treatments were started at Aquarium of Submerged Plants of Wuhan Botanical Garden, Chinese Academy of Sciences. Three pots per treatment and per species were randomly selected and immersed in 0.5 m, 1 m, 2 m deep pools, which are always filled with clean and fresh water by keeping tap open, the remaining three pots as control were placed in the open space beside the pools and were weeded and watered regularly. 2.2. Measurements About 6 months later (30 April 2008), all submerged pots were re-exposed to air and the recovery growth abilities and survival were estimated by observing the emergence of new leaves in an individual plant every other day [11]. After 2 weeks, the increments in branch number (including all tillers emerge from a rhizome and secondary branches from a node) and maximum stem length (the length from stem base to the top of the plant) of all survival plants were calculated by comparing with their initial values
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measured at the beginning of submergence (12 November 2007). After the measurements were made, all survival plants were dug up, washed carefully in a nylon bag, oven-dried at 80 °C for 24 h and weighted. 2.3. Statistical analysis Statistical analysis was conducted using SPSS 15.0 for windows (SPSS Inc., Chicago, USA). Differences of the increments in branch number and maximum stem length, root shoot ratio among species and treatments were tested by two-way ANOVA. Differences of root mass and shoot mass among treatments of the same species were tested by one-way ANOVA. When main effects were significant, significant differences between species in the same treatment and those between treatments in the same species were tested by Duncan’s multiple range test. 3. Results 3.1. Survival and recovery growth ability After submergence for about 6 months, most of P. distichum, C. dactylon and H. altissima could survive (Fig. 1). The survival of C. dactylon and H. altissima increased when submergence depth went higher. For P. distichum, however, the survival was the lowest after 2 m submergence depth. Among the three species, the survival of P. distichum was the highest after 0.5 m and 1 m submergence depths but the lowest after 2 m submergence depth. For any submergence depth treatments, the survival of C. dactylon was higher than H. altissima. After the long-term submergence, the recovery growth of C. dactylon first started and quickened with submergence depths (Fig. 1B). The recovery growth rate of H. altissima also increased with the increase of submergence depths, but it required the longest time, 6–8 d, to emerge new leaves and had the lowest recovery growth rate among the three species (Fig. 1C). P. distichum could start recovery growth after 4 days and its recovery growth rate decreased with the increase of submergence depths (Fig. 1A). 3.2. Increments in branch number and maximum stem length For survival P. distichum, branch number increment was not different among different treatments (Fig. 2A). For C. dactylon and H. altissima, branch number increment was tended to increase when submergence depth went higher. Compared with the control plants, branch number increment of C. dactylon decreased significantly only after 0.5 m submergence depth, while that of H. altissima decreased significantly after all submergence treatments. Among the three species, branch number increment of P. distichum was largest, intermediate for C. dactylon and smallest for H. altissima. In the order of control, 0.5 m, 1 m and 2 m submergence treatment, maximum stem length increment of the three species declined successively, but the difference in C. dactylon was most significant, followed by P. distichum, H. altissima was not significant (Fig. 2B). Among the three species, maximum stem length increment of C. dactylon was significantly lower than P. distichum and H. altissima, but the difference between the latter two species was not significant. 3.3. Root mass, shoot mass and root shoot ratio For any one species, the difference in root mass among different submergence treatments was not significant (Fig. 3A). Compared with the control plants, root mass of P. distichum did not differ after
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100
Percentage of plants with new emerging leaves (%)
(B)
(A)
80
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C. dactylon
60 40
0.5 m 1m 2m
20 0
0
2
4
6
8
10
100
12
14
0
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4
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8
8
10
12
14
16
10
12
14
16
(C)
80
H. altissima 60 40 20 0
0
2
4
6
Days after re-emergence (d) Fig. 1. Percentage of plants with new emerging leaves of P. distichum (A), C. dactylon (B) and H. altissima (C) during re-emergence after different submergence depths.
5
B a b ab a
CK 0.5 m 1 m C (B) a c bc b
4 3 2 1 0
P. distichum C. dactylon H. altissima
Maximum stem length increment (cm)
Branch number increment
(A)
A a a a a
2m
7.5
A a ab ab b
B a b b c
A a a a a
6.0 4.5 3.0 1.5 0.0
P. distichum C. dactylon H. altissima
Fig. 2. Increments in branch number (A) and maximum stem length (B) of P. distichum, C. dactylon and H. altissima subjected to different submergence depths. Different capital letters indicate significant differences among three species (p < 0.05). Different small letters indicate significant differences among four treatments of the same species (p < 0.05).
0.5 m submergence depth but decreased significantly after the other two submergence depths. In contrast, root mass of C. dactylon decreased significantly after 0.5 m and 1 m submergence treatments while no difference was found after 2 m submergence treatment. In H. altissima, root mass for all submergence treatments were significantly lower than that for the control treatment. For C. dactylon and H. altissima, shoot mass did not differ among different submergence treatments, and they were significantly lower than the control treatments (Fig. 3B). In P. distichum, shoot mass for the control and 0.5 m submergence treatments were significantly higher than that for 1 m and 2 m submergence treatments, but the difference between the control and 0.5 m submergence treatments was not significant. Root shoot ratio of the three species was not different among different submergence treatments (Fig. 3C). Compared with the
control plants, root shoot ratio of H. altissima was significantly higher while that of P. distichum remained almost unaffected after all submergence treatments. For C. dactylon, submergence treatments, except 0.5 m depth, increased significantly root shoot ratio. Among the three species, root shoot ratio of C. dactylon was the largest, and that of P. distichum was the smallest.
4. Discussion Survival and recovery growth after submergence are important factors to evaluate submergence-tolerance of plants [15,16]. After about 6 months of submergence (from 12 November 2007 to 30 April 2008) and subsequent re-emergence for 2 weeks, P. distichum, C. dactylon and H. altissima had different survival and recovery
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CK
(A)
0.15
a ab b b
a b b ab
a b b b
0.5 m
(B) 0.75
2m a a b b
a b b b
a b b b
0.60
Shoot mass (g)
0.12
Root mass (g)
1m
0.09 0.06
0.45 0.30
0.03
0.15
0.00
0.00
P. distichum C. dactylon H. altissima
Root shoot ratio
(C)
0.4
B a a a a
P. distichum C. dactylon H. altissima A b ab a a
AB b a a a
0.3
0.2
0.1
0.0
P. distichum C. dactylon H. altissima Fig. 3. Root mass (A), shoot mass (B) and root shoot ratio (C) of P. distichum, C. dactylon and H. altissima subjected to different submergence depths. Different capital letters indicate significant differences among three species (p < 0.05). Different small letters indicate significant differences among four treatments of the same species (p < 0.05).
growth ability. The survival was the highest for C. dactylon and the lowest for P. distichum after 2 m submergence treatment, while the highest for P. distichum and the lowest for H. altissima after 0.5 m and 1 m submergence treatments. The survival of the three species was lower than that reported in Wang et al. [14] that C. dactylon and H. altissima were subjected 6 months submergence of 2.5 m depth (from July 2006 to January 2007), but the minimum value was above 50% and they could start recovery growth within one week. Therefore, the three species are submergence-tolerant and can be applied in vegetation reconstruction in water fluctuation zone of the Three Gorges reservoir. Comparing their growth changes during submergence and re-emergence, however, the three species could develop different survival tactics during the long-term submergence. Due to reflection and absorption of water, and absorption and scattering of dissolved substances, suspended soil particles, debris particles and plankton in water, light intensity decrease significantly with the increase of submergence depths. Reduced light availability greatly limit photosynthesis of submerged plants [17,18]. In order to escape from low light and hypoxia environments as soon as possible, some species can increase shoot elongation in the early submergence [18–20], while some species reduce consumption of reserves and survive prolonged submergence through growth retardation [3,5,20]. After long-term complete submergence, initiation of new leaves and their subsequent growth require carbohydrate reserves and too low remaining reserves will lead to poor recovery growth and even death of plants [9–11,21,22]. Therefore, if shoot elongation depletes the existing reserves during long-term complete submergence, it will hasten death of submerged plants [10]. If the increases in elongation growth are very closely correlated with net photosynthesis,
however, it may increase survival [7]. In this study, P. distichum had the largest increments in branch number and maximum stem length, and the smallest root shoot ratio, suggesting it had more shoot growth than the other two species during submergence. Its higher survival after 0.5 m and 1 m submergence, however, indicated it may maintain a certain net photosynthesis during submergence, at least in the early stage. When submergence depth increased to 2 m, net photosynthesis of P. distichum may reduce to negative, resulting in decrease of shoot mass and consequently poor survival percentage. In contrast, survival of C. dactylon and H. altissima increased with the increase of submergence depths, which might attribute to less elongation growth and root mass changes during deeper submergences because less shoot consumption can result in submerged plants survival longer time when reserves are limited [8,14]. The submergence time in water fluctuation zone of the Three Gorges reservoir is artificially regulated in winter and spring. In the early stage, non-submerged P. distichum, C. dactylon and H. altissima were still green, but their growth had slowed or stopped and photosynthates began to transport to belowground organs. For the complete submerged plants, however, the rapid drop in photosynthetic rate hastened aging and death of aboveground parts and reduced photosynthate accumulation in belowground parts, resulting in their lower maximum stem length increment, shoot mass and root mass than non-submerged plants. In general, longterm complete submergence has a greater effect on aboveground than on belowground parts [8,14]. In this study, similar results were also found in C. dactylon and H. altissima, which led to their higher root shoot ratio than control plants. However, all submergence treatments did not change root shoot ratio of P. distichum, indicating that more shoot growth, at least in the early submer-
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gence, partially offset the different effects of submergence on aboveground and belowground parts. In conclusion, P. distichum, C. dactylon and H. altissima could start recovery growth within 1 week and most plants could survive after about 6 months of submergence. However, the three species developed different survival tactics during the long-term submergence. P. distichum could maintain a certain net photosynthesis during submergence and hence more shoot growth did not decrease its survival. C. dactylon reduced consumption of reserves and survived prolonged submergence through growth retardation. Shoot growth of H. altissima was intermediate between the former two species, and its survival was lower but more than 50%, indicating it is also submergence-tolerant. Since the simulated submergence was carried out in pools, in which water is relatively static, less silt and the maximum depth is only 2 m, the survival of the three species will be far lower than the results of this study when they apply in vegetation reconstruction in water fluctuation zone of the Three Gorges reservoir, which wave erosion and silt covering appear frequently. For P. distichum, its survival will be lower than H. altissima because its adaptation strategy to long-term submergence is unique and muddy river water will inhibit its submerged net photosynthesis. Therefore, to determine which plants are more suitable for the vegetation recovery in water fluctuation zone of the Three Gorges reservoir, more field submergence experiments are urgently required. Acknowledgments This project was supported by the National Natural Science Foundation of China (30670368), the National Key Technology R&D Program of China (2006BAC10B01), Western China Action Programme of Chinese Academy of Sciences (KZCX2-XB2-07) and Wuhan Garden & Forest Bureau Project (200817). References [1] Y. Wang, Y.F. Liu, S.B. Liu, et al., Vegetation reconstruction in the water-levelfluctuation zone of the Three Gorges reservoir, Chinese Bulletin of Botany 22 (5) (2005) 513–522. [2] H. Yuan, L.A. Wang, Y.H. Zhang, et al., Health evaluation system of the waterlevel-fluctuation zone in the Three Gorges area, Resources and Environment in the Yangtze Basin 15 (2) (2006) 249–253. [3] F.L. Luo, B. Zeng, T. Chen, et al., Response to simulated flooding of photosynthesis and growth of riparian plant Salix variegata in the Three Gorges reservoir region of China, Journal of Plant Ecology 31 (5) (2007) 910– 918.
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