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.