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Effects of warming temperatures on germination responses and trade-offs between seed traits in an alpine plant

Citation

Notarnicola, Rocco; Nicotra, Adrienne; Kruuk, Loeske; Arnold, Pieter (2022), Effects of warming temperatures on germination responses and trade-offs between seed traits in an alpine plant, Dryad, Dataset, https://doi.org/10.5061/dryad.np5hqbzx9

Abstract

1. Climate warming may affect multiple aspects of plant life history, including important factors such as germination responses and the key trade-off between offspring size and number. As a case study to address these concepts, we used an alpine plant (waxy bluebell, Wahlenbergia ceracea; Campanulaceae) that shows plasticity to warming in seed traits and in which seed dormancy status regulates germination. We chose an alpine species because alpine environments are ecosystems particularly under threat by climate change.

2. We conducted germination assays under cool and warm temperatures using seeds produced by individuals that were grown under historical (cooler) and future (warmer) temperature scenarios. We assessed the presence of a seed size vs number trade-off, and then examined the effects of seed number and size on germination percentage, the fractions of dormant and viable seeds, and germination velocity. Further, we examined whether warming during parental growth and during germination affected these relationships.

3. We found evidence for a seed size vs number trade-off only under historical parental temperatures. Indeed, under future growth temperatures, parental plants produced fewer and smaller seeds and there was no evidence of a trade-off. However, the reductions in both seed traits under warming did not affect germination, despite correlations of seed size and number with germination traits. Warming increased germination, particularly of larger seeds, but overall it resulted in more than fourfold reductions in parental fitness.

4. Synthesis. Our study shows the importance of growth conditions when evaluating the seed size vs number trade-off. Stressful conditions, such as warmer temperatures, can restrain the ability of plants to reach optimal investment in reproduction, masking the trade-off. By analysing responses across the whole life cycle, we show here an overall detrimental effect of warming, highlighting the potential risk of climate change for W. ceracea, and, potentially, for alpine plant communities more widely.

Methods

Experiments were conducted using the plant species Wahlnebrgia ceracea (waxy bluebells). Two datasets were used in this manuscript.

1) Seed size vs number trade-off: Parental individuals from a total of 30 lines ('Line') were grown in growth chambers for 191 days under temperature conditions of a historical/cooler (1960–1970) or a projected future/warmer (2090–2100) climate ('Parental_Temperature'). The parental individuals were randomly assigned to one of three blocks, which corresponded to positions inside the chambers, and each block was equivalent in all chambers ('Block'). Day and night temperatures during the experiment were changed every 15 days to mimic seasonality, with the maximum day temperatures during the peak of summer being 24°C and 29°C for the historical and future parental temperatures, respectively. After 100 days since the imposition of the temperature treatments (during the peak of the summer), half of the plants were moved for 5 days to new chambers, where the temperature was 5°C above the respective treatments, i.e., 29°C and 34°C ('Heatwave'). After this time, the parental individuals were moved back to their respective historical or future temperature treatments. We collected the seeds throughout the 191 days of parental growth, and we stored them in desiccators for at least 11 weeks. After this time, we calculated seed size ('Seed_Size') as the average mass of three lots of 50 seeds divided by 50. We calculated seed number ('Seed_Number') as the ratio between the cumulative mass of the seeds produced by each parental individual and seed size. The 30 lines of the parental individuals were obtained by crossing plants that originated from seeds that were collected at the same elevation, either high or low elevation ('Elevation') in sites within Kosciuszko National Park, NSW, Australia. Therefore, 14 lines originated from high elevations and 14 lines from low elevations.

2) Germination responses - seed traits correlations: The seeds were harvested from the parental individuals grown under historical/cooler or projected future/warmer temperatures ('Parental_Temperature') (see above) from a subset of 14 lines ('Line'). These seeds were used in germination assays in the glasshouse under cool (25°C) or warm temperatures (30°C) ('Germination_Temperature'). We measured seed size ('Seed_Size') as the average mass of three lots of 50 seeds; then these seeds were sowed in agar dishes (25 seeds per dish, 2 dishes per temperature treatment from each parental individual). Seed number (‘Seed_Number’) was the same as above. Dishes were left under temperature treatments for 4 weeks to allow germination of the non-dormant fraction of the seeds ('Not_Dormant_Seeds') and germination was checked once per week. Then, all the dishes were moved to a cold room at 4–5°C in the dark for 4 weeks to allow cold stratification. After this time, dishes were moved back to the glasshouse under the same temperature treatments as before to allow germination of the dormant seeds. We considered seeds to be dormant ('Dormant_seeds') if they germinated during or after cold stratification or if they did not germinate at all but were still determined to be viable at the end of the experiment. We considered seed to be viable (‘Viable_Seeds’) if they germinated (‘Germinated_Seeds’) as well as the seeds that contained an endosperm but still did not germinate (‘Not_Germinated_Seeds’), while we considered empty seeds as non-viable (‘Not_Viable_Seeds’). Germinated and not germinated seeds (as above) were used to calculate the germination percentage. We calculated germination velocity (‘Germination_Velocity’) as the reciprocal of the mean germination time (germination velocity (germination (%) week-1) GV = (G1 + G2 +…+ Gn) / (G1 x T1 + G2 x T2 +…+ Gn x Tn), where Gn is the number of new germinating seeds at each sampling point, and Tn is the time between each sampling point (= one week).

The files provided present the datasets in their first sheet and keys with the definitions of each term in the second sheet.

Usage Notes

Files can be opened using Excel and analysed using R.

Funding

Australian Research Council, Award: DP170101681