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Journal of Resources and Ecology >

2022 , Vol. 13 >Issue 2: 319 - 327

DOI: https://doi.org/10.5814/j.issn.1674-764x.2022.02.014

Soil and Agriculture-Forest Ecosystem

Seed and Fruiting Phenology Plasticity and Offspring Seed Germination Rate in Two Asteraceae Herbs Growing in Karst Soils with Varying Thickness and Water Availability

  • LIU Junting , 1, 2 ,
  • LI Suhui 1, 2 ,
  • SONG Haiyan 1, 2 ,
  • LEI Ying 1, 2 ,
  • CHEN Jinyi 1, 2 ,
  • WANG Jiamin 1, 2 ,
  • GUO Xuman 1, 2 ,
  • LIU Jinchun , 1, 2, *
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  • 1. Key Laboratory of Eco-Environments in Three Gorges Reservoir Region (Ministry of Education), Chongqing 400715, China
  • 2. Key Laboratory of Plant Ecology and Resources Research in Three Gorges Reservoir Region, School of Life Sciences, Southwest University, Chongqing 400715, China
* LIU Jinchun, E-mail:

LIU Junting, E-mail:

Received date: 2021-03-24

  Accepted date: 2021-10-19

  Online published: 2022-03-09

Supported by

The Chongqing Natural Science Foundation(cstc2020jcyj-msxmX0244)

The Special Funds for Basic Scientific Research in Central Universities(XDJK2020B037)

The National Natural Science Foundation of China(31500399)

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Abstract

Shallow soil with low water availability is the key limiting factor for plant growth and reproduction in vulnerable karst regions. Annual herbs are pioneers adapted to these areas; however, little is known about the responses of their seeds and infructescence, and the germination of their offspring to these limited water and soil resources. In this study, we investigated how the seed and fruiting phenology plasticity and offspring seed germination rates of two annual Asteraceae herbs (Xanthium sibiricum and Bidens pilosa) respond to the harsh karst soil environment, by assessing the seed number, seed biomass and offspring seed germination rate. X. sibiricum and B. pilosa were grown under three soil thicknesses and three water availability levels in a full two-way randomized block design. The key results were as follows: (1) The number and biomass of progenies (infructescence and seeds) of X. sibiricum decreased with the decline of soil thickness and/or water availability (P <0.05). The infructescence and seed biomass of B. pilosa increased with the decline of water availability. (2) Seed quantity and seed biomass of X. sibiricum showed no correlation after their parents experienced resource reductions. A significant positive relationship between seed number and seed biomass was observed in B. pilosa (P <0.05). (3) The offspring seed germination rate of X. sibiricum did not change with the decrease of soil thickness under three levels of water treatment. However, the offspring seed germination rate of B. pilosa decreased significantly with the decrease of soil thickness under the control water level (P<0.05). The results show that X. sibiricum tends to improve its competitiveness by ensuring the quantity and quality of offspring in order to adapt to the shallow karst soils and dry karst habitats. In contrast, B. pilosa adapts to the unfavorable karst habitats by a risk-sharing strategy. B. pilosa produces more and bigger seeds to in an attempt to expand its survival range and escape from the unfavorable living environment, but this results in a lower seed number and germination rate of its progeny under the karst soil resource reduction.

Key words： karst drought; shallow soil; seed number; parental environment; seed biomass

Cite this article

LIU Junting , LI Suhui , SONG Haiyan , LEI Ying , CHEN Jinyi , WANG Jiamin , GUO Xuman , LIU Jinchun . Seed and Fruiting Phenology Plasticity and Offspring Seed Germination Rate in Two Asteraceae Herbs Growing in Karst Soils with Varying Thickness and Water Availability[J]. Journal of Resources and Ecology, 2022 , 13(2) : 319 -327 . DOI: 10.5814/j.issn.1674-764x.2022.02.014

1 Introduction

Phenotypic plasticity is an important mechanism for organisms to adapt to heterogeneous environments and to enhance their viability (Ellers and Stuefer, 2010). It is also considered to be another possible path of biological evolution in addition to genetic variation (Li et al., 2018a). Thus, information on seed and infructescence phenology as related to fitness and reproductive strategy is important for understanding plant ecology and evolution (Megumi et al., 2020). Studies on seed and infructescence phenology plasticity of plants under different environmental conditions are likely to increase due to the ongoing global climatic change and the continuing pressure of human activities (Xiao et al., 2020; Luo et al., 2021).
The environment experienced by parental plants directly affects the seed and infructescence traits of those plants, so as to maintain the quality of their offspring at a stable level and ensure the seedlings which develop can adapt to the new environment (Sultan, 2000). For example, Chen et al. (2018) found that the water environment experienced by Caragana korshinskii parents directly affected the seed number and weight of the offspring—after the parents experienced a drought environment, the seed number and yield per branch length and the weight per single seed were significantly lower than those of the control. Another study found that different sowing times of parental plants resulted in different seed biomass allocations of the offspring. In that study, when the parents of Suaeda coeniculata were sown earlier, they allocated less biomass to their offspring seeds; and conversely, when the parents were sown later, their offspring had greater seed biomass (Liu et al., 2015). In addition, the CO2 partial pressure experienced by the parents has a significant effect on the progeny seeds of Arabidopsis thaliana. The individuals grown under high CO2 partial pressure have smaller seeds and lower reserves in the tissues (Teng, 2006). The seeds of Asteraceae plants with atypia can improve their seed dispersal ability under adverse conditions (Eric and Ophélie, 2001). It seems that parents hope to help their offspring escape from the bad habitats. In short, a large number of studies have shown that plants will actively adjust the phenotype of their offspring to adapt to the environment.
A complete plant life cycle includes seed germination, plant growth and development, flowering, infructescence, and then seed germination (Lv, 2015). The germination of seeds produced by parents is the most important and sensitive stage in the life history of plants (Zhu, 2019). It is this step that completes the entire life cycle, connecting parents and offspring. The changes in the environment experienced by parents, such as light, temperature and water, often have an important influence on the germination of offspring seeds (Kathleen, 2009). Some studies have shown that the light environment experienced by the female parent directly affects the seed germination rate and germination season of the offspring. The seeds produced by the parent plants growing under sufficient light have a higher germination rate in autumn, while the seeds produced by the parent plants growing under shade conditions have a higher germination rate in spring. In addition, the flowering time of parents can also affect the seed germination phenology. The offspring seeds produced by parents with early flowering are more likely to germinate in autumn compared with those seeds produced by parents with late flowering (Galloway, 2005). This may be due to the fact that the seeds produced by early flowering parents have a longer germination time before winter when the temperature and water conditions are also conducive to offspring germination in early autumn. Some studies have shown that the offspring seed germination rate is higher when the parent A. thaliana plants complete the growth process at 22 ℃, compared with the offspring seeds whose parents complete the growth process at 14 ℃. This is because A. thaliana seeds mature in May to June, when the daytime temperature in this area is about 20℃, consistent with the temperature experienced by most wild plants during seed maturation, which is the optimum germination temperature (Schmuths et al., 2006). Water is one of the prerequisites for plant germination. In the arid and semi-arid Ordos plateau, the water conditions of the female parent will determine the geographical distribution of a species. Three species of Caragana are distributed successively along the precipitation gradient from west to east. According to their drought tolerance, the order of distribution from east to west is Caragana korshinskii, Caragana intermedia, and then Caragana microphylla (Lai et al., 2015). In summary, different environments experienced by parent plants have different influences on the offspring seed germination.
Karst areas host a unique ecosystem, and are distributed widely around the world, especially in southwest China (Wang et al., 2019). Drought and soil heterogeneity are the key limiting factors that restrain plant regeneration in karst areas (Jiang et al., 2014). The special aboveground and underground two-layer structure of the karst leads to serious soil and water leakage and poor soil water retention (Chen et al., 2018). On the other hand, the karst area has complex and diverse habitats with obviously discontinuous and highly heterogeneous limestone soil due to the combination of exposed rocks, a tattered land surface, steep slopes, and soil accumulation in depressions (Jiang et al., 2020). Therefore, even though the total rainfall is sufficient in karst areas, plants here are often suffering from drought stress (He et al., 2011). In recent years, with the frequent occurrence of drought events in karst areas, studying how plants respond to “karst drought” has attracted more and more attention (Zhao et al., 2017a; Zhang et al., 2019). However, most research has focused on the nutritive organs (Zhang et al., 2019), while little is known about the responses of seed and fruiting traits, or the germination of their offspring to these limited water and soil resources. Therefore, it is important to investigate the differences in the plants from the aspects of fruiting and growth strategies of plants in karst areas.
Xanthium sibiricum Patrin ex Widder and Bidens pilosa L. are annual herbaceous plants in the family Asteraceae, and pioneer species in the early stage of vegetation succession. Both of them have well-developed roots, high tolerance to drought and barren conditions, and strong environmental adaptability, so they are common herbaceous species in karst areas. Many studies show that karst plants can adapt to the special habitats of this region by changing their morphological plasticity and physical and chemical properties (Zhao et al., 2017b). For example, the decrease of soil thickness and water availability reduce the photosynthesis (Zhao et al., 2017a; Li et al., 2018b) and the biomass allocation ratio in karst plants to improve the utilization efficiency of water and soil resources (Yan et al., 2008; Li et al., 2017). Our previous studies have also shown that reduced soil thickness and moisture inhibit the growth of X. sibiricum and B. pilosa, but both plants maintain stable investments in the roots and leaves at the expense of the stem investment in order to maintain stable light and water use efficiencies (unpublished data). However, little is known about whether the reduced soil thickness and water availability experienced by X. sibiricum and B. pilosa during the growth period have any impacts on the seed and infructescence phenology or on the germination rate of their offspring; or whether these effects further improve their adaptability to the special karst habitats.
Although both X. sibiricum and B. pilosa show relatively consistent adaptation strategies during the growth period, they tend to have “r-selected” life history strategies (Li et al., 2021). However, they are different in plant size, seed size and root depth. On the other hand, high heterogeneity in the thickness of the soil and water availability may lead to ecological niche differentiation in karst habitats (Liu et al., 2020). As a result, different plants with similar life history strategies may still have differences in their responses or strategies in their seed and infructescence traits and offspring seed germination. We speculate that B. pilosa, with a small plant size, a small seed size and shallow roots, may still produce smaller and more numerous seeds under the environment lacking in water and soil resources, but it would be unable to maintain a high germination rate, that is, it would choose an r-strategy. While X. sibiricum, which has a large plant size, a large seed size and deep roots, may tend to produce larger and fewer seed with a high germination rate, that is, there would be a tendency to approach the K end in the r-K continuum.
Therefore, on the basis of previous studies, we took X. sibiricum and B. pilosa as the research objects, and treated them with different water availability and soil thickness conditions. Then the seed (infructescence) number, seed biomass and the seed germination rate of their offspring were checked in order to explore the effects of the environmental variations experienced by parents on the seed and infructescence phenology plasticity and germination rate of their offspring in karst areas. The research results are expected to contribute to a theoretical basis for vegetation restoration, species recruitment strategies and rocky desertification management in karst areas.

2 Methods

2.1 Experimental materials and design

The experiment was carried out in the ecological garden of Southwest University (29°49'N, 106°25'E), at an altitude of 245 m. The annual average temperature is 18 ℃, the annual average relative humidity is 80%, and the annual average precipitation ranges from 1000 to 1350 mm. The test materials were X. sibiricum and B. pilosa, two common annual herbs in karst environments. The seeds of both were collected in Zhongliang Mountain (a typical karst area), located in Beibei, Chongqing during the seed maturity season. The mother plants of the collected seeds grow in areas with a similar middle amount of soil thickness. The experimental soil was yellow limestone soil, also taken from Zhongliang Mountain. The soil had a pH of 7.4±0.14, an organic matter content of 0.34%±0.02%, total nitrogen content of 0.28± 0.03 g kg-1, total phosphorus content of 0.39±0.02 g kg-1, total potassium content of 23.7±3.22 g kg-1, and field water holding capacity of 39.8%±2.23%.
On March 5, 2018, the seeds of X. sibiricum and B. pilosa were sown in seedling trays filled with karst soil. After 45 days, we selected the seedlings of similar size and transferred them to the center of each container designed as described below. Previous results have shown that more than 80% of the soil in the karst area is less than 40 cm deep, and 28.9% is less than 10 cm deep. In scattered places, the thickness of the soil is greater than 50 cm (Zhou et al., 2010; Zhao et al., 2019). We also found the same situation in the field investigation in Zhongliang Mountain. Therefore, in order to mimic the karst stone gullies and stone crevice niches, we set up three soil thickness levels: 10 cm (filled with 1000 g of dry soil, SL), 40 cm (4000 g of dry soil, SM), and 70 cm (7000 g of dry soil, SCK), in three kinds of home-made rectangular containers with the same bottom area (10 cm×10 cm). The bottoms of the containers were provided with water holes of the same size and number. The soil was kept moist to ensure the survival and growth of the seedlings before the treatments. On May 20, 2018, after a month of acclimation for the seedlings, three water treatments were set for plants of the same height (about 20 cm). According to the average daily rainfall per unit area of the Chongqing area from 1981 to 2011 and the basal area of the containers, we calculated the amount of water to apply. We set the average daily rainfall in Chongqing as the control (WCK), while a 50% reduction was the mild water reduction group (WM), and a 70% reduction was the severe water reduction group (WL) (Table 1). Each container was watered once every three days. Therefore, the full-factorial design included two factors, namely the thickness of the soil (SCK, SM and SL) and water availability (WCK, WM and WL). There were nine treatments in total, with 10 replicates of each treatment. In addition, during the pot experiment, the soil moisture content was tracked and determined. Before every water treatment, mixed soil samples from different soil layers were collected by a five-point sampling method at the four corners and in the middle of the container. Soil samples were mixed evenly, put in an aluminum box, and taken back to the laboratory for soil moisture content measurement.
Table 1 Water design during the test
Months WCK WM WL
Apr.-Jun. 130 ml (3 d) -1 65 ml (3 d) -1 43 ml (3 d) -1
Jul.-Sept. 125 ml (3 d) -1 63 ml (3 d) -1 38 ml (3 d) -1
Oct.-Dec. 56 ml (3 d) -1 28 ml (3 d) -1 19 ml (3 d) -1

Note: WCK, WM and WL: high (no reduction in water), moderate (50% reduction in water) and low (70% reduction in water) water availability, respectively.

After completing the life cycle, the infructescence and seeds of each plant were harvested separately, and the harvested seeds were used for the germination experiment. The seeds harvested after the treatment were cultivated in a light incubator. In order to simulate the field environment, the parameters of the light incubator were set to alternate light and dark for 12 hours and a temperature of 25 ℃. Taking 1 mm of the radicle exposed from the seed coat as the “germinated seeds”, the number of germinated seeds was counted every 24 h, until no new seeds germinated for 3 consecutive days. Then the germination rate of the progeny seeds was calculated.

2.2 Measurements

The infructescence sequences which matured naturally under the different treatments were collected and dried in an oven at 60 ℃ to constant weight, and then the dry weight of the infructescence sequences was measured. The seeds were separated from the infructescence sequence and the dry weight was determined. The infructescences and seeds of each plant were counted and weighed to obtain the total number of infructescences, total number of seeds, total infructescence biomass and total seed biomass of each plant.
$RWC = \frac{{SWC}}{{FC}} \times 100\%$
where, RWC indicate relative soil water content, SWC indicate soil water content, FC indicate field water holding capacity.
$GR = \frac{{{N_2}}}{{{N_1}}} \times 100\%$
where, GR indicate offspring seed germination rate, N1 indicate total number of seeds tested, N2 indicate total number of seeds germinated after the end of the germination test.

2.3 Statistical analysis

All data processing and statistical analyses were conducted using SPSS 22.0. The effects of water, the thickness of the soil and their interactions on soil moisture content, total numbers of infructescences and seeds, total infructescence biomass and total seed biomass were evaluated by Two-way ANOVA. The differences between the different soil thicknesses for each species under the same water treatment were analyzed by One-way ANOVA. Linear regression between seed number and seed biomass was analyzed by Origin 2021. Significant differences at the 0.05 level (P<0.05) were determined. All figures were produced using Origin 2021.

3 Results

3.1 Soil water content

The soil water content decreased significantly with the decrease of water supply at a given thickness of soil treatment. However, it did not show a significant difference between the soil thickness treatments under WCK and WM, but decreased significantly under WL (P<0.05) (Table 2).
Table 2 Relative soil water content under different treatments
Water application
The thickness of the soil WCK WM WL
Soil moisture
content (%)		 Relative water content Soil moisture content (%) Relative water content Soil moisture content (%) Relative water content
SCK 19.16±0.71Aa 48%FC 14.13±0.32Ba 36%FC 10.81±0.33Ca 27%FC
SM 20.55±0.53Aa 51%FC 12.86±0.46Ba 32%FC 9.36±0.70Ca 24%FC
SL 21.05±0.57Aa 52%FC 12.60±0.71Ba 32%FC 7.47±0.38Cb 19%FC

Note: WCK, WM and WL: high (no reduction in water), moderate (50% reduction in water) and low (70% reduction in water) water availability, respectively. SCK, SM and SL: deep (70 cm), moderate (40 cm) and shallow (10 cm) thickness of the soil, respectively. Lowercase letters indicate significant differences between different thicknesses of the soil at the 0.05 level, and uppercase letters indicate significant differences between different water availability levels at the 0.05 level. FC: field water holding capacity.

3.2 Infructescence (and seed) number and biomass

Under the three water conditions, the infructescence and seed number, and the infructescence and seed biomass of X. sibiricum and B. pilosa all decreased significantly with the reduction in the thickness of the soil (P<0.05). Further, the amplitudes of decrease for these four indexes in X. sibiricum were significantly lower than those in B. pilosa (Fig. 1).
Fig. 1 Effects of different thicknesses of the soil and water availability on infructescence (and seed) number and biomass of X. sibiricum and B. pilosa (M±SE)

Notes: WCK, WM and WL: high (no reduction in water), moderate (50% reduction in water) and low (70% reduction in water) water availability, respectively. SCK, SM and SL: deep (70 cm), moderate (40 cm) and shallow (10 cm) thickness of the soil, respectively. Different lowercase letters indicate significant differences between different thicknesses of the soil under the same water availability (P < 0.05). Different uppercase letters indicate significant differences between different water availability levels under the same thickness of the soil.

In X. sibiricum, the infructescence and seed number, and the infructescence and seed biomass showed decreasing trends with the decrease of water application (P<0.05). In B. pilosa, the infructescence and seed biomass actually tended to increase with the decrease of water application under SM and SL (Fig. 1).
Significant thickness of soil ×water availability interactions were observed for infructescence number, seed number, inflorescence biomass and seed biomass of X. sibiricum. A significant thickness of soil × water availability interaction was also observed for the infructescence number of B. pilosa, but not on the seed number, infructescence biomass or seed biomass (Table 3).
Table 3 Results of Two-way ANOVA on the infructescence and seed number and biomass of X. sibiricum and B. pilosa
Species Treatment F-value
Infructescence number Seed number Infructescence biomass Seed biomass
X. sibiricum Water 5.36** 11.72** 15.26** 6.29**
Soil 58.43** 78.79** 85.97** 118.18**
Water × Soil 3.18* 5.11** 3.26* 2.95*
B. pilosa Water 7.55** 4.56* 4.83* 4.97*
Soil 216.49** 142.38** 212.94** 162.22**
Water × Soil 4.10** 0.34ns 1.36ns 0.38ns

Note: *, ** indicate significant differences at P < 0.05, P < 0.01, respectively. ns: no significant difference at the 0.05 level.

3.3 The relationship between seed number and seed biomass

In X. sibiricum, there was almost no significant positive correlation between the seed number and seed biomass of the offspring after their parents experienced water reduction, except under SMWL and SLWCK (Fig. 2) (P>0.05). However, in B. pilosa, there was a significant positive correlation between the seed number and seed biomass of the offspring after their parents experienced water availability and thickness of soil reductions (P<0.05) (Fig. 2). Overall, the slope decreased with the reduction in resources.
Fig. 2 The relationship between seed number and seed biomass of X. sibiricum (a) and B. pilosa (b)

Note: r: Pearson’s correlation coefficient; α: Regression slope. WCK, WM and WL: high (no reduction in water), moderate (50% reduction in water) and low (70% reduction in water) water availability, respectively. SCK, SM and SL: deep (70 cm), moderate (40 cm) and shallow (10 cm) thickness of the soil, respectively; *, ** and *** indicate significant differences at P < 0.05, P < 0.01 and P < 0.001, respectively; ns: no significant difference at the 0.05 level.

3.4 Offspring seed germination rate

The seed germination rate of the offspring of X. sibiricum did not change with the decrease of soil thickness for their parents under the three water treatments tested in the experiment (P>0.05). Due to the limited seeds received in the shallow soil and moderate to severe drought conditions, the data for their germination rates were not available for B. pilosa. However, we found that the seed germination rate for the offspring of B. pilosa was significantly decreased with the decrease of the soil thickness experienced by their parents under the well-watered treatment, WCK (P<0.05) (Fig. 3).
Fig. 3 The effect of different thicknesses of soil and water availability levels on offspring seed germination rates of X. sibiricum and B. pilosa

Note: WCK, WM and WL: high (no reduction in water), moderate (50% reduction in water) and low (70% reduction in water) water availability, respectively. SCK, SM and SL: deep (70 cm), moderate (40 cm) and shallow (10 cm) thickness of the soil, respectively. Different lowercase letters indicate significant differences between the different thicknesses of soil under the same water availability level (P < 0.05).

Comparing the seed germination rates of the offspring of the two species, the rate of X. sibiricum was lower than that of B. pilosa after their parents experienced the control soil (deep soil) habitat; while it was higher after their parents experienced the shallow soil habitat (Fig. 3).

4 Discussion

Many studies focusing on plants in karst areas have shown that limited resources, including water and soil volume, can inhibit plant growth (Zhao et al., 2017). We also found that the vegetative growth of X. sibiricum and B. pilosa are constrained by drought or shallow soil in our other unpublished work. Reproduction is one of the most critical processes in a plant’s life history, and it is related with vegetative growth since the later provides nutrition for reproduction. Therefore, reproduction will also be inhibited by limited resources or stress. In our study, we did find the infructescence (and seed) number and biomass decreased significantly with the decline in soil thickness at any given water availability level, which is also consistent with previous research (Li et al., 2019). When plants suffer from resource reduction or environmental stress, both the vegetative growth of the parents and the production of offspring (infructescence or seeds) are inhibited to varying degrees. These results indicate that both the vegetative growth of the parents and the production of offspring (infructescence or seeds) are inhibited by both small soil volume and drought, i.e., the double stress that occurs in karst areas. However, we found a greater decrease in the infructescence (and seed) number and biomass from the decline in soil thickness at the lowest water application level (WL), as expected. Sufficient reproductive organs are the foundation of the development of plant populations, so that plants have to rely on the production of reproductive organs to avoid adverse environments. Under WL, even though the water content decreased significantly with the decreased thickness of soil, the two species changed their reproduction output ratios as much as possible to ensure the continuation and development of the population (Zhu and Wang, 2002; Galloway, 2005).
However, unlike in X. sibiricum, we did not find that B. pilosa reduced seed number, infructescence number, seed biomass or infructescence biomass under the condition of reduced moisture. On the contrary, it even chose to increase the biomass investment of infructescence and seeds when the water content was reduced. Many studies (for example, Liu et al., 2018; Yan, 2019) have shown that annual plants increase reproductive allocation under environmental stress to produce more seed and increase survival opportunities (Li et al., 2021). However, other studies have shown that the distribution of plant reproduction is also related to plant size, i.e., that larger plants usually have less investment in reproduction than smaller species (Dilixiati et al., 2014; Lin, 2018). Obviously, due to different plant sizes the two plants in this study have different biomass investment responses.
The fecundity of plants is not only related to the number of reproductive organs, but also to the size of the reproductive organs (Wu et al., 2006). In this study, the seed biomass and seed number of X. sibiricum did not show a significant positive correlation under the various treatments, while in B. pilosa they showed a positive correlation regardless of the decrease in soil thickness or the decrease in water content. Furthermore, the slope decreased with the decrease of resources. Studies have shown that due to extreme changes in habitat, plants will use different trade-off strategies, which will help maintain populations in unpredictable environments (Edwards et al., 2016). For X. sibiricum, choosing larger and fewer seeds or smaller and many seeds, it seems more random, while B. pilosa in the face of the double pressure, chooses seed size and number trade-offs, to ensure that there is always a part of the individuals that can successfully survive the adverse environment and ensure population reproduction. Therefore, to some extent, X. sibiricum produced smaller seeds, with stronger resistance to the unfavorable environmental conditions, and greater advantages in population establishment and regeneration (Mark et al., 1996). While B. pilosa used r-selection to produce a greater seed number, to invest in spreading the seeds on a large scale, this would expand their habitats and contribute more to vegetation (Coomes et al., 2003).
This study found that under any water conditions, the offspring seed germination rate of X. sibiricum was not affected by soil thickness, which was consistent with the study on Convallaria majalis (Ove, 1999). The competitive ability was still guaranteed to adapt to the pressure of reduced soil resources. For B. pilosa, the soil thickness and germination rate had a proportional relationship when the plants were grown in an environment with sufficient water, which was consistent with Greipsson and Davy (1995). This indicated that B. pilosa chose the risk-sharing strategy for the strategy of germination and growth (Pake, 1996; Bruno et al., 2003). When faced with severe stresses, B. pilosa even tried to produce bigger seeds. However, regretfully, we did not get enough seeds for germination so that we could not assess the possible strategy under severe stresses for B. pilosa.

5 Conclusions

In conclusion, X. sibiricum resists the pressure of karst soil resource reduction by ensuring the quantity and quality of offspring to improve its competitiveness, that is, tending towards the K end of the r-K continuum, as we expected. In contrast, B. pilosa adapts to unfavorable karst habitats by a risk-sharing strategy. It tries to produce more and bigger seeds to enhance its competitive ability and enhance its dispersal ability, that is, tending toward the r end of the r-K continuum. However, when the karst soil resources are reduced, it ends with a lower number of seeds and the germination rate of offspring. Overall, compared with X. sibiricum, B. pilosa has more flexible plasticity and a greater breeding investment in the dry and barren karst regions.

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