1 Introduction
As a crucial component of global environmental change, climate warming has been usually recognized as a graduated process (Seifert et al., 2015). However, in recent decades, an increase in the frequency and magnitude of climatic extremes has been obvious (Smith, 2011; Baldwin et al., 2019). Extreme weather event such as heat wave has swept the global and inflicted heavy losses on the human society (Sun et al., 2018; Baldwin et al., 2019). Meanwhile, extreme warming has been certified to impact the biosphere (Beznosov & Suzdaleva, 2004; Elias, 2018). The likelihood of extreme warming events has been predicted to increase and last longer, and the future scenario of climate warming may change (Sun et al., 2018).
As terrestrial ecosystem, aquatic ecosystem suffers the disturbance from climate warming as well (Beznosov & Suzdaleva, 2004; Audet et al., 2017). The effects of climate warming on the reproductive pattern of terrestrial plants have been equivocal – several studies have indicated a loss of sex and others reported the contrast (Dorken & Eckert, 2001; Pluess & Stöcklin, 2005; Klady et al., 2011; Dolezal et al., 2020). Shallow freshwater habitats are probably more sensitive to climate warming and their water temperature increases more rapidly than the temperature of deep water due to their close proximity to air (Mooij et al., 2008). Aquatic macrophytes are the major producers in the shallow freshwater habitats and prominently impacted by the climate warming (Beznosov & Suzdaleva, 2004). Climate warming has been proven to boost the growth and expansion of several macrophyte species (Wu & Ding, 2019; Zhang et al., 2019). As a crucial stage of life history, the reproductive strategy of clonal macrophytes has been a key point for the understanding of their ecology and evolution (Franklin et al., 2021). However, the reproductive strategy of clonal macrophytes under climate warming has been understudied. Asexual clonal reproduction dominates in the breeding system of macrophytes especially for the submerged life-form. Hence, previous researches on the warming effects on the reproduction of macrophytes have mainly concerned clonal propagation. For instance, You et al. (2013, 2014) and Liu et al. (2016) found that warming promoted the clonal propagation ofEichhornia crassipes . Silveira & Thiébaut (2017) found than an increase in temperature favored the recruitment of lateral branches ofElodea canadensis . Yan et al. (2021) showed that warming enhanced the clonal asexual propagation of Potamogeton crispus . Prior researches have investigated the sexual reproduction of macrophytes in response to warming and showed that extreme warming impeded the sexual reproduction (Li et al., 2016; Xu et al., 2020; Yan et al., 2021). Most clonal plants possess two reproductive modes – they generate offspring sexually through seeds and asexually through vegetative propagules (Franklin et al., 2021). Sexual reproduction promotes the genetic differentiation of sexual offspring while asexual propagation enables the clonal progeny to conserve the genetic information from their genets (McKey et al., 2010). Therefore, the reproductive pattern – how the reproductive efforts are assigned in different reproductive modes, determines the fitness of clonal plants and have further evolutionary impacts (Wang et al., 2021b). The effects of clonality on sexual reproduction has been drawn increased attention in recent years (Barrett, 2015). The phenotypic plasticity in plant allocation has been verified to be prominent in response to climate change factor such as warming in a meta-analysis (Stotz et al., 2021). However, an integral exploration on the reproductive pattern of clonal aquatic macrophytes – how the reproductive effort is assigned under climate warming is scarce. Moreover, due to the clonal characteristic of most aquatic macrophytes, the potential difference in genetic background has been commonly neglected especially for the submerged plants despite that the geographic distance correlates with the genetic variance (Wu et al., 2015; Franklin et al., 2021). Despite the likely genetic variance among different clonal macrophyte populations, above studies on the reproduction of macrophytes merely considered plant materials from single geographic source and a broad perspective on the effects of geographic source on the reproductive pattern of macrophytes is scarce. Ecological and genetic factors may jointly contribute to the formation of reproductive pattern for clonal plants; however, the relative importance of environment and genetic background has been insufficiently evaluated (Eckert, 2002; Gillard et al., 2020).
Potamogeton crispus L. or the curly pondweed is a cosmopolitan clonal submerged plant species native to Eurasia (Jian et al., 2003; Heuschele & Gleason, 2014). Both asexual and sexual reproduction are available for this species (Xu et al., 2020; Yan et al., 2021). Hence, this species is an ideal model species for the study of reproductive pattern. Despite that, asexual propagation has been the major concern for the researchers since vegetative reproduction is the main reproductive mode (Heuschele & Gleason, 2014). The asexual propagule ofP. crispus is called turion, which is a metamorphosis of shoot apices and a combination of modified, short leaves condensed on extremely shortened stems (Adamec, 2018). Turions or vegetative buds usually function as overwintering organs for many clonal macrophytes from temperate to polar regions (Adamec, 2018). However, the turion of P. crispus is dormant innately over the warm summer, germinates in winter or early spring, and the seeds and turions formed along with the decay of vegetative parts such as shoots in late spring or early summer (Sastroutomo, 1981; Heuschele & Gleason, 2014; Adamec, 2018; personal field observation). The formation of aestivated turions depends on the synergy of high temperature and long duration at high irradiance in early summer (Adamec, 2018). Therefore, climate warming can be expected to cause the transition of life history and alter the reproductive pattern ofP. crispus (Xu et al., 2020; Yan et al., 2021). The morphology of turion has been a focus in several prior researches. For instance, Xie & Yu (2011) and Xie et al. (2015) found that the nutrient level determined the size and number of turion production. Qian et al. (2014) proved that water column phosphorus rather than water column nitrogen determined the number and biomass of turion production, and water nutrient level influenced the scale leaf number of turion and thus the turion size. Moreover, the metamorphosed leaves or the condensed scale leaves contain chlorophyll and can photosynthesize, which confer an ecological advantage on this species and help produce new organs (Adamec, 2018). Few studies concerned the sexual production ofP. crispus . Xu et al. (2020) and Yan et al. (2021) reported that the sexual reproduction ofP. crispus weakened under warming.
In the present study, four populations of P. crispus originated from different geographic sources were subjected to different situations of unwarming and extreme warming (a 2–5.8 ℃ increase in water temperature) in a mesocosm experiment. The asexual propagation (turion production) and sexual reproduction (seed output) were evaluated to explore the reproductive pattern under warming. Following hypotheses were postulated:
1. Warming affects the production and morphology of asexual turions forP. crispus ;
2. A loss of sex exists under warming;
3. Fitness is optimized through alteration of reproductive pattern under warming.