Elevated temperature may accelerate invasive ...

5 downloads 0 Views 268KB Size Report
Subject Editor: Peter Zwerger, JKI, Germany. Summary. Numerous studies .... with diluted Hoagland solution (25% v ⁄v) once a week until harvest after 35 days.
DOI: 10.1111/j.1365-3180.2011.00884.x

Elevated temperature may accelerate invasive expansion of the liana plant Ipomoea cairica R-L WANG* à, R-S ZENG*à, S-L PENG , B-M CHEN , X-T LIANG*à & X-W XIN*à *Key Laboratory of Agroecology and Rural Environment of Guangdong Regular Higher Education Institutions, South China Agricultural University, Guangzhou, China,  State Key Laboratory of Biocontrol, School of Life Sciences, Sun Yat-Sen University, East Campus, Guangzhou, China, and àKey Laboratory of Ecological Agriculture, Ministry of Agriculture, South China Agricultural University, Guangzhou, China Received 15 March 2011 Revised version accepted 18 July 2011 Subject Editor: Peter Zwerger, JKI, Germany

Summary Numerous studies have shown that elevated CO2 levels promote liana establishment in forests, thus suggesting their increased prevalence in the future in these habitats. Limited information exists, however, concerning the effects of potentially increasing global temperatures on these plants. The invasive liana Ipomoea cairica has infested many forests in South China. We hypothesised that elevated temperature could change the resource allocation pattern and allelopathic potential of this plant, resulting in increased invasiveness. We compared seed germination, growth rates and leachate phytotoxicity of I. cairica at three temperatures (22, 26 and 30C). The seed germination rates of I. cairica were 11.6%, 21.2% and 26.4% at 22, 26 and 30C respec-

tively. Elevated temperature resulted in significant changes in morphology and biomass allocation of I. cairica. Shoot biomass of the plant increased, while the root biomass decreased with increasing temperature. Phytotoxicity of aqueous leachates from fresh leaves of I. cairica varied depending on receptor plants, but showed the strongest phytotoxic effects at the highest temperature (30C). Our results provide key information concerning the effects of elevated temperatures on the allelopathic potential, germination and growth of I. cairica and indicate that global warming could increase the invasiveness of this species. Keywords: global warming, allelopathy, aqueous leachate, biological invasion, morning glory, mile a minute vine.

WANG R-L, ZENG R-S, PENG S-L, CHEN B-M, LIANG X-T & XIN X-W (2011) Elevated temperature may accelerate invasive expansion of the liana plant Ipomoea cairica. Weed Research.

Introduction During the 20th century, global average surface temperature has increased by about 0.6C. Global circulation models predicted that the temperature will increase by 1.4–5.8C between 1990 and 2100 (Houghton et al., 2001; Schneider, 2001), which will in turn affect soil moisture, precipitation patterns and ice and snow cover (Houghton et al., 2001). Such a rapid climate change, unmatched at least in the past 1000 years, is likely to have significant impacts on biological species and ecosystems

(Jiang & Kulczycki, 2004; Klanderud, 2005). Climate change in combination with the direct warming effect may impact plant growth, reproduction and resource allocation, as well as interactions between species (Klanderud, 2005). Lianas (woody vines or climbers) are an abundant and diverse group of plants in forests throughout the world. They play a critical role in the maintenance of biological diversity and natural ecosystems and provide important food sources for wildlife (Grauel & Putz, 2004). Relative to shrubs and trees, lianas maximise

Correspondence: Shao-Lin Peng, State Key Laboratory of Biocontrol, School of Life Sciences, Sun Yat-Sen (Zhongshan) University, East Campus, Bei San Road, Guangzhou 510006, Guangdong, China. Tel: (+86) 20 39332983; Fax: (+86) 20 39332983; E-mail: [email protected]  2011 The Authors Weed Research  2011 European Weed Research Society Weed Research

2 R Wang et al.

resource investment in height and leaf area (LA) growth with minimal expenditure in supportive tissues (Putz, 1991; Condon et al., 1992). An increased investment in LA is expected to result in higher relative growth rates in terms of biomass increment (Den Dubbelden & Verburg, 1996). The success of lianas in tropical forests may result from their growth pattern and efficient resource allocation (Condon et al., 1992). Lianas may allocate a smaller proportion of biomass to roots, because they only have to produce fine roots for nutrient absorption (Putz, 1991; Schnitzer et al., 2005). As lianas grow very rapidly and remain on top of the canopy, they can shade out their host trees and subsequently reduce their growth (Schnitzer et al., 2005; Allen et al., 2007). In response to the global climate change of the last decades, the vast growth of lianas is out of control and has severely affected tree growth and biological diversity of forest ecosystems (Wright et al., 2004; Zotz et al., 2006). Ipomoea cairica (L.) Sweet (Convolvulaceae) is a perennial herbaceous twisting vine originating from North America and is listed as one of the worst invasive species in South China (Weber et al., 2008). Leaves are papery, digitate or pinnate with five leaflets. Ipomoea cairica flowers all year in Guangdong. The flowers gathered in cymes with a pink or purple bell-formed corolla (Jia et al., 2007). The seeds are black to tan, 4–6 mm long, densely short-tomentose and subglobose to ovoid. Stems are long, thin and glabrous, and readily set roots at the nodes when in contact with soil (Jia et al., 2007). In addition, I. cairica can reproduce easily through both sexual and asexual propagation (Liu et al., 2006). The asexual propagation of its stem segments is the main reproductive method in South China (Hu & Wang, 2001). Some plants species possess the ability to suppress the growth and development of neighbouring plants through the release of phytotoxic allelochemicals into the environment (Callaway & Aschehoug, 2000; Chen et al., 2009). The release of allelochemicals can occur via several mechanisms, including foliar leaching, root exudation, residue decomposition and volatilisation (Inderjit & Nilsen, 2003). Allelochemicals play an important role in mediating plant–plant interactions (Rice, 1984). Allelopathy has been regarded as a mechanism for successful exotic plant invasion (Callaway & Aschehoug, 2000; Chen et al., 2009; Wang et al., 2010). Ma et al. (2009) isolated two allelochemicals: 3-3¢5-trihydroxy-4¢-7-dimethoxyflavone-3-O-sulphate and 3-3¢-5-trihydroxy-4¢-7-dimethoxyflavone from I. cairica, which were shown to inhibit seed germination of Raphanus sativus L., Cucumis sativus L., Brassica pekinensis L. and Ligularia virgaurea L. Ipomoea cairicaÕs rampant growth, wide eco-physiological tolerance,

strong reproduction and allelopathic inhibitory effect on neighbouring plants (Wu et al., 2006; Zhao & Peng, 2008; Ma et al., 2009) have now caused substantial damage to natural ecosystems and biodiversity in Guangdong province of China (Zhao & Peng, 2008). Palaeoclimatic records and palaeoecological evidence indicate that there is a poleward plant migration in latitude and an upward shift in the elevation with increased temperatures (Li et al., 2006). In general, elevated temperature enhances photosynthesis and stimulates growth of organs and meristems, accelerating plant development (Klanderud, 2005). Both biotic factors (insects, herbivores, pathogens) and abiotic factors (temperature, moisture, light and nutrients status) can affect the synthesis of allelochemicals (Lobo´n et al., 2002; Wang et al., 2010), the allelopathic potential of the donor plant, as well as the sensitivity of the receptor (Lobo´n et al., 2002). Increased temperatures may also enhance the production of potential allelochemicals. For example, the production of indole alkaloid gramine in the leaves of barley (Hordeum vulgare) was increased under high temperature (Hanson et al., 1983). Based on field investigations, we have observed that I. cairica has become more and more prevalent in the forests in south China (Zhao & Peng, 2008). Studies show that the rise of the winter minimum temperatures in Guangdong began in the second half of the 1960s and the warming became more evident since the 1980s (Liang & Wu, 2000). The observed increase in I. cairica may be attributed to global warming and especially temperature. We hypothesised that elevated temperature may change the allocation pattern between shoot and root and enhance the allelopathic potential of I. cairica, which may further increase its invasiveness. To better understand the effects of high temperature on germination rate, seedling development and allelopathic potential of the invasive alien plant I. cairica, three experiments were set-up testing different temperatures, including a germination experiment, a growth and morphology experiment and a bioassay experiment on allelopathic potential of I. cairica.

Materials and methods Germination experiment

Seeds of I. cairica were collected in May 2009 from Baiyun Mountain, Guangzhou (N 2320¢, E 11329¢) and stored in desiccators for 1 month. Uniform seeds were selected and surface-sterilised with 0.5% KMnO4 for 15 min and then washed with sterile water. Fifty seeds were sown on two layers of filter paper (15 cm diameter, qualitative filter paper, No.1) in a glass Petri dish (15 cm diameter) containing 10 mL of sterile water.  2011 The Authors Weed Research  2011 European Weed Research Society Weed Research

Temperature and invasion of Ipomoea cairica 3

The Petri dishes were placed in growth chambers in the dark at three constant temperatures (22, 26 and 30C) respectively. Seeds with a minimal root length of 1 mm were considered as germinated. The germination rate was recorded over a 9-day incubation period. There were five independent replicates. Growth and morphology experiment

Plants of I. cairica (6.5 m in height) at the vegetative stage were collected in April of 2010 from a natural population from the farm of South China Agriculture University, Guangzhou (N 2316¢, E 11334¢). They were cut into 10-cm-long pieces before uniform cuttings were selected, transplanted into plastic pots (9 cm diameter, 12.5 cm high, one plant per pot) and allowed to climb on 1.2 m bamboo stakes. The pots were filled with equal proportions of pool mud and river sand. Plants were grown in a glasshouse (25 ± 1C, 14 ⁄ 10 h day ⁄ night cycle, 75% ± 2% relative humidity, 400 mmol m)2 s)1). When the seedlings grew about 15 cm high, they were selected for the experiments. Plants (10 pots each) were transferred into growth chambers (14 ⁄ 10 h day ⁄ night cycle, 75% ± 2% relative humidity, 400 mmol m)2 s)1) at three constant temperatures (22, 26 and 30C) respectively. All plants were randomised twice a week to avoid internal chamber effects. The plants were watered with diluted Hoagland solution (25% v ⁄ v) once a week until harvest after 35 days. At harvest, five pots were used for analysis of the allelopathic potential of its aqueous leachates. Plants from the remaining five pots were harvested and divided into leaves, stems and roots for each treatment. The total stem length was measured, and the total LA was determined using a LA meter (CI-203 Area-meter; CID, USA). After drying at 70C for 72 h, leaves, stems and roots were weighed. The specific leaf area was calculated as the ratio of LA to leaf dry mass. The leaf (stem, root) mass ratio (LMR, SMR and RMR) was calculated by dividing the dry mass by total plant dry mass.

respectively, to produce aqueous leachates with a concentration of 0.1 g mL)1 (Wu et al., 2006; Zhao & Peng, 2008). The leachates were filtered through two layers of filter paper and then diluted with distilled water to prepare concentrations of 0.1, 0.05 and 0.025 g mL)1. The pH value of the leachates was adjusted to 6.8 with 1 M HCl or NaOH, and distilled water was used as control. Seeds of R. sativus and B. campestris were surfacesterilised using the same method as described above. Twenty-five seeds of each test plant were sown on the top of two layers of filter paper (9 cm diameter, qualitative filter paper, No.1) in a glass Petri dish (9 diameter) containing 5 mL of each aqueous leachate of I. cairica. The controls were made with distilled water. Plants were cultivated in growth chambers (dark condition) at three constant temperatures (22, 26 and 30C) respectively. Germination rates were recorded at 3 days after incubation and seedling shoot height and root length were recorded 5 days after incubation. Sprouted seeds with minimal root length of 1 mm were considered as germinated. All experiments were conducted with three independent replicates. The inhibition or stimulation of response parameters in the bioassay was evaluated using a response index (RI) that was calculated as follows: RIð%Þ ¼ ðT =C  1Þ  100

ð1Þ

where C was control value and T was treatment value. If RI is >0, the effect is stimulatory, while RI is