4.1 Warming shifted the metamorphosis for turion formation
The warming effects on the reproductive quantity of vegetative
propagules were complex, since a decreased turion number for population
1, an increased turion number for population 2 and a consistency in
turion number for population 3 and 4 were observed under warming.
However, considering turion biomass, despite an increased turion biomass
was shown for population 2 under warming, the remaining three
populations displayed a coherence in turion biomass production under
warming. Namely, the turion biomass production did not decrease under
warming. In terms of turion biomass production, at least P.
crispus could maintain its fitness under warming. Extreme warming was
observed to advance the initial time for turion formation in the present
study. However, the turion biomass production was unresponsive to
warming. The decay rate of macrophyte shoots has been positively
correlated with temperature (Li et al., 2021). Extreme warming may boost
the decay of plant shoots, which may disturb the formation of turions
since turions are innately the metamorphosis of apical shoots (Adamec,
2018). P. crispus has been widely considered to be tolerable to
environmental stresses such as eutrophication and heavy metal pollution
(Cao et al., 2009; Qiao et al., 2015). From the perspective of asexual
reproduction, the strong tolerance capacity of P. crispusappeared to be certified once again. However, the present result for
turion production has certain discrepancy with a recent report from Yan
et al. (2021), which showed an increasing turion production under
warming. The turions used for the experimental set-up in our experiment
originated from four distant populations in the warm temperate zone
while the material source of Yan et al. (2021)’s study was populations
in Liangzi Lake from sub-tropical region. The linear distance between
the sampling site of Yan et al. (2021)’s study and that of Population 1
in our study is over 1300 km. Local adaptation and differentiation of
plant ecotypes may occur among these populations since geographic
distance is relevant with genetic variance (Wu et al., 2015; Franklin et
al., 2021). Thus, the plant populations from warm temperate region are
speculated to be less sensitive in clonal reproduction to warming than
that from subtropical zone. The underlying mechanism for the difference
in clonal reproduction among different climate zones has not been
explored for clonal macrophytes and needs further investigation.
Intriguingly, P. crispus decreased the production of
oligophyllous turions and produced more pleiophyllus turions under
warming. Considering the modular structure of turion, this strategy may
contribute to the enhancement of fitness for P. crispus . First,
pleiophyllous turions are larger in size than oligophyllous turions.
Larger vegetative propagules with more scale leaves likely have larger
nutrient reserve and thus possess greater longevity and vitality (Xie &
Yu, 2011). Second, the scale leaves possess the photosynthetic
potential. Pleiophyllous turions may take the advantage of higher
photosynthetic efficiency and thus help produce new organs more rapidly
(Adamec, 2018). Lastly, a pleophyllous turion has more axillary buds
than an oligophyllous turion. The axillary buds are the sole modules for
the generation of young seedlings. Therefore, pleiophyllous turions
probably have greater germination potential than oligophyllous turions
(Qian et al., 2014). Overall, the production of more pleiophyllous
turions is likely a strategy of fitness optimization for P.
crispus under warming.