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.