Using B. pilosa seeds from three locations, this paper reports a study of the effects of high temperature and water stress on seed viability and germination. It was found that B. pilosa seeds had a wide temperature range for germination, as all seeds germinated when incubated at 10–30°C, although germination was repressed at 35°C and inhibited at 40°C, with the extent varying with the provenance of the seeds. Regardless of whether it was generated with PEG or NaCl, the seedling percentage was 100% under water potentials up to − 0.6 MPa, with this tolerance extending to − 0.8 MPa for seeds from Xishuangbanna. Depending on the seed provenance, the seeds could tolerate a 6–21 h daily thermoperiod at 40°C without effects on germination and 8–10 days of continuous heating at 40°C and 30 min of heating at 50–70°C without a loss of viability. Marked variation in tolerance to high temperature for B. pilosa seeds was revealed based on these three provenances, with seeds from Xishuangbanna always demonstrating the highest viability after heat treatment and the highest tolerance during germination under stress.
Germination is a key step for the invasion of most plants. The ability of a species to germinate rapidly under a wide set of environmental conditions has been regarded as an important trait for invasive species (Vilà and Weiner, 2004), which was supported by the results of the present study. Compared to the results of our previous studies on invasive plant seeds in Xishuangbanna, such as Piper aduncum (Wen et al., 2015), Tithonia diversifolia (Wen, 2015), Amaranthus spinosus (Ye and Wen, 2016), and three invasive Asteraceae weeds (Crassocephalum crepidioides, Conyza canadensis and Ageratum conyzoides, Yuan and Wen, 2017), B. pilosa seeds from the Xishuangbanna provenance had an even broader temperature range for maximum germination, demonstrating complete germination when incubated at temperatures ranging from 10 to 30°C, almost complete germination at 35°C, and partial germination at 40°C. Only C. crepidioides and C. canadensis were found to have a similar minimum temperature for germination, whereas A. spinosus has a similar maximum temperature.
In addition to germination traits, seeds tolerance to high temperature is regarded as a key factor contributing to invasive capacity (Wen, 2015; Wen et al., 2015), as the seeds must survive before germination. Plant invasion mostly occurs in open habitats, where the maximum ground temperature reached by open soil can be very high on sunny days, such as > 60°C in Xishuangbanna with an extreme value of 71.4°C recorded (Wen et al., 2015). Meanwhile, after dispersal, seeds usually need to remain on the ground for a period until conditions become suitable for germination, and thus, high-temperature stress is unavoidable, making high-temperature tolerance necessary for successful invasion. This was supported by our previously mentioned studies and again by the present study, as air-dried seeds of all of these invasive species tolerated 30 min of heating at 70°C without substantial viability loss, except for P. aduncum, which tolerated temperatures up to 65°C and exhibited a small viability loss at 70°C as an invader occurring at the forest edge in Xishuangbanna (Wen et al., 2015). This inter-specific convergence of high-temperature tolerance for seeds indicates its importance for plant invasion success in Xishuangbanna.
Comparative studies between invasive and native species and between invasive species and non-invasive congeners, as well as comparative studies of invasive species between native and invasive origins/populations, comprise an important approach to identify plant traits conferring invasion capability and include those of Bochenek et al. (2016), Cervera and Parra-Tabla (2009), Erfmeier and Bruelheide (2005), Mandák (2003), van Clef and Stiles (2001) and Wainwright and Cleland (2013). Many studies have found adaptive evolutionary changes in seed dormancy and germination traits, which contribute to plant invasion capability, as demonstrated by Beckmann et al. (2011), Hierro et al. (2009), Kudoh et al. (2007), Leiblein-Wild et al. (2014), Udo et al. (2017) and Xia et al. (2011). By comparison among seeds from different provenances, this study found intra-specific variation in B. pilosa germination traits, mainly for germination at 35 and 40°C and with a 9–24 h daily thermoperiod, indicating the effects of seed provenance on germination traits. This intra-specific variation also provides an explanation for the reported discrepancy/inconsistency in the germination temperature range of B. pilosa seeds in the literature. For example, Reedy and Singh (1992) reported that its optimum temperature range for germination was 25/20 to 35/30°C (day/night, 12/12 h) and that temperatures below 15/10°C were unfavourable for germination. Yan et al. (2013) recorded germination percentages of 70% at 10°C and 45% at 40°C, whereas these were less than 20% at 35°C in another study (Hong et al., 2004). However, previous studies paid little attention to the variation in seed tolerance to high temperatures even though invasive plants usually have a wide geographical distribution. This study revealed marked intra-specific variation in B. pilosa seeds among the three provenances, with their tolerance to high temperature always correlated with the climate conditions in the provenance of seed origin; for example, seeds from Xishuangbanna demonstrated the highest adaption to high temperatures, whereas those from Diqin had the least and those from Chuxiong had intermediate tolerance, assessed by both endurable maximum temperature for 30-min heating and longest continuous heating duration at 40°C. This variation with that in the germination trait together revealed that B. pilosa seeds have the ability to evolve fast and adapt to local conditions. We suggest that both inter-specific convergence among weedy seeds in Xishuangbanna and intra-specific B. pilosa divergence with respect to seed high-temperature tolerance and germination traits reflect the adaption of invasive plants to local conditions.
As a common invasive plant in tropical and subtropical regions (Huang and Kao, 2014), B. pilosa has been extensively studied, and it exhibits many seed traits contributing to its invasiveness. For example, this weed is comprised of individuals exhibiting two types of flowering-fruiting phenologies, which differ in seed production, seed mass, germination speed, and total germination (Gurvich et al., 2004). It is also a prolific seed producer, as its entire reproductive cycle can be completed in 57–70 days and can occur 5–6 times per years in some areas, depending on local climates; thus, a single plant can produce 3000–6000 seeds per year. Their burred seeds can be spread by attaching to animals’ fur, birds’ feathers, and people’s clothing. Moreover, this plant produces dimorphic seeds, which differ in dispersal capability, with central achenes often dispersed earlier than peripheral achenes (Rocha, 1996), and dormancy status, with long achenes germinating readily under a wide range of conditions and short achenes showing fairly precise germination requirements (Forsyth and Brown, 1982), as well as a light requirement that can be induced by leaf canopy shade (Fenner, 1984a,b). Furthermore, this study revealed the divergence in germination traits and high-temperature tolerance of B. polosa seeds. It appears that the variation in germination traits and seed high-temperature tolerance as an evidence of adaptation to local environments, in addition to the previously investigated features, could be responsible for the invasiveness of Bidens polosa transferred by seeds. We concluded that high-temperature tolerance is an important seed trait that contributes to plant invasion success in open habitats in Xishuangbanna.